GB2378469A

GB2378469A - Flexible electrically sequenced tractor

Info

Publication number: GB2378469A
Application number: GB0226839A
Authority: GB
Inventors: Duane Bloom; N Bruce Moore; Ronald E Beaufort
Original assignee: WWT International Inc
Current assignee: WWT International Inc
Priority date: 1998-12-18
Filing date: 1999-12-17
Publication date: 2003-02-12
Anticipated expiration: 2019-12-17
Also published as: GB0226835D0; GB2378469B; GB2378468B; GB0226839D0; GB2378468A

Abstract

A tractor for moving within a borehole which comprises an elongated body configured to pull equipment within said borehole, said equipment exerting a longitudinal load on said body; a gripper 104 longitudinally movably engaged with said body, said gripper having an actuated position in which said gripper limits movement between said gripper and an inner surface of said borehole, and a retracted position in which said gripper permits substantially free relative movement between said gripper and said inner surface; and a propulsion system 108,110 on said body for propelling said body through said borehole while said gripper is in said actuated position; wherein said body is sufficiently flexible such that said tractor can turn up to 30{ per 30m (100 feet) of travel, while said longitudinal load is at least 23kN (5250 pounds).

Description

i. ELECTRICALLY SEQUENCED TRACTOR

5 Background

Field of the Invention

The present invention relates to downhole drilling and, in particular, to an electrically sequenced tractor (EST) for controlling the motion of a downhole drilling tool in a borehole.

Description of the Related Art

The art of drilling vertical, inclined, and horizontal boreholes plays an important role in many industries, such as the petroleum, mining, and communications industries.

In the petroleum industry, for example, a typical oil well comprises a vertical borehole 15 which is drilled by a rotary drill bit attached to the end of a drill string: The drill string is typically constructed of a series of connected links of drill pipe which extend between ground surface equipment and the drill bit. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the bit, and to 20 remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole.

The method described above is commonly termed "rotary drilling" or 25 "conventional drilling." Rotary drilling often requires drilling numerous boreholes to recover oil, gas, and mineral deposits. For example, drilling for oil usually includes drilling a vertical borehole until the petroleum reservoir is reached, often at great depth.

Oil is then pumped from the reservoir to the ground surface. Once the oil is completely recovered from a first reservoir, it is typically necessary to drill a new vertical borehole 30 from the ground surface to recover oil from a second reservoir near the first one. Often a large number of vertical boreholes must be drilled within a small area to recover oil

from a plurality of nearby reservoirs. This requires a large investment of time and resources. In order to recover oil from a plurality of nearby reservoirs without incurring the costs of drilling a large number of vertical boreholes from the surface, it is desirable to 5 drill inclined or horizontal boreholes. In particular, it is desirable to initially drill vertically downward to a predetermined depth, and then to drill at an inclined angle therefrom to reach a desired target location. This allows oil to be recovered from a plurality of nearby underground locations while minimizing drilling. In addition to oil recovery, boreholes with a horizontal component may also be used for a variety of other 10 purposes, such as coal exploration and the construction of pipelines and communications lines.

Two methods of drilling vertical, inclined, and horizontal boreholes are the aforementioned rotary drilling and coiled tubing drilling. In rotary drilling, a rigid drill string, consisting of a series of connected segments of drill pipe, is lowered from the 15 ground surface using surface equipment such as a derrick and draw works. Attached to the lower end of the drill string is a bottom hole assembly, which may comprise a drill bit, drill collars, stabilizers, sensors, and a steering device. In one mode of use, the upper end of the drill string is connected to a rotary table or top drive system located at the ground surface. The top drive system rotates the drill string, the bottom hole 20 assembly, and the drill bit, allowing the rotating drill bit to penetrate into the formation.

In a vertically drilled hole, the drill bit is forced into the formation by the weight of the drill string and the bottom hole assembly. The weight on the drill bit can be varied by controlling the amount of support provided by the derrick to the drill string. This allows, for example, drilling into different types of formations and controlling the rate at 25 which the borehole is drilled.

The inclination of the rotary drilled borehole can be gradually altered by using known equipment such as a downhole motor with an adjustable bent housing to create inclined and horizontal boreholes. Downhole motors with bent housings allow the ground surface operator to change drill bit orientation, for example, with pressure pulses 30 from the surface pump. Typical rates of change of inclination of the drill string are relatively small, approximately 3 degrees per 30m (100 feet) of borehole depth. Hence,

the drill string inclination can change from vertical to horizontal over a vertical distance of about 900m (3000 feet). The ability of the substantially rigid drill string to turn is often too limited to reach desired locations within the earth. In addition, friction of the drilling assembly on the casing or open hole frequently limits the distance that can be 5 achieved with this drilling method.

As mentioned above, another type of drilling is coiled tubing drilling. In coiled tubing drilling, the drill string is a non-rigid, generally compliant tube. The tubing is fed into the borehole by an injector assembly at the ground surface. The coiled tubing drill string can have specially designed drill collars located proximate the drill bit that apply 10 weight to the drill bit to penetrate the formation. The drill string is not rotated. Instead, a downhole motor provides rotation to the bit. Because the coiled tubing is not rotated or not normally used to force the drill bit into the formation, the strength and stiffness of the coiled tubing is typically much less than that of the drill pipe used in comparable rotary drilling. Thus, the thickness of the coiled tubing is generally less than the drill 15 pipe thickness used in rotary drilling, and the coiled tubing generally cannot withstand the same rotational, compression, and tension forces in comparison to the drill pipe used in rotary drilling.

One advantage of coiled tubing drilling over rotary drilling is the potential for greater flexibility of the drilling assembly, to permit sharper turns to more easily reach 20 desired locations within the earth. The capability of a drilling tool to turn from vertical to horizontal depends upon the tool's flexibility, strength, and the load which the tool is carrying. At higher loads, the tool has less capability to turn, due to friction between the borehole and the drill string and drilling assembly. Furthermore, as the angle of turning increases, it becomes more difficult to deliver weight on the drill bit. At loads of only 25 9000 N (2000 pounds) or less, existing coiled tubing tools, which are pushed through the hole by the gravity of weights, can turn as much as 90 per 30m (100 feet) of travel but are typically capable of horizontal travel of only 750m (2500 feet) or less. In comparison, at loads up to 13000N (3000 pounds), existing rotary drilling tools, whose drill strings are thicker and more rigid than coiled tubing, can only turn as much as 30 30 40 per 30m (100 feet) of travel and are typically limited to horizontal distances of

1500-1800m (5000-6000 feet). Again, such rotary tools are pushed through the hole by the gravity force of weights.

In both rotary and coiled tubing drilling, downhole tractors have been used to apply axial loads to the drill bit, bottom hole assembly, and drill string, and generally to 5 move the entire drilling apparatus into and out of the borehole. The tractor may be designed to be secured between the lower end of the drill string and the upper end of the bottom hole assembly. The tractor may have anchors or grippers adapted to grip the borehole wall just proximal the drill bit. When the anchors are gripping the borehole, hydraulic power from the drilling fluid may be used to axially force the drill bit into the 10 formation. The anchors may advantageously be slidably engaged with the tractor body, so that the drill bit, body, and drill string (collectively, the "drilling tool") can move axially into the formation while the anchors are gripping the borehole wall. The anchors serve to transmit axial and torsional loads from the tractor body to the borehole wall.

- One example of a downhole tractor is disclosed in allowed U.S. Patent Application No. 15 08/694,910 to Moore ("Moore '910"). Moore '910 teaches a highly effective tractor design as compared to existing alternatives.

It is known to have two or more sets of anchors (also referred to herein as "grippers") on the tractor, so that the tractor can move continuously within the borehole.

For example, Moore '910 discloses a tractor having two grippers. Longitudinal (unless 20 otherwise indicated, the teens "longitudinal" and "axial" are hereinafter used interchangeably and refer to the longitudinal axis of the tractor body) motion is achieved by powering the drilling tool forward with respect to a first gripper which is actuated (a "power stroke"), and simultaneously moving a retracted second gripper forward with respect to the drilling tool ("resetting"), for a subsequent power stroke. At the 25 completion of the power stroke, the second gripper is actuated and the first gripper is retracted. Then, the drilling tool is powered forward while the second gripper is actuated, and the retracted first gripper is simultaneously reset for a subsequent power stroke. Thus, each gripper is operated in a cycle of actuation, power stroke, retraction, and reset, resulting in longitudinal motion of the drilling tool.

30 It has been proposed that the power required for actuating the anchors, axially thrusting the drilling tool, and axially resetting the anchors may be provided by the

drilling fluid. For example, in the tractor disclosed by Moore '910, the grippers comprise inflatable engagement bladders. The Moore tractor uses hydraulic power from the drilling fluid to inflate and radially expand the bladders so that they grip the borehole walls. Hydraulic power is also used to power forward cylindrical pistons 5 residing within propulsion cylinders slidably engaged with the tractor body. Each such cylinder is rigidly secured to a bladder, and each piston is axially fixed with respect to the tractor body. When a bladder is inflated to grip the borehole, drilling fluid is directed to the proximal side of the piston in the cylinder that is secured to the inflated bladder, to power the piston forward with respect to the borehole. The forward 10 hydraulic thrust on the piston results in forward thrust on the entire drilling tool.

Further, hydraulic power is also used to reset each cylinder when its associated bladder is deflated, by directing drilling fluid to the distal side of the piston within the cylinder.

Tractors may employ a system of pressure-responsive valves for sequencing the distribution of hydraulic power to the tractor's anchors, thrust, and reset sections. For 15 example, the Moore '910 tractor includes a number of pressure-responsive valves which shuttle between their various positions based upon the pressure of the drilling fluid in various locations of the tractor. In one configuration, a valve can be exposed on both sides to different fluid skeams. The valve position depends on the relative pressures of the fluid skeams. A higher pressure in a first stream exerts a greater force on the valve 20 than a lower pressure in a second stream, forcing the valve to one extreme position. The valve moves to the other extreme position when the pressure in the second stream is greater than the pressure in the first shream. Another type of valve is spring-biased on one side and exposed to fluid on the other, so that the valve will be actuated against the spring only when the fluid pressure exceeds a threshold value. The Moore tractor uses 25 both of these types of pressure-responsive valves.

It has also been proposed to use solenoid-conkolled valves in tractors. In one configuration, solenoids electrically trigger the shuttling of the valves from one extreme position to another. Solenoid-controlled valves are not pressure-actuated. Instead, these valves are controlled by electrical signals sent from an electrical control system at the 30 ground surface.

Various types of radially expanding anchors have been used in downhole tractors, such as rigid friction blocks, flexible beams, and engagement bladders. Some advantages of bladders are that they are more radially expandable and thus can operate within certain voids in the earth. Also, bladders can conform to various different 5 geometries of the borehole wall. One known bladder configuration comprises a combination of fiber and rubber. Previous designs utilized Nylon fibers and Nitrile Butadiene Rubber (NBR). The fatigue life of current bladder designs is such that the bladders are able to achieve as much as 7400 cycles of inflation.

One problem with bladders is that they do not resist torque in the tractor body.

10 As the drill bit rotates into the formation, the earth transmits a reactive torque to the bit, which is transmitted proximally through the tractor body. When an engagement bladder is inflated to grip the borehole wall, the compliant bladder tends to permit the tractor body to twist to some degree due to the torque therein. Such rotation can confuse tool direction sensors, requiring an approximation of such reverse twist in the dull direction 15 control algorithm.

Prior art tractors have utilized anchors which permit at least some degree of

rotation of the tractor body when the anchor is engaged with an underground borehole wall. A disadvantage of this configuration is that it causes the drill string to absorb reaction torque from the formation. When drilling, the drill bit exerts a drilling torque 20 onto the formation. Simultaneously, the formation exerts an equal and opposite torque to the tractor body. This torque is absorbed partially by the drill string, since the configuration allows rotation of the tractor body when the anchor is actuated. This causes the drill string to twist. If all of the anchors are retracted, which may occur when the tool is to be retrieved, the drill string tends to UIltWiSt, which can result in 25 inconsistent advance during walking.

Thus, there is a need for a downhole drilling tractor which overcomes the above mentioned limitations of the prior art.

Summary of the Invention

30 Accordingly, it is a principle advantage of the present invention to overcome some or all of these limitations and to provide an improved downhole drilling tractor.

The invention provides a tractor for moving within a borehole, comprising: an elongated body configured to pull equipment within said borehole, said equipment exerting a longitudinal load on said body; a gripper longitudinally movably engaged with said body, said gripper having an actuated position in which said gripper limits 5 movement between said gripper and an inner surface of said borehole, and a retracted position in which said gripper permits substantially free relative movement between said gripper and said inner surface; and a propulsion system on said body for propelling said body through said borehole while said gripper is in said actuated position; wherein said body is sufficiently flexible such that said tractor can turn up to 30 per 30m (100 feet) 10 of travel, while said longitudinal load is at least 23kN (5250 pounds).

The structural configuration of the tractor, which allows it to work within the harsh environment and limited space within the bore of an oil well, is an important aspect of the invention. A preferred embodiment of the invention has a structural configuration that permits the tractor to fit within an envelope no more than 22cm (8.5 15 inches) in diameter and, preferably, no more than 7.30cm (2.875 inches) in diameter.

This relatively small diameter permits the tractor to work with standard oil well equipment that is designed for 7.30-22cm (2.875-8.5 inch) diameter well bores.

Another important aspect is the structural configuration that permits the tractor to make relatively sharp turns. Specifically, the tractor desirably has a length of no more than 20 46m (150 feet), more desirably no more than 30rn (100 feet), more desirably no more than 23m (75 feet), more desirably no more than 15m (50 feet), and even more desirably no more than 12m (40 feet). Preferably the length of the tractor is approximately tom (32 feet). Advantageously, the tractor can turn at least 60 per 30m (100 feet) of travel.

Yet another important aspect is a structure that permits the tractor to operate at 25 downhole pressures up to 1.1 x lO8Nm 2 (16,000 psi) and, preferably, 3.4 x 107 - 6.8 x 107Nm 2 (5,000-lO,OOO psi), and downhole temperatures up to 149 C (300 F) and, preferably, 93 - 121 C (200-250 F). Preferably, the tractor can operate at differential pressures of 1.4 x 106 - 1.7 x 107Nm 2 (200-2500 psi), and more preferably within a range of 3.4 x 106 - 1.1 x 107Nm 2 (500-1600 psi) (the pressure differential between the 30 inside and outside of the EST, thus across the internal flow channel and the annulus surrounding the tractor).

l Advantageously, the body is sufficiently flexible such that the tractor can preferably turn up to 30 , more preferably 45 , and even more preferably 60 per 30m (100 feet) of travel, while pushing or pulling a longitudinal load of at least 231 - (5250 pounds). The particular load which the body can push or pull while exhibiting this 5 turning capability depends upon the body diameter. Various embodiments of the invention include tractors having diameters of 5.5cm (2.175 inches), 8. 6cm (3.375 inches), 12.1cm (4.75 inches), and 15.2cm (6.0 inches). Note that other embodiments are also conceived. A tractor having a diameter of 5.5cm (2.175 inches) desirably has the above-mentioned turning capability while pushing or pulling loads up to 4.4kN 10 (1000 pounds), and more desirably up to 8.8kN (2000 pounds). The same information for other embodiments is summarized in the following table: EST Diameter Load at which tractor can turn un to 30 . 45 . or 60 per 30m (100 feet) 5.5cm (2. 175 inches) Preferably 4.4kN (1000 pounds), and more preferably 8.8kN (2000 pounds) 8.6cm (3.375 inches) Preferably 23kN (5250 pounds), and more preferably 46kN (1O,500 pounds) 12.1cm (4.75 inches) Preferably 58kN (13,000 pounds), and more preferably 120kN (26,000 pounds) 15.2cm (6.0 inches) Preferably lOOkN (22,500 pounds), and more preferably 200kN (45, 000 pounds) The flexibility of the tractor is preferably such that said tractor can bend so as to IS turn up to 60 per 30m (100 feet) of travel, while said longitudinal load is as high as 47kN (1O,500 pounds). The body may have a diameter of at least 2.5cm (one inch).

The tractor may have a length of at least 3m (10 feet).

The gripper may have a length of at least Sum (10 inches).

The grippers are preferably configured to transmit a torque of up to 680Nm (500 20 ft.lbs) to the borehole.

It should be noted that as the maximum diameter of the tractor's pistons, shafts, and control assembly increase so also shall the maximum thrustpull and speed. These and other design considerations can be adjusted for optimum performance with respect to maximum and minimum speed, maximum and minimum pull-thrust, control response 25 times, turning radius, and other desirable performance characteristics.

In one embodiment, the tractor has large diameter segments and small diameter segments. The large diameter segments include one or more of (1) a valve housing

having valves configured to control the flow of fluid to components of the propulsion system, (2) a motor housing having motors configured to control the valves, (3) an electronics housing having logic componentry configured to control the motors, (4) one or more propulsion chambers configured to receive fluid to propel the body, (5) pistons 5 axially movable within the propulsion chambers, and (6) the gripper. For the tractor having a diameter of 8.6cm (3.375 inches), the large diameter segments have a diameter of at least 7.9cm (3.125 inches). The small diameter segments have a diameter of 5.2cm (2.05 inches) or less and a modulus of elasticity of 4.6 x 10 2Nm 3 (19,000,000) or more.

Substantially all of the bending of the tractor occurs in the small diameter segments.

10 Thus the tractor preferably has large diameter segments and small diameter segments, said large diameter segments including one or more of a valve housing having valves configured to control the flow of fluid to components of said propulsion system, a motor housing having motors configured to control said valves, an electronics housing having logic componentry configured to control said motors, one or more propulsion chambers 15 configured to receive fluid to propel said body, pistons axially movable within said propulsion chambers, and said gripper, said large diameter segments having a diameter of at least 7.9cm (3.125 inches) , substantially all of the bending of said tractor occurring in said small diameter segments.

Preferably, said large diameter segments include said propulsion chambers and 20 said pistons inside said propulsion chambers, said pistons being fixed to said body, each of said pistons surrounding a segment of said body and having an annular surface generally perpendicular to said body, said annular surface being configured to receive thrust from a fluid flowing to said piston, said pistons having a diameter of at least 3.8cm (1.5 inches).

25 The invention also provides a tractor for moving within a borehole, comprising: an elongated body defining a longitudinal axis and being configured to transmit torque through said body, said body configured so that when said body is subjected to a torque about said longitudinal axis of as high as 680Nm (500 ft-lbs), twisting of said body is limited to 5 per step of the tractor; a first gripper axially movably engaged with said 30 body, said first gripper having an actuated position in which said first gripper limits movement of said first gripper relative to an inner surface of said borehole and a

retracted position in which said first gripper permits substantially free relative movement between said first gripper and said inner surface, said first gripper being rotationally fixed with respect to said body so that said first gripper resists rotation of said body with respect to said borehole when said first gripper is in said actuated 5 position; a second gripper axially movably engaged with said body, said second gripper having an actuated position in which said second gripper limits movement of said second gripper relative to an inner surface of said borehole and a retracted position in which said second gripper permits substantially free relative movement between said second gripper and said inner surface, said second gripper being rotationally fixed with 10 respect to said body so that said second gripper resists rotation of said body with respect to said borehole when said second gripper is in said actuated position; and a propulsion system on said body, for propelling said body when at least one of said grippers is in said actuated position; wherein said body is sufficiently flexible such that said tractor can turn up to 60 per 30m (100 feet) of travel.

15 All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the

preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.

Brief Description of the Drawings

Figure 1 is a schematic diagram of the major components of one embodiment of a coiled tubing drilling system of the present invention; Figure 2 is a front perspective view of the electrically sequenced tractor of the 25 present invention (EST); Figure 3 is a rear perspective view of the control assembly of the EST; Figures 4A-F are schematic diagrams illustrating an operational cycle of the EST; Figure 5 is a rear perspective view of the aft transition housing of the EST; 30 Figure 6 is a front perspective view of the aft transition housing of Figure 5;

i Figure 7 is a sectional view of the aft transition housing, taken along line 7-7 of Figure 5; Figure 8 is a rear perspective view of the electronics housing of the EST; Figure 9 is a front perspective view of the forward end of the electronics housing 5 of Figure 8; Figure 10 is a front view of the electronics housing of Figure 8; Figure 11 is a longitudinal sectional view of the electronics housing, taken along line 11-11 of Figure8; Figure 12 is a cross-sectional view of the electronics housing, taken along line lO 12-12 of Figure 8; Figure 13 is a rear perspective view of the pressure transducer manifold of the EST; Figure 14 is a front perspective view of the pressure transducer manifold of Figure 13; 15 Figure 15 is a cross-sectional view of the pressure transducer manifold, taken along line 15-15 of Figure 13; Figure 16 is a cross-sectional view of the pressure transducer manifold, taken along line 16-16 of Figure 13; Figure 17 is a rear perspective view of the motor housing of the EST; 20 Figure 18 is a front perspective view of the motor housing of Figure 17; Figure 19 is a rear perspective view of the motor mount plate of the EST; Figure 20 is a front perspective view of the motor mount plate of Figure l 9; Figure 21 is a rear perspective view of the valve housing of the EST; Figure 22 is a front perspective view of the valve housing of Figure 21; 25 Figure 23 is a front view of the valve housing of Figure 21; Figure 24 is a side view of the valve housing, showing view 24 of Figure 23; Figure 25 is a side view of the valve housing, showing view 25 of Figure 23; Figure 26 is a side view of the valve housing, showing view 26 of Figure 23; Figure 27 is a side view of the valve housing, showing view 27 of Figure 23; 30 Figure 28 is a rear perspective view of the forward transition housing of the EST;

Figure 29 is a front perspective view of the forward transition housing of Figure 28; Figure 30 is a cross-sectional view of the forward transition housing, taken along line 30-30 of Figure 28; 5 Figure 31 is a rear perspective view of the diffuser of the EST; Figure 32 is a sectional view of the diffuser, taken along line 32-32 of Figure 31; Figure 33 is a rear perspective view of the failsafe valve spool and failsafe valve body of the EST; Figure 34 is a side view of the failsafe valve spool of Figure 33; 10 Figure 35 is a bottom view of the failsafe valve body; Figure 36 is a longitudinal sectional view of the failsafe valve in a closed . position; Figure 37 is a longitudinal sectional view of the failsafe valve in an open position; 15 Figure 38 is a rear perspective view of the aft propulsion valve spool and aft propulsion valve body of the EST; Figure 39 is a cross-sectional view of the aft propulsion valve spool, taken along line 39-39 of Figure 38; Figure 40 is a longitudinal sectional view of the aft propulsion valve in a closed 20 position; Figure 41 is a longitudinal sectional view of the aft propulsion valve in a first open position; Figure 42 is a longitudinal sectional view of the aft propulsion valve in a second open position; 25 Figures 43A-C are exploded longitudinal sectional views of the aft propulsion valve, illustrating different flow-restricting positions of the valve spool; Figure 44A is a longitudinal partially sectional view of the EST, showing the leadscrew assembly for the aft propulsion valve; Figure 44B is an exploded view of the leadscrew assembly of Figure 44A; 30 Figure 45 is a longitudinal partially sectional view of the EST, showing the failsafe valve spring and pressure compensation piston;

Figure 46 is a longitudinal sectional view of the relief valve poppet and relief valve body of the EST; Figure 47 is a rear perspective view of the relief valve poppet of Figure 46; Figure 48 is a longitudinal sectional view of the EST, showing the relief valve 5 assembly; Figure 49A is a front perspective view of the aft section of the EST, shown disassembled; Figure 49B is an exploded view of the forward end of the aft shaft shownin Figure 49A 10 Figure 50 is a side view of the aft shaft of the EST; Figure 51 is a front view of the aft shaft of Figure 50; Figure 52 is a rear view of the ah shaft of Figure 50; Figure 53 is a side view of the aft shaft of Figure 50, shown rotated 180 about its longitudinal axis; 15 Figure 54 is a front view of the aft shaft of Figure 53; Figure 55 is a cross-sectional view of the aft shaft, taken along line 55-55 shown in Figures 49 and 50; Figure 56 is a cross-sectional view of the aft shaft, taken along line 56-56 shown in Figures 49 and 50; 20 Figure 57 is a cross-sectional view of the aft shaft, taken along line 57-57 shown in Figures 49 and 50; Figure 58 is a cross-sectional view of the aft shaft, taken along line 58-58 shown in Figures 49 and 50; Figure 59 is a crosssectional view of the aft shaft, taken along line 59-59 shown 25 in Figures 49 and 50; Figure 60 is a rear perspective view of the aft packerfoot of the EST, shown disassembled; Figure 61 is a side view of the aft packerfoot of the EST; Figure 62 is a longitudinal sectional view of the aft packerfoot of Figure 61; 30 Figure 63 is an exploded view of the aft end of the aft packerfoot of Figure 62;

Figure 64 is an exploded view of the forward end of the aft pac erfoot of Figure 62; Figure 6S is a rear perspective view of an aft flextoe packerfoot of the present invention, shown disassembled; 5 Figure 66 is a rear perspective view of the mandrel of the flextoe packerfoot of Figure 65; Figure 67 is a cross-sectional view of the bladder of the flextoe packerfoot of Figure 65; Figure 68 is a cross-sectional view of a shaft of the EST, formed by diffusion 10 bonding; Figure 69 schematically illustrates the relationship of Figures 69A-D; Figures 69A-D are a schematic diagram of one embodiment of the electronic configuration of the EST; Figure 70 is a graph illustrating the speed and load-carrying capability range of 15 the EST; Figure 71 is an exploded longitudinal sectional view of a stepped valve spool; Figure 72 is an exploded longitudinal sectional view of a stepped tapered valve spool; Figure 73A is a chord illustrating the turning ability of the EST; 20 Figure 73B is a schematic view illustrating the flexing characteristics of the aft shaft assembly of the EST; Figure 74 is a rear perspective view of an inflated packerfoot of the present invention; Figure 75 is a cross- sectional view of a packerfoot of the present invention; 25 Figure 76 is a side view of an inflated flextoe packerfoot of the present invention; Figure 77A is a front perspective view of a Wiegand wheel assembly, shown disassembled; Figure 77B is a front perspective view of the Wiegand wheel assembly of Figure 30 77A, shown assembled;

\ Figure 77C is front perspective view of a piston having a Wiegand displacement sensor; Figure 78 is a graph illustrating the relationship between longitudinal displacement of a propulsion valve spool of the EST and f owrate of fluid admitted to 5 the propulsion cylinder; and Figure 79 is a perspective view of a notch of a propulsion valve spool of the EST. Detailed Description of the Preferred Embodiment

10 It must be emphasized that the following describes one configuration of the EST.

However, numerous variations are possible. These variations in structure result in various ranges of performance characteristics. Several physical constraints require the EST to be innovative with respect to the use of available space within the borehole. The physical constraints are the result of the drilling environment. First, the maximum 15 diameter of the tool is restricted by the diameter of the drilled hole and the amount and pressure of the drilling fluid pumped through the internal bore of the tool and returning to the ground surface with drill cuttings. Next, the physical length of the tractor is restricted by the size of surface handling equipment and rig space. The temperature and pressure downhole are the result of rock formation conditions. The desired thrust 20 capacity of the EST is defined by the size of the drill bit, the downhole motor thrust capacity, and rock characteristics. The desired pull capacity of the tool is defined by the weight of the drill string and the bottom hole assembly in the drilling fluid considering the friction of the components against the borehole wall or casing wall or by the desired functional requirements, such as the amount of force required to move a sliding sleeve 25 in a casing. The desired maximum speed is influenced by rig economics that include the associated costs of drilling labor, material, facilities, cost of money, risk, and other economic factors. The lowest desired speed is defined by the type of operation, such as rate of penetration in a particular formation or rate of milling casing. In addition, drilling convention has resulted in numerous default sizes used in drilling. These size 30 constraints are generally a function of the size of drill bit available, the size of casing available, the size of ground surface equipment, and other parameters.

l For example, the EST design described herein has a maximum diameter of 8.57cm (3.375 inches) for use in a 9.53cm (3.75-inch) hole. However, several other designs are conceived, including a 5.4cm (2.125 inch) diameter tool for use in a 7.3cm (2.875 inch) hole, a 12.1cm (4.75 inch) diameter tool for use in a 15.2cm (6.0 inch) 5 hole, and a 15.2cm (6.0 inch) diameter tool for use in a 21.6cm (8.5 inch) hole.

It is believed, however, that for a given set of operating criteria, such as a requirement that the tool operate within a 8.57cm (3.75 inch) diameter borehole and have a given maximum length, that the present invention has numerous advantages over prior art tractors. For example, having a single tractor which can fit within a given

10 borehole and which can sustain both slow speeds for activities such as milling and high speeds for activities such as tripping out of a borehole is extremely valuable, in that it saves both the expense of having another tractor and the time which would otherwise be required to change tractors.

Figure 1 shows an electrically sequenced tractor (EST) 100 for moving 15 equipment within a passage, configured in accordance with a preferred embodiment of the present invention. IN the embodiments shown in the accompanying figures, the electrically sequenced tractor (EST) of the present invention may be used in conjunction with a coiled tubing drilling system 20 and a bottom hole assembly 32. System 20 may include a power supply 22, tubing reel 24, tubing guide 26, tubing injector 28, and 20 coiled tubing 30, all of which are well known in the art. Assembly 32 may include a measurement while drilling (MOOD) system 34, downhole motor 36, and drill bit 38, all of which are also known in the art. The EST is configured to move within a borehole having an inner surface 42. An annulus 40 is def ned by the space between the EST and the inner surface 42.

25 It will be appreciated that the EST may be used to move a wide variety of tools and equipment within a borehole. Also, the EST can be used in conjunction with numerous types of drilling, including rotary drilling and the like. Additionally, it will be understood that the EST may be used in many areas including petroleum drilling, mineral deposit drilling, pipeline installation and maintenance, communications, and the 30 like. Also, it will be understood that the apparatus and method for moving equipment within a passage may be used in many applications in addition to drilling. For example,

these other applications include well completion and production work for producing oil from an oil well, pipeline work, and communications activities. It will be appreciated that these applications may require the use of other equipment in conjunction with an EST according to the present invention. Such equipment, generally referred to as a 5 working unit, is dependent upon the specific application undertaken.

For example, one of ordinary skill in the art will understand that oil and gas well completion typically requires that the reservoir be logged using a variety of sensors.

These sensors may operate using resistivity, radioactivity, acoustics, and the like. Other logging activities include measurement of formation dip and borehole geometry, 10 formation sampling, and production logging. These completion activities can be accomplished in inclined and horizontal boreholes using a preferred embodiment of the EST. For instance, the EST can deliver these various types of logging sensors to regions of interest. The EST can either place the sensors in the desired location, or the EST may idle in a stationary position to allow the measurements to be taken at the 15 desired locations. The EST can also be used to retrieve the sensors from the well.

Examples of production work that can be performed with a preferred embodiment of the EST include sands and solids washing and acidizing. it is known that wells sometimes become clogged with sand, hydrocarbon debris, and other solids that prevent the free flow of oil through the borehole 42. To remove this debris, 20 specially designed washing tools known in the industry are delivered to the region, and fluid is injected to wash the region. The fluid and debris then return to the surface.

Such tools include acid washing tools. These washing tools can be delivered to the region of interest for performance of washing activity and then returned to the ground surface by a preferred embodiment of the EST.

25 In another example, a preferred embodiment of the EST can be used to retrieve objects, such as damaged equipment and debris, from the borehole. For example, equipment may become separated from the drill string, or objects may fall into the borehole. These objects must be retrieved, or the borehole must be abandoned and plugged. Because abandonment and plugging of a borehole is very expensive, retrieval 30 of the object is usually attempted. A variety of retrieval tools known to the industry are available to capture these lost objects. The EST can be used to transport retrieving tools

! to the appropriate location, retrieve the object, and return the retrieved object to the surface. In yet another example, a preferred embodiment of the EST can also be used for coiled tubing completions. As known in the art, continuous-completion drill string 5 deployment is becoming increasingly important in areas where it is undesirable to damage sensitive formations in order to run production tubing. These operations require the installation and retrieval of fully assembled completion drill string in boreholes with surface pressure. The EST can be used in conjunction with the deployment of conventional velocity string and simple primary production tubing installations. The 10 EST can also be used with the deployment of artificial lift devices such as gas lift and downhole flow control devices.

In a further example, a preferred embodiment of the EST can be used to service plugged pipelines or other similar passages. Frequently, pipelines are difficult to service due to physical constraints such as location in deep water or proximity to metropolitan 15 areas. Various types of cleaning devices are currently available for cleaning pipelines.

These various types of cleaning tools can be attached to the EST so that the cleaning tools can be moved within the pipeline.

In still another example, a preferred embodiment of the EST can be used to move communication lines or equipment within a passage. Frequently, it is desirable to 20 run or move various types of cables or communication lines through various types of conduits. The EST can move these cables to the desired location within a passage.

Overview of EST Components Figure 2 shows a preferred embodiment of an electrically sequenced tractor 25 (EST) of the present invention. The EST 100 comprises a central control assembly 102, an uphole or aft packerfoot 104, a downhole or forward packerfoot 106, aft propulsion cylinders 108 and 110, forward propulsion cylinders 112 and 114, a drill string connector 116, shafts 118 and 124, flexible connectors 120, 122, 126, and 128, and a bottom hole assembly cormector 129. Drill string connector 116 connects a drill string, 30 such as coiled tubing, to shaft 118. Aft packerfoot 104, aft propulsion cylinders 108 and 110, and connectors 120 and 122 are assembled together end to end and are all axially

\ slidably engaged with shaft 118. Similarly, forward packerfoot 106, forward propulsion cylinders 112 and 114, and connectors 126 and 128 are assembled together end to end and are slidably engaged with shaft 124. Connector 129 provides a connection between EST 100 and downhole equipment such as a bottom hole assembly. Shafts 118 and 124 5 and control assembly 102 are axially fixed with respect to one another and are sometimes referred to herein as the body of the EST. The body of the EST is thus axially fixed with respect to the drill string and the bottom hole assembly.

EST Schematic Configuration and Operation 10 Figures 4A-4F schematically illustrate a preferred configuration and operation of the EST. Aft propulsion cylinders 108 and 110 are axially slidably engaged with shaft 118 and form annular chambers surrounding the shaft. Annular pistons 140 and 142 reside within the annular chambers formed by cylinders 108 and 110, respectively, and are axially fixed to shaft 118. Piston 140 fluidly divides the annular chamber formed by 15 cylinder 108 into a rear chamber 166 and a front chamber 168. Such rear and front chambers are fluidly sealed to substantially prevent fluid flow between the chambers or leakage to annulus 40. Similarly, piston 142 fluidly divides the annular chamber formed by cylinder 110 into a rear chamber 170 and a front chamber 172.

The forward propulsion cylinders 112 and 114 are configured similarly to the aft 20 propulsion cylinders. Cylinders 112 and 114 are axially slidably engaged with shaft 124. Annular pistons 144 and 146 are axially fixed to shaft 124 and are enclosed within cylinders 112 and 114, respectively. Piston 144 fluidly divides the chamber formed by cylinder 112 into a rear chamber 17 and a front chamber 176. Piston 146 fluidly divides the chamber formed by cylinder 114 into a rear chamber 178 and a front 25 chamber 180. Chambers 166, 168, 170, 172, 174, 176, 178, and 180 have varying volumes, depending upon the positions of pistons 140, 142, 144, and 146 therein.

Although two aft propulsion cylinders and two forward propulsion cylinders (along with two corresponding aft pistons and forward pistons) are shown in the illustrated embodiment, any number of aft cylinders and forward cylinders may be 30 provided, which includes only a single aft cylinder and a single forward cylinder. As described below, the hydraulic thrust provided by the EST increases as the number of

propulsion cylinders increases. In other words, the hydraulic force provided by the cylinders is additive. Four propulsion cylinders are used to provide the desired thrust of approximately 471(N (10,500 pounds) for a tractor with a maximum outside diameter of 8.6cm (3.375 inches). It is believed that a configuration having four propulsion 5 cylinders is preferable, because it permits relatively high thrust to be generated, while limiting the length of the tractor. Alternatively, fewer cylinders can be used, which will decrease the resulting maximum tractor pullthrust. Alternatively, more cylinders can be used, which will increase the maximum output force from the tractor. The number of cylinders is selected to provide sufficient force to provide sufficient force for the 10 anticipated loads for a given hole size.

The EST is hydraulically powered by a fluid such as drilling mud or hydraulic fluid. Unless otherwise indicated, the terms "fluids and "drilling fluid" are used interchangeably hereinafter. In a preferred embodiment, the EST is powered by the same fluid which lubricates and cools the drill bit. Preferably, drilling mud is used in an 15 open system. This avoids the need to provide additional fluid channels in the tool for the fluid powering the EST. Alternatively, hydraulic fluid may be used in a closed system, if desired. Referring to Figure 1, in operation, drilling fluid flows from the drill string 30 through EST 100 and down to drill bit 38. Referring again to Figures 4A-F, diffuser 148 in control assembly 102 diverts a portion of the drilling fluid to power the 20 EST. Preferably, diffuser 148 filters out larger fluid particles which can damage internal components of the control assembly, such as the valves.

Fluid exiting diffuser 148 enters a spring biased failsafe valve 150. Failsafe valve 150 serves as an entrance point to a central galley 155 (illustrated as a flow path in Figures 44-F) in the control assembly which communicates with a relief valve 152, 25 packerfoot valve 154, and propulsion valves 156 and 158. When the differential pressure (unless otherwise indicated, hereinafter "differential pressure" or "pressure" at a particular location refers to the difference in pressure at that location from the pressure in annulus 40) of the drilling fluid approaching failsafe valve 150 is below a threshold value, failsafe valve 150 remains in an off position, in which fluid within the central 30 galley vents out to exhaust line E, i.e., to annulus 40. When the differential pressure rises above the threshold value, the spring force is overcome and failsafe valve 150

opens to permit drilling fluid to enter central galley 155. Failsafe valve 150 prevents premature starting of the EST and provides a fail-safe means to shut down the EST by pressure reduction of the drilling fluid in the coiled tubing drill string. Thus, valve 150 operates as a system on/off valve. The threshold value for opening failsafe valve 150, 5 i.e., for turning the system on, is controlled by the stiffness of spring 151 and can be any value within the expected operational drilling pressure range of the tool. A preferred threshold pressure is about 500 psi".

Drilling fluid within central galley 155 is exposed to all of the valves of EST 100. A spring-biased relief valve 152 protects over-pressurization of the fluid within 10 the tool. Relief valve 152 operates similarly to failsafe valve 150. When the fluid pressure in central galley 155 is below a threshold value, the valve remains closed.

When the fluid pressure exceeds the threshold, the spring force of spring 153 is overcome and relief valve 152 opens to permit fluid in galley 155 to vent out to annulus 40. Relief valve 152 protects pressure-sensitive components of the EST, such as the 15 bladders of packerfoot 104 and 106, which can rupture at high pressure. In the illustrated embodiment, relief valve 152 has a threshold pressure of about 1.1 x 107Nm 2 (1600 psi").

Packerfoot valve 154 controls the inflation and deflation of packerfeet 104 and 106. Packerfoot valve 154 has three positions. In a first extreme position (shown in 20 Figure 4A), fluid from central galley 155 is permitted to flow through passage 210 into aft packerfoot 104, and fluid from forward packerfoot 106 is exhausted through passage 260 to annulus 40. When valve 154 is in this position aft packerfoot 104 tends to inflate and forward packerfoot 106 tends to deflate. In a second extreme position (Figure 4D), fluid from the central galley is permitted to flow through passage 260 into forward 25 packerfoot 106, and fluid from aft packerfoot 104 is exhausted through passage 210 to annulus 40. When valve 154 is in this position aft packerfoot 104 tends to deflate and forward packerfoot 106 tends to inflate. central third position of valve 154 permits restricted flow from galley 155 to both packerfeet. In this position, both packerfeet can be inflated for a double-thrust stroke, described below.

30 In normal operation, the aft and forward packerfoot are alternately actuated. As aft packerfoot 104 is inflated, forward packerfoot 106 is deflated, and vice-versa. The

position of packerfoot valve 154 is controlled by a packerfoot motor 160. In a preferred embodiment, motor 160 is electrically controllable and can be operated by a programmable logic component on EST 100, such as in electronics housing 130 (Figures 8-12), to sequence the inflation and deflation of the packerfeet. Although the 5 illustrated embodiment utilizes a single packerfoot valve controlling both packerfeet, two valves could be provided such that each valve controls one of the packerfeet. An advantage of a single packerfoot valve is that it requires less space than two valves. An advantage of the two-valve configuration is that each packerfoot can be independently controlled. Also, the packerfeet can be more quickly simultaneously inflated for a 10 double thrust stroke.

Propulsion valve 156 controls the flow of fluid to and from the aft propulsion cylinders 108 and 110. In one extreme position (shown in Figure 4B), valve 156 permits fluid from central galley 155 to flow through passage 206 to rear chambers 166 and 170. When valve 156 is in this position, rear chambers 166 and 170 are connected 15 to the drilling fluid, which is at a higher pressure than the rear chambers. This causes pistons 140 and 142 to move toward the downhole ends of the cylinders due to the volume of incoming fluid. Simuitaneously, front chambers i68 and i72 reduce in volume, and fluid is forced out of the front chambers through passage 208 and valve 156 out to annulus 40. If packerfoot 104 is inflated to grip borehole wall 42, the pistons 20 move downhole relative to wall 42. If packerfoot 104 is deflated, then cylinders 108 and 110 move uphole relative to wall 42.

In its other extreme position (Figure 4E), valve 156 permits fluid from central galley 155 to flow through passage 208 to front chambers 168 and 172. When valve 156 is in this position, front chambers 168 and 172 are connected to the drilling fluid, 25 which is at a higher pressure than the front chambers. This causes pistons 140 and 142 to move toward the uphole ends of the cylinders due to the volume of incoming fluid.

Simultaneously, rear chambers 166 and 170 reduce in volume, and fluid is forced out of the rear chambers through passage 206 and valve 156 out to annulus 40. In a central position propulsion valve 156 blocks any fluid communication between cylinders 108 30 and 110, galley 155, and annulus 40. If packerfoot 104 is inflated to grip borehole wall

42, the pistons move uphole relative to wall 42. If pac erfoot iO4 is der aled, then cylinders 108 and 110 move downhole relative to wall 42.

Propulsion valve 158 is configured similarly to valve 156. Propulsion valve 158 controls the flow of fluid to and from the forward propulsion cylinders 112 and 114. In 5 one extreme position (Figure 4E), valve 158 permits fluid from central galley 155 to flow through passage 234 to rear chambers 174 and 178. When valve 156 is in this position, rear chambers 174 and 178 are connected to the drilling fluid, which is at a higher pressure than the rear chambers. This causes the pistons 144 and 146 to move toward the downhole ends of the cylinders due to the volume of incoming fluid.

10 Simultaneously, front chambers 176 and 180 reduce in volume, and fluid is forced out of the front chambers through passage 236 and valve 158 out to annulus 40. If packerfoot 106 is inflated to grip borehole wall 42, the pistons move downhole relative to wall 42. If packerfoot 106 is deflated, then cylinders 108 and 110 move uphole relative to wall 42.

15 In its other extreme position (Figure 4B), valve 158 permits fluid from central galley 155 to flow through passage 236 to front chambers 176 and 180 are connected to the drilling fluid, which is at a higher pressure than rear chambers 174 and 178. This causes the pistons 144 and 146 to move toward the uphole ends of the cylinders due to the volume of incoming fluid. Simultaneously, rear chambers 174 and 178 reduce in 20 volume, and fluid is forced out of the rear chambers through passage 234 and valve 158 out to annulus 40. If packerfoot 106 is inflated to grip borehole wall 42, the pistons move uphole relative to wall 42. If packerfoot 106 is deflated, then cylinders 108 and 110 move downhole relative to wall 42. In a central position, propulsion valve 158 blocks any fluid communication between cylinders 112 and 114, galley 155, and 25 annulus 40.

In a preferred embodiment, propulsion valves 156 and 158 are configured to form a controllable variable flow restriction between central galley 155 and the chambers ofthe propulsion cylinders. The physical configuration of valves 156 and 158 is described below. To illustrate the advantages of this feature, consider valve 156. As 30 valve 156 deviates slightly from its central position, it permits a limited volume flowrate from central galley 155 into the aft propulsion cylinders. The volume flowrate can be

l precisely increased or decreased by varying the flow restriction, i.e., by opening further or closing further the valve. By carefully positioning the valve, the volume flowrate of fluid into the aft propulsion cylinders can be controlled. The flow-restricting positions of the valves are indicated in Figures 4 -F by flow lines which intersect X's. The flow 5 restricting positions permit precise control over (1) the longitudinal hydraulic force received by the pistons; (2) the longitudinal position of the pistons within the ah propulsion cylinders; and (3) the rate of longitudinal movement of the pistons between positions. Propulsion valve 158 may be similarly configured, to permit the same degree of control over the forward propulsion cylinders and pistons. As will be shown below, 10 controlling these attributes facilitates enhanced control of the thrust and speed of the EST and, hence, the drill bit.

In a preferred embodiment, the position of propulsion valve 156 is controlled by an aft propulsion motor 162, and the position of propulsion valve 158 is controlled by a forward propulsion motor 1 64. Preferably, these motors are electrically controllable and 15 can be operated by a programmable logic component on EST 100, such as in electronics unit 92 (Figure 3), to precisely control the expansion and contraction of the rear and front chambers of the aft and forward propulsion cylinders.

The above-described configuration of the EST permits greatly improved control over tractor thrust, speed, and direction of travel. EST lOO can be moved downhole 20 according to the cycle illustrated in Figures 4A-F. As shown in Figure 4A, packerfoot valve 154 is shuttled to a first extreme position, permitting fluid to flow from central galley 155 to aft packerfoot 104, and also permitting fluid to be exhausted from forward packerfoot 106 to annulus 40. Aft packerfoot 104 inflates and grips borehole wail 42, anchoring aft propulsion cylinders 108 and 110. Forward packerfoot 106 deflates, so 25 that forward propulsion cylinders 112 and 114 are free to move axially with respect to borehole wall 42. Next, as shown in Figure 4B, propulsion valve 156 is moved toward its first extreme position, permitting fluid to flow from central galley 155 into rear chambers 166 and 170, and also permitting fluid to be exhausted from front chambers 168 and 172 to annulus 40. The incoming fluid causes rear chambers 166 and 170 to 30 expand due to hydraulic force. Since cylinders 108 and llO are fixed with respect to borehole wall 42, pistons 140 and 142 are forced downhole to the forward ends of the

i pistons, as shown in Figure 4C. Since the pistons are fixed to she* 118 of the LST body, the forward movement of the pistons propels the EST body downhole. This is known as a power stroke.

Simultaneously or independently to the power stroke of the aft pistons 140 and 5 142, propulsion valve 158 is moved to its second extreme position, shown in Figure 4B.

This permits fluid to flow from central galley 155 into front chambers 176 and 180, and from rear chambers 174 and 178 to annulus 40. The incoming fluid causes front chambers 176 and 180 to expand due to hydraulic force. Accordingly, forward propulsion cylinders 112 and 114 move downhole with respect to the pistons 144 and 10 146, as shown in Figure 4C. This is known as a reset stroke. After the aft propulsion cylinders complete a power stroke and the forward

propulsion cylinders complete a reset stroke, packerfoot valve 154 is shuttled to its second extreme position, shown in Figure 4D. This causes forward packerfoot 106 to inflate and grip borehole wall 42, and also causes aft packerfoot 104 to deflate. Then, 15 propulsion valves 156 and 158 are reversed, as shown in Figure 4E. This causes cylinders 112 and 114 to execute a power stroke and also causes the cylinders 108 and 110 to execute a reset stroke, shown in Figure 4F. Packerfoot valve 154 is then shuttled back to its first extreme position, and the cycle repeats.

Those skilled in the art will understand that EST 100 can move in reverse, i.e., 20 uphole, simply by reversing the sequencing of packerfoot valve 154 or propulsion valves 156 and 158. When packerfoot 104 is inflated to grip borehole wall 42, propulsion valve 156 is positioned to deliver fluid to front chambers 168 and 172. The incoming fluid imparts an uphole hydraulic force on pistons 140 and 142, causing cylinders 108 and 110 to execute an uphole power stroke. Simultaneously, propulsion 25 valve 158 is positioned to deliver fluid to rear chambers 174 and 178, so that cylinders 112 and 114 execute a reset stroke. Then, packerfoot valve 154 is moved to inflate packerfoot 106 and deflate packerfoot 104. Then the propulsion valves are reversed so that cylinders 112 and 114 execute an uphole power stroke while cylinders 108 and 110 execute a reset stroke. Then, the cycle is repeated.

30 Advantageously, the EST can reverse direction prior to reaching the end of any particular power or reset stroke. The tool can be reversed simply by reversing the

j positions of the propulsion valves so that hydraulic power is provided on the opposite sides of the annular pistons in the propulsion cylinders. This feature prevents damage to the drill bit which can be caused when an obstruction is encountered in the formation.

The provision of separate valves controlling (1) the inflation of the packerfeet, 5 (2) the delivery of hydraulic power to the aft propulsion cylinders, and (3) the delivery of hydraulic power to the forward propulsion cylinders permits a dual power stroke operation and, effectively, a doubling of axial thrust to the EST body. For example, packerfoot valve 154 can be moved to its central position to inflate both packerfeet 104 and 106. Propulsion valves 156 and 158 can then be positioned to deliver fluid to the 10 rear chambers of their respective propulsion cylinders. This would result in a doubling of downhole thrust to the EST body. Similarly, the propulsion valves can also be positioned to deliver fluid to the front chambers of the propulsion cylinders, resulting in double uphole thrust. Double thrust may be useful when penetrating harder formations.

As mentioned above, packerfoot valve motor 160 and propulsion valve motors 15 162 and 164 may be controlled by an electronic control system. In one embodiment, the control system of the EST includes a surface computer, electric cables (fiber optic or wire), and a programmable logic component 224 (Figure 63) located in electronics housing 130. Logic component 224 may comprise electronic circuitry, a microprocessor, EPROM and/or tool control software. The tool control software is 20 preferably provided on a programmable integrated chip (PIC) on an electronic control board. The control system operates as follows: An operator places commands at the surface, such as desired EST speed, direction, thrust, etc. Surface software converts the operator's commands to electrical signals that are conveyed downhole through the electric cables to logic component 224. The electric cables are preferably located within 25 the composite coiled tubing and connected to electric wires within the EST that run to logic component 224. The PIC converts the operator's electrical commands into signals which control the motors.

As part of its control algorithm, logic component 224 can also process various feedback signals containing information regarding tool conditions. For example, logic 30 component 224 can be configured to process pressure and position signals from pressure transducers and position sensors throughout the EST, a weight on bit (WOB) signal

from a sensor measuring the load on the drill bit, and/or a pressure signal from a sensor measuring the pressure difference across the drill bit. In a preferred embodiment, logic component 224 is programmed to intelligently operate valve motors 160, 162, and 164 to control the valve positions, based at least in part upon one or both of two different 5 properties - pressure and displacement. From pressure information the control system can determine and control the thrust acting upon the EST body. From displacement information, the control system can determine and control the speed of the EST. In particular, logic component 224 can control the valve motors in response to (1) the differential pressure of fluid in the rear and front chambers of the propulsion cylinders 10 and in the entrance to the failsafe valve, (2) the positions of the annular pistons with respect to the propulsion cylinders, or (3) both.

The actual command logic and software for controlling the tractor will depend on the desired performance characteristics of the tractor and the environment in which the tractor is to be used. Once the performance characteristics are determined, it is 15 believed that one skilled in the art can readily determine the desired logical sequences and software for the controller. It is believed that the structure and methods disclosed herein offer numerous advantages over the prior art, regardless of the performance

characteristics and software selected. Accordingly, while a prototype of the invention uses a particular software program (developed by Halliburton Company of Dallas, 20 Texas), it is believed that a wide variety of software could be used to operate the system Pressure transducers 182,184, 186, 188, and 190 may be provided on the tool to measure the differential fluid pressure in (1) rear chambers 166 and 170, (2) front chambers 168 and 172, (3) rear chambers 174 and 178, (4) front chambers 176 and 180, and (5) in the entrance to failsafe valve 150, respectively. These pressure transducers 25 send electrical signals to logic component 224, which are proportional to the differential fluid pressure sensed. In addition, as shown in Figures 4A-F, displacement sensors 192 and 194 may be provided on the tool to measure the positions of the annular pistons with respect to the propulsion cylinders. In the illustrated embodiment, sensor 192 measures the axial position of piston 140 with respect to cylinder 110, and sensor 194 30 measures the axial position of piston 144 with respect to cylinder 112. Sensors 192 and

194 can also be positioned on pistons 140 and 146, or additional displacement sensors can be provided if desired.

Rotary accelerometers or potentiometers are preferably provided to measure the rotation of the motors. By monitoring the rotation of the motors, the positions of the 5 motorized valves 154, 156, and 158 can be determined. Like the signals from the pressure transducers and displacement sensors, the signals from the rotary accelerometers or potentiometers are fed back to logic component 224 for controlling the valve positions.

10 Detailed Structure of the EST The major subassemblies of the EST are the aft section, the control assembly, and the forward section. Referring to Figure 2, the major components of the aft section comprise shaft 118, aft packerfoot 104, aft propulsion cylinders 108 and l l O. connectors 120 and 122, and aft transition housing 131. The aft section includes a central conduit 15 for transporting drilling fluid supply from the drill string to control assembly 102 and to the drill bit. The aft section also includes passages for fluid flow between control assembly 102 and aft packerfoot 104 and ad propulsion cylinders 108 and 110. The aft section further includes at least one passage for wires for transmission of electrical signals between the ground surface, control assembly 102, and the bottom hole 20 assembly. A drill string connector 116 is attached to the aft end of the aft section, for fluidly connecting a coiled tubing drill string to shaft 118, as known in the art.

The forward section is structurally nearly identical to the aft section, with the exceptions that the components are inverted in order and the forward section does not include an aft transition housing. The forward section comprises shaft 124, forward 25 propulsion cylinders 112 and 114, connectors 126 and 128, and forward packerfoot 106.

The forward section includes a central conduit for transporting drilling fluid supply to the drill bit. The forward section also includes passages for fluid flow between control assembly 102 and forward packerfoot 106 and forward propulsion cylinders 112 and 114. The forward section further includes at least one passage for wires for 30 transmission of electrical signals between the ground surface, control assembly 102, and the bottom hole assembly. A connector 129 is attached to the forward end of the

forward section, for connecting shaft 124 to downhole components such as the bottom hole assembly, as known in the art.

Control Assembly 5 Referring to Figures 2 and 3, control assembly 102 comprises an aft transition housing 131 (Figure 2), electronics unit 92, motor unit 94, valve unit 96, and forward transition unit 98. Electronics unit 92 includes an electronics housing 130 which contains electronic components, such as logic component 224, for controlling the EST.

Motor unit 94 includes a motor housing 132 which contains motors 160, 162, and 164.

10 These motors control packerfoot valve 154 and propulsion valves 156 and 158, respectively. Valve unit 96 includes a valve housing 134 containing these valves, as well as failsafe valve 150. Forward transition unit 98 includes a forward transition housing 136 which contains diffuser 148 (not shown) and relief valve 152.

The first component of control assembly 102 is aft transition unit 90. Aft 15 transition housing 131 is shown in Figures 5-7. Housing 131 is connected to and is supplied with drilling fluid from shaft 118. Housing 131 shifts the drilling fluid supply from the center of the tool to a side, to provide space for an electronics package 224 in electronics unit 92. Figure 5 shows the aft end of housing 131, and Figure 6 shows its forward end. The aft end of housing 131 attaches to flange 366 (Figures 49A-B) on 20 shaft 118. In particular, housing 131 has pentagonally arranged threaded connection bores 200 which align with similar bores 365 in flange 366. High strength connection studs or bolts are received within bores 365 and bores 200 to secure the flange and housing 131 together. Flange 366 has recesses 367 through which nuts can be fastened onto the aft ends of the connection studs, against surfaces of recesses 367. Suitable 25 connection bolts are MP33 non-magnetic bolts, which are high in strength, elongation, and toughness. At its forward end, housing 131 is attached to electronics housing 130 in a similar manner, which therefore need not be described in detail. Furthermore, all of the adjacent housings may be attached to each other and to the shafts in a like or similar manner, and, therefore, also need not be described in detail.

30 It will be appreciated that the components of the EST include numerous passages for transporting drilling fluid and electrical wires through the tool. Aft

transition housing 131 includes several longitudinal bores which comprise a portion of these passages. Lengthwise passage 202 transports the drilling fluid supply (from the drill string) downhole. As shown in Figure 7, passage 202 shifts from the center axis of the tool at the aft end of housing 131 to an offcenter position at the forward end.

5 Longitudinal wire passage 204 is provided for electrical wires. A longitudinal wire passage 205 is provided in the forward end of housing 131, extending about half of the length of the housing. Passages 204 and 205 communicate through an elongated opening 212 in housing 131. In a preferred embodiment, wires from the surface are separated at opening 212 and connected to a 7-pin boot 209 (Figure 69) and a 10-pin 10 boot 211. Boots 209 and 211 fit into passages 204 and 205, respectively, at the forward end of housing 131 and connect to corresponding openings in electronics housing 132.

Passage 206 permits fluid communication between aft propulsion valve 156 and rear chambers 166 and 170 of aft propulsion cylinders 108 and 110. Passage 208 permits fluid communication between valve 156 and front chambers 168 and 172 of cylinders 15 108 and 110. Passage 210 permits fluid communication between packerfoot valve 154 and aft packerfoot 104.

Figures 8-12 show electronics housing 130 of electronics unit 92, which contains an electronic logic component or package 224. Housing 130 includes longitudinal bores for passages 202, 204, 205, 206, 208, and 210. Electronics package 224 resides in a 20 large diameter portion of passage 205 inside housing 130. The above-mentioned 10-pin boot 211 at the forward end of aft transition housing 131 is connected to electronics package 224. Passage 205 is preferably sealed at the aft and forward ends of electronics housing 130 to prevent damage to electronics package 224 caused by exposure to high pressure from annulus 40, which can be as high as 1.1 x lO8Nm 2 (16,000 psi). A 25 suitable seal, rated at 1.4 x lO8Nm 2 (20,000 psi), is sold by Green Tweed, Inc., having offices in Houston, Texas. Preferably, housing 130 is constructed of a material which is sufficiently heat-resistant to protect electronics package 224 from damage which can be caused by exposure to high downhole temperatures. A suitable material is Stabaloy AG 17. 30 As shown in Figures 9 and 11, a recess 214 is provided in the forward end of electronics housing 130, for receiving a pressure transducer manifold 222 (Figures 13

i 16) which includes pressure transducers 182, 184, 186, 188, and 190 (Figure 3).

Passages 206, 208, and 210 are shifted transversely toward the central axis of electronics housing 130 to make room for the pressure transducers. Referring to Figure 12, transverse shift bores 216, 218, and 220 are provided to shift passages 206, 208, and 5 210, respectively, to their forward end positions shown in Figures 9 and 10. Shift bores 216, 218, and 220 are plugged at the radial exterior of housing 130 to prevent leakage of fluid to annulus 40.

Figures 13-16 show pressure transducer manifold 222, which is configured to house pressure transducers for Tneasuring the differential pressure of drilling fluid 10 passing through various manifold passages. Pressure transducers 182, 184, 186, 188, and 190 are received within transducer bores 225, 226, 228, 230, and 232, respectively, which extend radially inward from the outer surface of manifold 222 to longitudinal bores therein. Longitudinal bores for passages 205, 206, 208, and 210 extend through the length of manifold 222 and align with corresponding bores in electronics housing 15 130. In addition, longitudinal bores extend rearward from the forward end of manifold 222 without reaching the aft end, forming passages 234, 236, and 238. Passage 234 fluidly communicates with rear chambers 174 and i78 of forward propulsion cylinders 112 and 114. Passage 236 fluidly communicates with front chambers 176 and 180 of cylinders 112 and 114. Passage 238 fluidly communicates with forward packerfoot 106.

20 As shown in Figures 15 and 16, transducer bores 225, 226, 228, 230, and 232 communicate with passages 206, 208, 234, 236, and 238, respectively. As will be described below, the pressure transducers are exposed to drilling fluid on their inner sides and to oil on their outer sides. The oil is maintained at the pressure of annulus 40 via a pressure compensation piston 248 (Figure 45), in order to produce the desired 25 differential pressure measurements.

Figures 17 and 18 show motor housing 132 of motor unit 94. Attached to the forward end of electronics housing 130, housing 132 includes longitudinal bores for passages 202, 204, 206, 208, 210,234, 236, and 238 which align with the corresponding bores in electronics housing 130 and pressure transducer manifold 222. Housing 132 30 also includes longitudinal bores for passages 240, 242, and 244, which respectively house packerfoot motor 160, aft propulsion motor 162, and forward propulsion motor

164. In addition, a longitudinal bore for a passage 246 houses a pressure compensation piston 248 on its aft end and failsafe valve spring lS 1 (Figure 45) on its forward end.

The assembly and operation of the motors, valves, pressure compensation piston, and failsafe valve spring are described below.

5 motor mount plate 250, shown in Figures 19 and 20, is secured between the forward end of motor housing 132 and the aft end of valve housing 134. The motors are enclosed within leadscrew housings 318 (described below) which are secured to mount plate 250. Plate 250 includes bores for passages 202, 204, 206, 208, 210, 234, 236,238, 240, 242, 244, and 246 which align with corresponding bores in motor housing 132 and 10 valve housing 134. As shown in Figure 20, on the forward side of plate 250 the bores for passages 240 (packerfoot motor), 242 (aft propulsion motor), and 244 (forward propulsion motor) are countersunk to receive retaining bolts 334 (Figure 44). Bolts 334 secure leadscrew housings 318 to the aft side of plate 250.

Figures 21-27 show valve housing 134 of valve unit 96. Attached to the forward 15 end of motor mount plate 250, housing 134 has longitudinal recesses 252, 254, 256, and 258 in its outer radial surface which house failsafe valve 15O, packerfoot valve 154, aft propulsion valve i56, and forward propulsion valve 158, respectively. Housing 134 has bores for passages 202, 204, 206, 208, 210, 234, 236, 238, 240, 242, 244, and 246, which align with corresponding bores in motor mount plate 250. At the forward end of 20 housing 134, a central longitudinal bore is provided which forms an aft portion of galley 155. Galley 155 does not extend to the aft end of housing 134, since its purpose is to feed fluid from the exit of failsafe valve lSO to the other valves. addition, a longitudinal bore is provided at the forward end of housing 134 for a passage 260.

Passage 260 permits fluid communication between packerfoot valve 154 and forward 25 packerfoot 106.

As shown in Figures 24-27, valve housing 134 includes various transverse bores which extend from the valve recesses to the longitudinal fluid passages, for fluid communication with the valves. As described below, valves 150, 154, 156, and 158 are spool valves, each comprising a spool configured to translate inside of a valve body.

30 During operation, the spools translate longitudinally within the bores in the valve bodies and communicate with the fluid passages to produce the behavior schematically shown

in Figures 4A-F. Figure 24 shows the openings of transverse bores within failsafe valve recess 252 which houses failsafe valve 150^ The bores form passages 262, 264, 266, and 268 which extend inward between failsafe valve 150 and various internal passages.

In particular, passages 262 and 266 extend inward to passage 238 (the exit of diffuser 5 148), and passages 264 and 268 extend to galley 155. As will be described below, failsafe valve 150 distributes fluid from passage 238 to galley 155 when the fluid pressure in passage 238 exceeds the desired "on/off" threshold.

Figure 25 shows the openings of transverse bores within forward propulsion valve recess 258. The bores form passages 270, 272, and 274 which extend from lO forward propulsion valve 158 to passage 236, galley 155, and passage 234, respectively.

Figure 26 shows the openings of transverse bores within aft propulsion valve recess 256. The bores form passages 276, 278, and 280 which extend from aft propulsion valve 156 to passage 208, galley 155, and passage 206, respectively. Figure 27 shows the openings of transverse bores within packerfoot valve recess 254. The bores form 15 passages 282, 284, and 286 which extend from packerfoot valve 154 to passage 260, galley 155, and passage 210, respectively. As mentioned above, propulsion valves 156 and 158 distribute fluid from galley 155 to the rear and from chambers of aft and forward propulsion cylinders 108, llO, 112, and 114. Packerfoot valve 154distributes fluid from galley 155 to aft and forward packerfeet 104 and 106.

20 Figures 28-30 show forward transition housing 136 of forward transition unit 98, which connects valve housing 134 to forward shad 124 and houses relief valve 152 and diffuser 148. To simplify manufacturing of the tool, aft and forward shafts 118 and 124 are preferably identical. Thus, housing 136 repositions the various passages passing through the tool, via transverse shift bores (Figure 30) as described above, to align with 25 corresponding passages in forward shaft 124. Note that the shift bores are plugged on the exterior radial surface of housing 136, to prevent leakage of fluid to annulus 40. As seen in the figures, the aft end of housing 136 has longitudinal bores for passages 155, 202, 204, 234, 236, 238, and 260, which align with the corresponding bores in valve housing 134. Supply passage 202 transitions from the lower portion of the housing at 30 the aft end to the central axis of the housing at the forward end, to align with a central bore in forward shaft 124. Wire passage 204 is enlarged at the forward end of housing

I 136, to facilitate connection with wire passages in forward shaft l:z4. Also, note that passage 238 does not extend to the forward end of housing 136. The purpose of passage 238 is to feed fluid from the diffuser to failsafe valve 150.

Referring still to Figures 28-30, diffuser 148 (Figures 31 and 32) is received in 5 passage 202, at the forward end of housing 136. Fluid passing through the diffuser wall enters passage 238 and flows back toward valve housing 134 and to failsafe valve 150.

An additional passage 238A fluidly communicates with passage 238 via a transverse shift bore. Fluid in passage 238A exerts an uphole axial force on the failsafe spool and hence on spring 151 (Figure 45), to open the valve. Galley 155 extends forward to 10 upper orifice 288 of housing 136, within which relief valve 152 (Figures 46-48) is received. The configuration and operation of diffuser 148 and the valves of the tool are described below.

One embodiment of diffuser 148 is shown in Figures 31 and 32. As shown, diffuser 148 is a cylindrical tube having a flange at its forward end and rearwardly 15 angled holes 290 in the tube. The majority of the drilling fluid flowing through passage 202 of forward transition housing 136 flows through the tube of diffuser 148 down to the bottom hole assembly. However, some of the fluid flows back uphole.1 ough holes 290 and into passage 238 which feeds failsafe valve 150. It is believed that the larger fluid particles will generally not make a reversal in direction, but will be forced 20 downhole by the current. Holes 290 form an angle of approximately 135 with the flow of fluid, though an angle of at least 110 with the flow of fluid is believed sufficient to reduce blockage. Further, rear angled holes 290 are sized to restrict the flow of larger fluid particles to valve housing 134. Preferably, holes 290 have a diameter of 0.32cm (0.125 inch) or less. Those skilled in the art will appreciate that a variety of different 25 types of diffusers or filters may be used, giving due consideration to the goal of preventing larger fluid particles from entering and possibly plugging the valves. Of course, if the valves are configured so that pluggage is not a significant concern, or if the fluid is sufficiently devoid of harmful larger fluid particles, then diffuser 148 may be omitted from the EST.

30 Referring to Figures 33-37, failsafe valve 150 comprises valve spool 292 received within valve body 294. Spool 292 has segments 293 of larger diameter. Body

) 2g4 has a central bore 298 which receives spool 292, and rigid ports in its lower wall for fluid passages 262, 264, 266, and 268, described above. The diameter of bore 298 is such that spool 292 can be slidably received therein, and so that segments 293 of spool 298 can slide against the inner wall of bore 298 in an effectively fluid-sealing 5 relationship. Central bore 298 has a slightly enlarged diameter at the axial positions of passages 264 and 268. These portions are shown in the figures as regions 279. Regions 279 allow entering fluid to move into or out of the valve with less erosion to the valve body or valve spool. Body 294 is sized to fit in a fluid-tight axially slidable manner in failsafe valve recess 252 in valve housing 134. Body 294 has angled end faces 296 10 which are compressed between similarly angled portions of valve housing 134 and forward transition housing 136 which define the ends of recess 252. Such compression keeps body 294 tightly secured to the outer surface of valve housing 134. Further, a spacer, such as a flat plate, may be provided in recess 252 between the forward end of valve body 294 and forward transition housing 136. The spacer can be sanded to absorb 15 tolerances in construction of such mating parts. In an EST having a diameter of 3.375 inches, ports 262, 264, 266, and 268 of valve body 294 have a diameter of preferably 2.5rn n (0.1 inches) to 1.2cm (0.5 inches), and more preferably of 5mm (0.2 inches) to 6.4mm (0.25 inches). In the same embodiment, passage 298 preferably has a diameter of tom (0.4 inches) to 1.2cm (0.5 inches).

20 Vent 300 of valve body 294 permits fluid to be exhausted from passage 298 to annulus 40. The ports of valve body 294 fluidly communicate with one another depending upon the position of spool 292. Figures 36 and 37 are longitudinal sectional views of failsafe valve 150. Note that ports 264 and 268 are shown in phantom because these ports do not lie on the central axis of body 294. Nevertheless, the positions of 25 ports 264 and 268 are indicated in the figures. In a closed position, shown in Figure 36, spool 292 permits fluid flow from passage 268 (which communicates with galley 155) to vent 300 (which communicates with annulus 40). In an open position, shown in Figure 37, spool 292 permits fluid flow from passages 264 and 268 (which communicates with galley 155) to passages 262 and 266 (which communicates with 30 diffuser exit 238).

As mentioned above, failsafe valve 150 permits fluid to flow into the galley 155 of valve unit 96. The desired volume fiowrate into galley 155 depends upon the tractor size and activity to be performed, and is summarized in the table below. The below listed ranges of values are the flowrates through valve 150 into galley 155 for milling, 5 drilling, tripping into an open or cased borehole, for various EST diameters. The flowrate into galley 155 depends upon the dimensions of the failsafe valve body and ports. Flow Rate (U.S. Gallons per minute) (Litres per minute! EST Diameter Milling fig Tripping Milling Drilling Tripping 5. 52cm(2.175inches) 0.003-1 0-6 8-100 0.01-3.8 0-23 30-380 8.57cm (3.375 inches) 0.006-1 0-12 8-200 0.02 - 3.8 0 - 45 30 - 760 12.1cr. (4.75 inches) 0.06-3 0-25 8-350 0.2 - 7.6 0 -I- 95 30 - 1300 15.2cm (6.0 inches) 0.6-10 0 55 10-550 2 - 38 0 - 210 38 - 2100 If desired, the stroke length of failsafe valve 150 may be limited to a 3.2mm (1/8 10 inch) stroke (from its closed to open positions), to minimize the burden on relief valve 152. The failsafe valve spool's stroke is limited by the compression of spring 151. For an EST having a diameter of 8.6cm (3.375 inches), this stroke results in a maximum volume fiowrate of approximately 45t (12 gallons) per minute from diffuser exit 238 to galley 155, with an average flowrate of approximately 30t (8 gallons) per minute. The 15 volume flowrate capacity of failsafe valve 150 is preferably significantly more than, and preferably twice, that of propulsion valves 154 and 156, to assure sufficient flow to operate the tool. In the illustrated embodiment, propulsion valves 156 and 158 are

identical, and packerfoot valve 154 is structurally similar. In particular, as shown in Figures 23-28, 20 the locations of the fluid ports of packerfoot valve 154 are slightly different from those of propulsion valves 156 and 158, due to space limitations which limit the positioning of the internal fluid passages of valve housing 134. However, it will be understood that packerfoot valve 154 operates in a substantially similar manner to those of propulsion valves 156 and 158. Thus, only aft propulsion valve 156 need be described in detail 25 herein.

Figures 38-42 show aft propulsion valve 156, which is configured substantially similarly to failsafe valve 150. Valve 156 is a 4-way valve comprising spool 304 and valve body 306. Spool 304 has larger diameter segments 309 and smaller diameter segments 311. As shown in Figure 39, segments 309 include one or more notches 312 5 which permit a variable flow restriction between the various flow ports in valve body 306. Valve body 306 has a configuration similar to that of failsafe valve body 294, with the exception that body 306 has three ports in its lower wall for fluid passages 276, 278, and 280, described above, and two vents 308 and 310 which fluidly communicate with annulus 40. A central bore 307 has a diameter configured to receive spool 304 so that 10 segments 309 slide along the inner wall of bore 307 in an effectively fluid-sealing relationship. Since the positions of the notches 312 along the circumference of the segments 309 may or may not be adjacent to the fluid ports in the valve body, bore 307 preferably has a slightly enlarged diameter at the axial positions of passages 276 and 280, so that the ports can communicate with all of the notches. That is, the inner radial 15 surface of the valve body 306 defining bore 307 has a larger diameter than the other inner radial surfaces constraining the path of movement of segments 309 of spool 304.

These enlarged diameter portions are shown in the figures as regions 279. Valve body 306 is sized to fit tightly in aft propulsion valve recess 256 in valve housing 134. A spacer may also be provided as described above in connection with failsafe valve body 20 294.

Figures 40-42 are longitudinal sectional views of the aft propulsion valve 156.

Note that ports 276 and 280 are shown in phantom because these ports do not lie on the central axis of valve body 3Q6. Nevertheless, the positions of ports 276 and 280 are indicated in the figures. The ports of body 306 fluidly communicate with one another 25 depending upon the axial position of spool 304. In a closed position of aft propulsion valve 156, shown in Figure 40, spool 304 completely restricts fluid flow to and from the aft propulsion cylinders. In another position, shown in Figure 41, spool 304 permits fluid flow from passage 278 (which communicates with galley 155) to passage 280 (which communicates with rear chambers 166 and 170 of aft propulsion cylinders 108 30 and 110), and from passage 276 (which communicates with front chambers 168 and 172 of cylinders 108 and 110) to vent 310 (which communicates with annulus 40). In this

position, valve 156 supplies hydraulic power for a forward thrust stroke of the a* propulsion cylinders, during which fluid is supplied to rear chambers 166 and 170 and exhausted from front chambers 168 and 172. In another position, shown in Figure 42, spool 304 permits fluid flow from passage 278 (which corurnunicates with galley 155) 5 to passage 276 (which communicates with front chambers 168 and 172), arid from passage 280 (which communicates with rear chambers 166 and 170) to vent 308 (which communicates with annulus 40). In this position, valve 156 supplies hydraulic power for a reset stroll of the aft propulsion cylinders, during which fluid is supplied to front chambers 168 and 172 and exhausted from rear chambers 166 and 170.

10 It will be appreciated that the volume flowrate of drilling fluid into aft propulsion cylinders 108 and 110 can be precisely controlled by controlling the axial position of valve spool 304 within valve body 306. The volume flowrate of fluid through any given fluid port of body 306 depends upon the extent to which a large diameter segment 309 of spool 304 blocks the port.

15 Figures 43A-C illustrate this concept. Figure 43A shows the spool 304 having a position such that a segment 309 completely blocks a fluid port of body 306. In this position, there is no flow through the port. As spool 304 slides a certain distance in one direction, as shown in Figure 43B, some fluid flow is permitted through the port via the notches 312. In other words, segment 309 permits fluid flow through the port only 20 through the notches. This means that all of the fluid passing through the port passes through the regions defined by notches 312. The volume flowrate through the port is relatively small in this position, due to the small opening through the notches. In general, the flo vrate depends upon the shape, dimensions, and number of the notches 312. Notches 312 preferably have a decreasing depth and width as they extend toward 25 the center of the length of the segment 309. This permits the flow restriction, and hence the volume flowrate, to be very finely regulated as a function of the spool's axial position. In Figure 43C, spool 304 is moved further so that the fluid is free to flow past segment 309 without necessarily flowing through the notches 312. In other words, 30 segment 309 permits fluid flow through the port at least partially outside of the notches.

This means that some of the fluid passing through the port does not flow through the

regions defined by notches 3i2. In this position fine flow restriction is signirlcan ly decreased, resulting in a greater flowrate through the port. Thus, the valve configuration of the EST permits more precise control over the fluid flowrate to the annular pistons in the propulsion cylinders, and hence the speed and thrust of the tractor.

5 Figure 78 graphically illustrates how the fluid flowrate to either the rear or front chambers of the propulsion cylinders varies as a function of the axial displacement of the propulsion valve spool. Section A of the curve corresponds to the valve position shown in Figure 43B, i.e., when the fluid flows only through the notches 312. Section B corresponds to the valve position shown in Figure 43C, i.e., when the fluid is free to lO flow past the edge of the large diameter segment 309 of the spool. As shown, the flowrate gradually increases in Section A and then increases much more substantially in Section B. Thus, Section A is a region which corresponds to f ne-tuned control over speed, thrust, and position of the EST.

Valve spool 304 preferably includes at least two, advantageously between two 15 and eight, and more preferably three, notches 312 on the edges of the large diameter segments 309. As shown in Figure 79, each notch 79 has an axial length L extending inward fiom the edge of the segment 309, a width W at the edge of the segr,ent 309, and depth D. For an EST having a diameter of 8.6cm (3.375 inches), L is preferably about 1.4 - 1.8mm (0. 055-0.070 inches), W is preferably about 2.9 - 3.8mm (0.115 20 0.150 inches), and D is preferably about 1.5 - 1.8mm (0.058-0.070 inches). For larger sized ESTs, the notch sizes are preferably larger, and/or more notches are provided, so as to produce larger flowrates through the notches. The notch size significantly affects the ability for continuous flow of fluid into the pistons, and hence continuous motion of the tractor at low speeds. In fact, the notches allow significantly improved control over 25 the tractor at low speeds, compared to the prior art. However, some drilling fluids

(especially barite muds) have a tendency to stop flowing at low flow rates and bridge shut small channels such as those in these valves. Greater volume of the notches allows more mud to flow before bridging occurs, but also results in less control at lower speeds. As an alternative means of controlling the tractor at very low speeds, the spool 30 can be opened for a specified interval, then closed and reopened in a "dithering" motion, producing nearly continuous low speed of the tractor.

The valve spools can also have alternative configurations. For example, the segments 309 may have a single region of smaller diameter at their axial ends, to provide an annular flow conduit for the drilling fluid. In other embodiments, the spools can be provided with a multiplicity of steps and shapes that would allow different 5 mudflow rates through the EST. For example, multiple steps 550 can be provided as shown in Figure 71. Alternatively, multiple tapered steps 552 may provided as shown in Figure 72. The spool configurations shown in Figures 71 and 72 allow the spool to more quickly "dither" into and out of different positions. Dithering would add surges of pressure to the propulsion cylinders, which may provide a more responsive tool 10 advance, but less fine-tuned control. A stepwise formation of tapers on the spool also tends to prevent drilling mud from plugging gaps between the spool and valve body.

Although the above-described spool conf gZLrations can be used to provide different flowrate regulation capabilities, the notched configuration of Figure 38 is preferred. Notches 312 have a larger minimum dimension than steps or tapered steps as 15 shown in Figures 71 and 72. Thus, notches 312 are less likely to become plugged by larger fluid particles, which could render the spool ineffective. Also, the notches are less affected by fluid boundary layers on the spools because the fluid boundary layer represents a smaller percentage of the total cross-sectional area defined by the notches.

Of significance in the design for the spool valves is the radial clearance between 20 the valve body and spool. The clearance is preferably made sufficiently large to resist potential plugging by large particles in the drilling fluid, but sufficiently small to prevent leakage which could inhibit control of the EST. This behavior is attributable to the tendency of some muds (especially those containing barite) to bridge or seal small openings. The clearance is sized within the typical operational characteristics of most 25 drilling fluids. Preferably, the clearance is about 15 - 58 microns (0.0006-0023 inches).

As mentioned above, the configuration of valves 154, 156, and 158 permits precise control over the volume flowrate of fluid to propulsion cylinders 108, 110, 112, and 114 and packerfeet 104 and 106. In the illustrated embodiment of the EST, the volume flowrate of fluid to the propulsion cylinders can be more precisely controlled 30 and maintained at any flowrate to a minimum of preferably 2t (0.6 gallons) per minute, more preferably 0.2t (0.06 gallons) per minute, and even more preferably 0.02t (0.006

gallons) per minute, corresponding to fluid flow only through the notches 312. The ability to control and maintain a substantially constant volume flowrate at such small flow levels permits the EST to operate at slow speeds. For an EST having a diameter of 8.6cm (3.375 inches), the stroke length of the propulsion valve spools is preferably 5 limited so that the maximum volume flowrate into the propulsion cylinders is approximately 034t (0-9 gallons) per minute. Preferably, the maximum stroke length from the closed position shown in Figure 40 is 6.4mm (0.25 inches).

As mentioned above, packerfoot valve 154 and aft and forward propulsion valves 156 and 158 are controlled by motors. In a preferred embodiment, the structural 10 configuration which permits the motors to communicate with the valves is similar for each motorized valve. Thus, only that of aft propulsion valve 156 is described herein.

Figures 44A and B illustrate the structural configuration of the EST which peTrnits aft propulsion motor 162 to control valve 156. This configuration transforms torque output from the motor into axial translation of valve spool 304. Motor 162 is cylindrical and is 15 secured within a tubular leadscrew housing 318. Motor 162 and leadscrew housing 318 reside in bore 242 of motor housing 132. The forward end of leadscrew housing 318 is retained in abutment with motor mount plate 250 via a retaining bolt 334 which extends through mount plate 250 and is threadingly engaged with the internal surface of housing 318. 20 Inside leadscrew housing 318, motor 162 is coupled to a leadscrew 322 via motor coupling 320, so that torque output from the motor causes leadscrew 322 to rotate. A bearing 324 is provided to maintain leadscrew 322 along the center axis of housing 318, which is aligned with aft propulsion valve spool 304 in valve housing 134.

Leadscrew 322 is threadingly engaged with a leadscrew nut 326. A longitudinal key 25 325 on leadscrew nut 326 engages a longitudinal slot 328 in leadscrew housing 318.

This restricts nut 326 from rotating with respect to leadscrew housing 318, thereby causing nut 326 to rotate along the threads of leadscrew 322. Thus, rotation of leadscrew 322 causes axial translation of nut 326 along leadscrew 322. A stem 330 is attached to the forward end of nut 326. Stem 330 extends forward through annular 30 restriction 333, which separates oil in motor housing 132 from drilling fluid in valve housing 134. The drilling fluid is sealed from the oil via a tee seal 332 in restriction

333. The forward end of stem 330 is attached to valve spool 304 via a spool bolt 336 and split retainer 338. Stem 330 is preferably relatively thin and flexible so that it can compensate for any misalignment between the stem and the valve spool.

Thus, it can be seen that torque output from the motors is converted into axial 5 translation of the valve spools via leadscrew assemblies as described above. The displacement of the valve spools is monitored by constantly measuring the rotation of the motors. Preferably, rotary accelerometers or potentiometers are built into the motor cartridges to measure the rotation of the motors, as known in the art. The electrical signals f om the accelerometers or potentiometers can be transmitted back to logic 10 component 224 via electrical wires 536 and 538 (Figure 69).

Preferably, motors 160, 162, and 164 are stepper motors, which require fewer wires. Advantageously, stepper motors are brushless. If, in contrast, brush-type motors are used, filaments from the breakdown of the metal brushes may render the oil electrically conductive. Importantly, stepper motors can be instructed to rotate a given 15 number of steps, facilitating precise control of the valves. Each motor cartridge may include a gearbox to generate enough torque and angular velocity to turn the leadscrew at the desired rate. Fine motor gear box assembly should be aDie to generate desirably at least 22N (5 pounds), more desirably at least 44N ( 10 pounds), and even more desirably at least 220N (50 pounds) of force and angular velocity of at least 75-180 rpm output.

20 The motors are preferably configured to rotate 12 steps for every complete revolution of the motor output shafts. Further, for an EST having a diameter of 8.57cm (3.375 inches), the motor, gear box, and accelerometer assembly desirably has a diameter no greater than 2.22crr. (0.875 inches) (and preferably l.9cm (0.75 inches)) and a length no longer than 7.7cm (3.05 inches). A suitable motor is product no. DF7-A sold by CD 25 Astro Intercorp, Inc. of Deerfield, Florida.

In order to optimally control the speed and thrust of the EST, it is desirable to know the relationships between the angular positions of the motor shafts and the flowrates through the valves to the propulsion cylinders. Such relationships depend upon the cross-sectional areas of the flow restrictions acting on the fluid flows through 30 the valves, and thus upon the dimensions of the spools, valve bodies, and fluid ports of the valve bodies. Such relationships also depend upon the thread pitch of the

) leadscrews. In a preferred embodiment, the leadscre vs have about 8-32 threads per inch. Inside motor housing 132, bores 240, 242, and 244 contain the motors as well as electrical wires extending rearward to electronics unit 92. For optimal performance, 5 these bores are preferably filled with an electrically nonconductive fluid, to reduce the risk of ineffective electrical transmission through the wires. Also, since the pressure of the motor chambers is preferably equalized to the pressure of annulus 40 via a pressure compensation piston (as described below), such fluid preferably has a relatively low compressibility, to minimize the longitudinal travel of the compensation piston A 10 preferred fluid is oil, since the compressibility of oil is much less than that of air. At the aft end of motor housing 132, these bores are fluidly open to the space surrounding pressure transducer manifold 222. Thus, the outer ends of pressure transducers 182, 184, 186, 188, and 190 are also exposed to oil.

Figure 45 illustrates the assembly and operation of failsafe valve 150. The aft 15 end of failsafe valve spool 292 abuts a spring guide 340 that slides inside passage 246 within motor housing 132, motor mount plate 250, and valve housing 134. Inside motor housing 132 passage 246 has an annular spring stop 342 which is fixed with respect to housing 132. Guide 340 has an annular flange 344. Failsafe valve spring 151, preferably a coil spring, resides within passage 246 so that its ends abut stop 342 and 20 flange 344. Fluid within passage 238A (from the exit of diffuser 148) exerts an axial force on the forward end of spool 292, which is countered by spring 151. As shown, a spacer having a passage 238B may be provided to absorb tolerances between the mating surfaces of valve housing 134 and forward transition housing 136. Passage 238B fluidly communicates with passage 238A and with spool passage 298 of failsafe valve body 25 294. When the fluid pressure in passage 238A exceeds a particular threshold, the spring force is overcome to open failsafe valve 150 as shown in Figure 37. Spring 151 can be carefully chosen to compress at a desired threshold fluid pressure in passage 238A.

When the EST is removed from a borehole, drilling fluid residue is likely to remain within passage 246 of motor housing 132 As shown in Figures 1718, a pair of 30 cleaning holes 554 may be provided which extend into passage 246. Such holes permit passage 246 to be cleaned by spraying water through the passage, so that spring 153

i operates properly during use. During use' holes 554 may be plugged so that the drilling fluid does not escape to annulus 40.

Referring to Figures 44A-B, the leadscrew assemblies for the motorized valves contain drilling fluid from annulus 40. Such fluid enters the leadscrew assemblies via 5 the exhaust vents in the valve bodies, and surrounds portions of the valve spools and stems 330 forward of annular restrictions 333. As mentioned above, the chambers rearward of restrictions 333 are filled with oil. In order to move the valve spools, the motors must produce sufficient torque to overcome (1) the pressure difference between the drilling fluid and the oil, and (2) the seal friction caused by tee seals 332. Since the 10 fluid pressure in annulus 40 can be as high as 1.1 x lO8Nm 2 (16,000 psi), the oil pressure is preferably equalized with the fluid pressure in annulus 40 so that the pressure difference across seals 332 is zero. Absent such oil pressure compensation, the motors would have to work extremely hard to advance the spools against the high pressure drilling fluid. A significant pressure difference can cause the motors to stall.

15 Further, if the pressure difference across seals 332 is sufficiently high, the seals would have to be very tight to prevent fluid flow across the seals. However, if the seals were very tight they would hinder and, probably, prevent movement of the stems 330 and hence the valve spools.

With reference to Figure 45, a pressure compensation piston 248 is preferably 20 provided to avoid the above-mentioned problems. Preferably, piston 248 resides in passage 246 of motor housing 132. Piston 248 seals drilling fluid on its forward end from oil on its aft end, and is configured to slide axially within passage 246. As the pressure in annulus 40 Increases, piston 248 slides rearward to equalize the oil pressure with the drilling fluid pressure. Conversely, as the pressure in annulus 40 decreases, 25 piston 248 slides forward. Advantageously, piston 248 effectively neutralizes the net longitudinal fluid pressure force acting on each of the valve spools by the drilling fluid and oil. Piston 248 also creates a zero pressure difference across seals 332 of the leadscrew assemblies of the valves.

Figures 46-48 illustrate the configuration and operation of relief valve 152.

30 Relief valve 152 comprises a valve body 348, poppet 350, and coil spring 153. Body 348 is generally tubular and has a nose 351 and an internal valve seat 352. Poppet 350

has a rouIlded end 354 configured to abut valve seat 352 to close the valve. Poppet 350 also has a plurality of longitudinal ribs 356 between which fluid may flow out to annulus 40. Aside forward transition housing 136, relief valve body 348 resides within a diagonal portion 349 of galley 155 which extends to orifice 288 and out to annulus 40.

5 Body 348 is tightly and securely received within the aft end of diagonal bore 349. A tube 351 resides forward of body 348. Tube 351 houses relief valve spring 153. Poppet 350 is slidably received within body 348. The forward end of poppet 350 abuts the aft end of spring 153. The forward end of spring 153 is held by an internal annular flange of tube 351. In operation, the drilling fluid inside galley 155 exerts a force on rounded 10 end 354 of poppet 350, which is countered by spring 153. As the fluid pressure rises, the force on end 354 also rises. If the fluid pressure in galley 155 exceeds a threshold pressure, the spring force is overcome, forcing end 354 to unseat from valve seat 352.

This permits fluid from galley 155 to exhaust out to annulus 40 through bore 349 and between the ribs 356 of poppet 350.

15 In a preferred embodiment, control assembly 102 is substantially cylindrical with a diameter of about 8.57crn (3.375 inches) and a length of about 1.2m (46.7 inches).

Housings 130, 131, 132, 134, and 136 are preferably constructed of a high strength material, to prevent erosion caused by exposure to harsh drilling fluids such as calcium bromide or cesium formate muds. In general, the severity and rate of erosion depends 20 on the velocity of the drilling fluid to which the material is exposed, the solid material within the fluid, and the angle at which the fluid strikes a surface. In operation, the control assembly housings are exposed to drilling mud velocities of O to 16.8m (55 feet) per second, with typical mean operating speeds of less than 9. lm (30 feet) per second (except within the valves). Under these conditions, a suitable material for the control 25 assembly housings is Stabaloy, particularly Stabaloy AG 17. In the valves, mud flow velocities can be as high as 46rn (150 feet) per second. Thus, the valves and valve bodies are preferably formed from an even more erosion-resistant material, such as tungsten carbide, Ferro-Tec (a proprietary steel formed of titanium carbide and available from Alloy Technologies International, Inc. of West Nyack, New York), or similar 30 materials. The housings and valves may be constructed from other materials, giving due consideration to the goal of resisting erosion.

) Shaft Assemblies In a preferred embodiment, the aft and forward shaft assemblies are structurally similar. Thus, only the aft shaft assembly is herein described in detail. Figure 49 shows the configuration of the aft shaft assembly. Aft packerfoot 104, flexible connector 120, 5 cylinder 108, flexible connector 122, and cylinder 110 are connected together end to end and are collectively slidably engaged on aft shaft 118. Annular pistons 140 and 142 are attached to shaft 118 via bolts secured into bolt holes 360 and 362, respectively. O rings or specialized elastomeric seals may be provided between the pistons and the shaft to prevent flow of fluid under the pistons. Cylinders 108 and 110 enclose pistons 140 10 and 142, respectively. The forward and aft ends of each propulsion cylinder are sealed, via tee-seals, O-rings, or otherwise, to prevent the escape of fluid from within the cylinders to annulus 40. Also, seals are provided between the outer surface of the pistons 140 and 142 and the inner surface of the cylinders 108 and 110 to prevent fluid from flowing between the front and rear chambers of the cylinders.

15 Connectors 120 and 122 may be attached to packerfoot 104 and cylinders 108 and l lO via threaded engagement, to provide high-pressure integrity and avoid using a multiplicity of bolts or screws. Tapers may be provided on the leading edges of connectors 120 and 122 and seal cap 123 attached to the forward end of cylinder 110.

Such tapers help prevent the assembly from getting caught against sharp surfaces such 20 as milled casing passages.

A plurality of elongated rotation restraints 364 are preferably attached onto shaft 118, which prevent packerfoot 104 from rotating with respect to the shaft. Restraints 364 are preferably equally spaced about the circumference of shaft 118, and can be attached via bolts as shown. Preferably four restraints 364 are provided. Packerfoot 25 104 is configured to engage the restraints 364 so as to prevent rotation of the packerfoot with respect to the shaft, as described in greater detail below.

Figures 50-59 illustrate in greater detail the configuration of shaft 118. At its forward end, shaft 118 has a flange 366 which is curved for more even stress distribution. Flange 366 includes bores for fluid passages 202, 206, 208, and 210, 30 which align with corresponding bores in aft transition housing 131. Note that the sizes of these passages may be varied to provide different flowrate and speed capacities of the

EST. addition, a pair of wire passages 204A is provided, one or both of the passages aligning with wire bore 204 of housing 131. Electrical wires 502, 504, 506, and 508 (Figure 69), which run up to the surface and, in one embodiment, to a position sensor on piston 142, reside in passages 204A. As shown in Figure 52, only wire passages 204A 5 and supply passage 202 extend to the aft end of shaft 118.

As shown in Figure 55, within shaft 118 fluid passages 206, 208, and 210 each comprise a pair of passages 206A, 208A, and 210A, respectively. Preferably, the passages split into pairs inside of flange 366. In the illustrated embodiment, pairs of gun-drilled passages are provided instead of single larger passages because larger 10 diameter passages could jeopardize the structural integrity of the shaft. With reference to Figure 53, passages 206A deliver fluid to rear chambers 166 and 170 of propulsion cylinders 108 and 110 via fluid ports 368 and 370, respectively. Figure 58 shows ports 370 which communicate with rear chamber 170 of cylinder 110. These ports are transverse to the longitudinal axis of shaft 118. Ports 368 are configured similarly to 15 ports 370. With reference to Figure 50, passages 208A deliver fluid to front chambers 168 and 172 of cylinders 108 and 110 via fluid ports 372 and 374, respectively. Ports 374 are shown in Figure 56. Ports 372 are configured similarly to ports 374. Passages 206A and 208A are provided for the purpose of delivering fluid to the propulsion cylinders. Hence, passages 206A and 208A do not extend rearwardly beyond 20 longitudinal position 380.

* With reference to Figure 53, passages 210A deliver fluid to aft packerfoot 104, via a plurality of fluid ports 378. Ports 378 are preferably arranged linearly along shaft 118 to provide fluid throughout the interior space of packerfoot 104. the preferred embodiment, nine ports 378 are provided. Figure 59 shows one of the ports 378, which 25 fluidly communicates with each of passages 210A. Since passages 210A are provided for the purpose of delivering fluid to aft packerfoot 104, such passages do not extendrearwardly beyond longitudinal position 382.

With reference to Figure 50, a wire port 376 is provided in shaft 118. Port 376 permits electrical communication between control assembly 102 and position sensor 30 192 (Figures 4A-F) or piston 142. For example, a Wiegand sensor or magnetometer

device (described below) may be located on piston 142. Port 376 is also shown in Figure 57.

In a preferred embodiment, some of the components of the EST are formed from a flexible material, so that the overall flexibility of the tool is increased. Also, the 5 components of the tool are preferably non-magnetic, since magnetic materials can interfere with the performance of magnetic displacement sensors. Of course, if magnetic displacement sensors are not used, then magnetic materials are not problematic. A preferred material is copper-beryllium (CuBe) or CuBe alloy, which has trace amounts of nickel and iron. This material is non-magnetic and has high strength 10 and a low tensile modulus. With reference to Figure 2, shafts 118 and 124, propulsion cylinders 108, 110, 112, and 114, and connectors 120, 122, 126, and 128 may be formed from CuBe. Pistons 140 and 142 may also be formed from CuBe or CuBe alloy. The cylinders are preferably chrome- plated for maximum life of the seals therein.

In a preferred embodiment, each shaft is about 3.7m (12 feet) long, and the total 15 length of the EST is about 9.8m (32 feet). Preferably, the propulsion cylinders are about 65cm (25.7 inches) long and 7.95cm (3.13 inches) in diameter. Connectors 120, 122, 126, and 128 are preferably smaller in diameter than the propulsion cylinders and packerfeet at their center. The connectors desirably have a diameter of no more than 7cm (2. 75 inches) and, preferably, no more than 5.2cm (2.05 inches). This results in 20 regions of the EST that are more flexible than the propulsion cylinders and control assembly 102. Consequently, most of the flexing of the EST occurs within the connectors and shafts. In one embodiment, the EST can turn up to 60 per 30m (100 feet) of drilled arc. Figure 73A shows an arc curved to schematically illustrate the turning capability of the tool. Figure 73B schematically shows the flexing of the aft 25 shaft assembly of the EST. The degree of flexing is somewhat exaggerated for clarity.

As shown, the flexing is concentrated in aft shaft 1 18 and connectors 120 and 122.

Shafts 118 and 124 can be constructed according to several different methods.

One method is diffusion bonding, wherein each shaft comprises an inner shaft and an outer shaft, as shown in Figure 68. Inner shaft 480 includes a central bore for fluid 30 supply passage 202, and ribs 484 along its length. The outer diameter of inner shaft 480 at the ribs 484 is equal to the inner diameter of outer shaft 482, so that ironer shaft 480

\ fits tightly into outer shaft 482. Substantially the entire outer surface of ribs 484 mates with the inner surface of shaft 482. Longitudinal passages are formed between the shafts. In aft shaft 118, these are passages 204 (wires), 206 (fluid to rear chambers of aft propulsion cylinders), 208 (fluid to front chambers of aft propulsion cylinders), and 5 210 (fluid to aft packerfoot).

The inner and outer shafts 480 and 482 may be formed by a co-extrusion process. Shafts 480 and 482 are preferably made from CuBe alloy and annealed with a "drill string" temper process (annealing temper and thermal aging) that provides excellent mechanical properties (tensile modulus of 7.6 - 9.0 x lO?Nm 2 (110,000 10 130,000 psi), and elongation of 8-10% at room temperature). The inner and outer shafts are then diffusion bonded together. Accordingly, the shafts are coated with silver, and the inner shaft is placed inside the outer shaft. The assembly is internally pressurized, externally constrained, and heated to approximately 816 C (1500 F). The CuBe shafts expand under heat to form a tight fit. Heat also causes the silver to diffuse into the 15 CuBe material, forming the diffusion bond. Experiments on short pieces of diffusion bonded shafts have demonstrated pressure integrity within the several passages. Also, experiments with short pieces have demonstrated diffusion bond shear strengths of 2.9 3.4 x lO8Nm 2 (42,000 to 49,000 psi) .

After the shafts are bonded together, the assembly is electrolitically chrome 20 plated to increase the life of the seals on the shaft. Special care is made to minimize the thickness of the chrome to allow both long life and shaft flexibility. The use of diffusion bonding permits the unique geometry shown in Figure 68, which maximizes fluid flow channel area and simultaneously maximizes the torsional rigidity of the shaft.

In a similar diffusion bonding process, the flange portion 366 (Figures 49A-B) can be 25 bonded to the end of the shaft.

Alternatively, other materials and constructions can be used. For example, Monel or titanium alloys can be used with appropriate welding methods. Monet is an acceptable material because of its non-magnetic characteristics. However, Monel's high modulus of elasticity or Young's Modulus tends to restrict turning radius of the tractor 30 to less than 40 per 30m (100 feet) of drilled arc. Titanium is an acceptable material because of its non-magnetic characteristics, such as high tensile strength and low

Young's modulus (compared to steel). [However, titanium welds are known to have relatively short fatigue life when subjected to drilling environments.

In another method of constructing shafts 118 and 124, the longitudinal wire and fluid passages are formed by "gun-drilling," a well-known process used for drilling long 5 holes. Advantages of gun-drilling include moderately lower torsional and bending stiffness than the diffusion-bonded embodiment, and lower cost since gun-drilling is a more developed art. When gun-drilling a hole, the maximum length and accuracy of the hole depends upon the hole diameter. The larger the hole diameter, the longer and more accurately the hole can be gun-drilled. However, since the shafts have a relatively small 10 diameter and have numerous internal passages, too great a hole diameter may result in inability of the shafts to withstand operational bending and torsion loads. Thus, in selecting an appropriate hole diameter, the strength of the shaft must be balanced against the ability to gun-drill long, accurate holes The shaft desirably has a diameter of 2.5 - 9cm (1-3.S inches) and a fluid supply 15 passage of preferably 1.5-4.4cm (0.6-1.75 inches) in diameter, and more preferably at least 2.5cm (0.99 inches) in diameter. In a preferred embodiment of the EST, the shaft diameter is 4.43-4.44cm (1.746-1.748 inches), and the diameter of fluid supply passage 202 is 2.5cm (1 inch). For an EST having a diameter of 8.57cm (3.375 inches), the shafts are designed to survive the stresses resulting from the combined loads of 1350Nm 20 (1000 ft-lbs) of torque, pulling-thrusting load up to 29kN (6500 pounds), and bending of 60 per 30m (100 feet) of travel. Under these constraints, a suitable configuration is shown in Figure 55, which shows aft shaft 118. Passages 204A, 206A, 208A, and 210A comprise pairs of holes substantially equally distanced between the inner surface of passage 202 and the outer surface of shaft 118. For each passage, a pair of holes is 25 provided so that the passages have sufficient capacity to accommodate required operational drilling fluid flowrates. This configuration is chosen instead of a single larger hole, because a larger hole may undesirably weaken the shaft. Each hole has a diameter of 4.8mm (0.188 inch). The holes of each individual pair are spaced apart by approximately one hole diameter. For a hole diameter of 4.8mm (0.188 inch) , it may 30 not be possible to gun-drill through the entire length of each shaft 118 and 124. In that

case, each shaft can be made by gun-drilling the holes into two or more shorter shafts and then electron beam (EB) welding them together end to end.

The welded shaft is then preferably thermally annealed to have desired physical properties, which include a tensile modulus of approximately 1.3 x 10 Nm 2 5 (19,000,000 psi), tensile strength of approximately 7.6-9.0 x 108Nm 2 (110,000 130,000 psi), and elongation of about 8-12%. The shaft can be baked at 777 C (1430 F) for 1 -8 hours depending upon the desired characteristics. Details of post-weld annealing methods are found in literature about CuBe. After the thermal annealing step, the welded shaft is then finished, machined, ground, and chrome-plated.

Packerfeet Figures 60-64 and 74-75 show one embodiment of aft packerfoot 104. The major components of packerfoot 104 comprise a mandrel 400, bladder assembly 404, end clamp 414, and connector 420. Mandrel 400 is generally tubular and has internal 15 grooves 402 sized and configured to slidably engage rotation restraints 364 on aft shaft 118 (Figure 49A). Thus, mandrel 400 can slide longitudinally, but cannot rotate, with respect to shaft 118. Bladder assembly 404 comprises generally rigid tube portions 416 and 417 attached to each end of a substantially tubular inflatable engagement bladder 406. Assembly 404 generally encloses mandrel 400. On the aft end of packerfoot 104, 20 assembly 404 is secured to mandrel 400 via eight bolts 408 received within bolt holes 410 and 412 in assembly 404 and mandrel 400, respectively. An end clamp 414 is used as armor to protect the leading edge of the bladder 406 and is secured via bolts onto end 417 of assembly 404. If desired, an additional end clamp can be secured onto end 416 of assembly 404 as well. Connector 420 is secured to mandrel 400 via eight bolts 422 25 received within bolt holes 424 and 426. Connector 420 provides a connection between packerfoot 104 and flexible connector 120 (Figure 49A).

The ends of bladder assembly 404 are preferably configured to move longitudinally toward each other to enhance radial expansion of bladder 406 as it is inflated. In the illustrated embodiment, aft end 416 of assembly 404 is fixed to mandrel 30 400, and forward end 417 is slidably engaged with segment 418 of mandrel 400. This permits forward end 417 to slide toward aft end 416 as the packerfoot is inflated,

thereby increasing the radial expansion of bladder 406. The EST's packerfeet are designed to traverse holes up to 10% larger than the drill bit without losing traction. For example, a typical drill bit size, and the associated drilled hole, is 9.5cm (3.75 inches) in diameter. A correspondingly sized packerfoot can traverse a 10.4cm (4.1 inch) diameter 5 hole. Similarly, a 11.4cm (4.5-inch) diameter hole will be traversed with a packerfoot that has an expansion capability to a minimum of 12.7cm (5.0 inches). Further, the slidable connection of bladder assembly 404 with segment 418 tends to prevent the fibers in bladder 406 from overstraining, since the bladder tends not to stretch as much.

Alternatively, the bladder assembly can be configured so that its forward end is fixed to 10 the mandrel and its aft can slide toward the forward end. However, this may cause the bladder to undesirably expand when pulling the tractor upward out of a borehole, which can cause the tractor to "stick" to the borehole walls. Splines 419 on the forward end of assembly 404 engage grooves inside connector 420 so that end 417 cannot rotate with respect to mandrel 400.

15 One or more fluid ports 428 are provided along a length of mandrel 400, which communicate with the interior of bladder 406. Ports 428 are preferably arranged about the circumference of mandrel 400, so that fluid is introduced uniformly throughout the bladder interior. Fluid from aft packerfoot passage 210 reaches bladder 406 by flowing through ports 378 in shaft 118 (Figures 53 and 59) to the interior of mandrel 400, and 20 then through ports 428 to the interior of bladder 406. Suitable fluid seals, such as O rings, are provided at the ends of packerfoot 104 between mandrel 400 and bladder assembly 404 to prevent fluid within the bladder from leaking out to annulus 40.

MA a preferred embodiment, bladder 406 is constructed of high strength fibers and rubber in a special orientation that maximizes strength, radial expansion, and 25 fatigue life. The rubber component may be nitrite butadiene rubber (NBR) or a tetra fluor-ethylene (TFE) rubber, such as the rubber sold under the trade name AFLAS.

NBR is preferred for use with invert muds (muds that have greater diesel oil content by volume than water). AFLAS material is preferred for use with some specialized drilling fluids, such as calcium formate muds. Other additives may be added to the rubber to 30 improve abrasion resistance or reduce hysterisis, such as carbon, oil, plasticizers, and various coatings including bonded Teflon type materials.

High strength fibers are included within the bladder, such as S-glass, Lglass, Kevlar (polyamides), and various graphites. The preferred material is S-glass because of its high strength (3.7 x lO9Nm 2) (530,000 psi) and high elongation (5-6%), resulting in greatly improved fatigue life compared to previous designs. For instance, if the 5 fatigue life criterion for the bladders is that the working strain will remain below approximately 25-35% of the ultimate strain of the fibers, previous designs were able to achieve about 7400 cycles of inflation. In contrast, the expected life of the bladders of the present invention under combined loading is estimated to be over 25,000 cycles.

Advantageously, more inflation cycles results in increased operational downhole time 10 and lower rig costs.

The fibers are advantageously arranged in multiple layers, a cross-ply pattern.

The fibers are preferably oriented at angles of +a relative to the longitudinal axis of the tractor, where cc is preferably between 0 and 45 , more preferably between 7 and 30 , even more preferably between 15 and 20 , and most preferably about 15 . This allows 15 maximal radial expansion without excessive bulging of the bladder into the regions between the packerfoot toes, described below. It also allows optimal fatigue life by the criterion described above.

When bladder 406 is inflated to engage a borehole wall 42, it is desirable that the bladder not block the uphole return flow of drilling fluid and drill cuttings in annulus 20 40. To prevent this, elongated toes 430 are bonded or otherwise attached to the outer surface of the rubber bladder 406, as shown in Figures 60 and 75. Toes 430 may have a triangular or trapezoidal cross-section and are preferably arranged in a rib-like manner.

When the bladder engages the borehole wall, crevices are formed between the toes 430 and the wall, permitting the flow of drilling fluid and drill cuttings past the packerfoot.

25 Toes 430 are preferably designed to be (1) sufficiently large to provide traction against the hole wall, (2) sufficiently small in crosssection to maximize uphole return flow of drilling fluid past the packerfoot in annulus 40, (3) appropriately flexible to deform during the inflation of the bladder, and (4) elastic to assist in the expulsion of drilling fluid from the packerfoot during deflation. Preferably, each toe has an outer radial 30 width of 2.5-15.2mm (0.1-0.6 inches), and a modulus of elasticity of about 4.6 x lOi2Nm-3 (19,000,000). Toes 430 may be constructed of CuBe alloy. The ends of toes

43() are secured onto ends 416 and 4i7 of bladder assembly 404 by 'oands ul material 432, preferably a high-strength non-magnetic material such as Stabaloy. Bands 432 prevent toes 430 from separating from the bladder during unconstrained expansion, thereby preventing formation of "fishhooks" which could undesirably restrict the 5 extraction of the EST from the borehole. Figure 74 shows packerfoot l 04 inflated.

A protective shield of plastic or metal may be placed in front of the leading edge of the packerfoot, to channel the annulus fluid flow up onto the inflated packerfoot and thereby protect the leading edge of the bladder from erosion by the fluid and its particulate contents.

10 Figures 65-67 and 76 illustrate an alternative embodiment of an aft packerfoot, referred to herein as a "flextoe packerfoot." Aft and forward flextoe packerfeet can be provided in place of the previously described packerfoot 104 and 106. Unlike prior art

bladder-type anchors, the flextoe packerfoot of the invention utilizes separate components for radial expansion force and torque transmission of the anchors. In 15 particular, bladders provide force for radial expansion to grip a borehole wall, while "flextoes" transmit torque from the EST body to the borehole. The flextoes comprise beams -which elastically bend within z plane parallel to the tractor body the tractor body.

Advantageously, the flextoes substantially resist rotation of the body while the packerfoot is engaged with the borehole wall. Other advantages of the flextoe 20 packerfoot include longer fatigue life, greater expansion capability, shorter length, and less operational costs.

The figures show one embodiment of an aft flextoe packerfoot 440. Since the forward flextoe packerfoot is structurally similar to aft flextoe packerfoot 440, it is not described herein. The major components of aft flextoe packerfoot 440 comprise a 25 mandrel 434, fixed endpiece 436, two dowel pin assemblies 438, two jam nuts 442, shuttle 444, spline endpiece 446, spacer tube 44S, connector 450, four bladders 452, four bladder covers 454, and four flextoes 456.

With reference to Figure 66, mandrel 434 is substantially tubular but has a generally rectangular bladder mounting segment 460 which includes a plurality of 30 elongated openings 462 arranged about the sides of segment 460. In the EST, bladders 452 are clamped by bladder covers 454 onto segment 460 so as to cover and seal shut

openings 462. In operation, fluid is delivered to the interior space of mandrel 434 via ports 378 in shaft 118 (Figures 53 and 59) to inflate the bladders. Although four bladders are shown in the drawings, any number of bladders can be provided. In an alternative embodiment, shown in Figure 76, one continuous bladder 452 is used. This 5 configuration prevents stress concentrations at the edges of the multiple bladders and allows greater fatigue life of the bladder.

Referring to Figure 65, bladder covers 454 are mounted onto mandrel 434 via bolts 468 which pass through holes on the side edges of covers 454 and extend into threaded holes 464 mandrel 434. Bolts 468 fluidly seal bladders 452 against mandrel 10 434, and prevent the bladders from separating from mandrel 434 due to the fluid pressure inside the bladders. Since the pressure inside the bladders can be as high as 1.65 x 107Nm 2 (2400 psi), a large number of bolts 468 are preferably provided to enhance the strength of the seal. In the illustrated embodiment, 17 bolts 468 are arranged linearly on each side of the covers 454. Jam nuts 442 clamp the aft and 15 forward ends of bladder covers 454 onto mandrel 434, to fluidly seal the aft and forward ends of the bladders. The individual bladders can easily be replaced by removal of the associated bladder cover 454, substantially reducing replacement costs and time compared to prior art configurations. Bladder covers 454 are preferably constructed of

CuBe or CuBe alloy.

20 Referring to Figure 65, fixed endpiece 436 is attached to the aft end of mandrel 434 via bolts extending into holes 437. Forward of the bladders, shuttle 444 is slidably engaged on mandrel 434. One dowel pin assembly 438 is mounted onto endpiece 436, and another assembly 438 is mounted onto shuttle 444. In the illustrated embodiment, assemblies 438 each comprise four dowel pin supports 439 which support the ends of 25 the dowel pins 458. The dowel pins hingedly support the ends of flextoes 456.

Endpiece 436 and shuttle 444 each have four hinge portions 466 which have holes that receive the dowel pins 458. During operation, inflation of the bladders 452 causes bladder covers 454 to expand radially. This causes the flextoes 456 to hinge at pins 458 and bow outward to engage the borehole wall. Figure 76 shows an inflated flextoe 30 packerfoot (having a single continuous bladder), with flextoes 456 gripping borehole wall 42. Shuttle 444 is free to slide axially toward fixed endpiece 436, thereby

/ enhancing radial expansion or the ilextoes. 1 nose skilled In me art win unaerslana mat either end of the flextoes 456 can be permitted to slide along mandrel 434. However, it is preferred that the forward ends of the flextoes be permitted to slide, while the aft ends are fixed to the mandrel. This prevents the slidable end of the flextoes from being 5 axially displaced by the borehole wall during tool removal, which could cause the flextoes to flex outwardly and interfere with removal of the tractor.

Spline end piece 446 is secured to mandrel 434 via bolts extending into threaded holes 472. At the point of attachment, the inner diameter of end piece 446 is approximately equal to the outer diameter of mandrel 434. Rear of the point of 10 attachment, the inner diameter of end piece 446 is slightly larger, so that shuttle 444 can slide within end piece 446. End piece 446 also has longitudinal grooves in its inner diameter, which receive splines 470 on the outer surface of shuttle 444. This prevents shuttle 470, and hence the forward ends of the flextoes 456, from rotating with respect to mandrel 434. Thus, since both the forward and aft ends of flextoes 456 are prevented 15 from rotating with respect to mandrel 434, the flextoes substantially prevent the tool from rotating or twisting when the packerfoot is engaged with the borehole wall.

In the sanJe manner as described above with regard to mandrel 40n of packerfoot 104, mandrel 434 of flextoe packerfoot 440 has grooves on its internal surface to slidably engage rotation restraints 364 on aft shaft 118. Thus, mandrel 434 can slide 20 longitudinally, but cannot rotate, with respect to shaft 118. Restraints 364 transmit torque from shaft 118 to a borehole wall 42. The components of packerfoot 440 are preferably constructed of a flexible, non-ma etic material such as CuBe. Flextoes 456 may include roughened outer surfaces for improved traction against a borehole wall.

The spacer tube 448 is used as an adapter to allow interchangeability of the 25 Flextoe packerfoot 440 and the previous described packerfoot 104 (Figure 60). The connector 450 is connected to the mandrel via the set screws. Connector 450 connects packerfoot 440 with flexible connector 120 (Figure 49A) of the EST.

Figure 67 shows the cross-sectional configuration of one of the bladders 452 utilized in flextoe packerfoot 440. In its uninflated state, bladder 452 has a multi-folded 30 configuration as shown. This allows for greater radial expansion when the bladder is inflated, caused by the unfolding of the bladder. Also, the bladders do not stretch as

much during use, compared lo prior bladders. This results in longer life of tllje bladders.

The bladders are made from fabric reinforced rubber, and may be constructed in several configurations. From the inside to the outside of the bladder, a typical construction is rubber/fiber/rubber/fiber/rubber. Rubber is necessary on the inside to maintain pressure.

5 Rubber is necessary on the outside to prevent fabric damage by cuttings passing the bladder. The rubber may be NBR or AFLAS (TFE rubber). Suitable fabrics include S glass, E-glass, Kevlar 29, Kevlar 49, steel fabric (for ESTs not having magnetic sensors), various types of graphite, polyester-polarylate fiber, or metallic fibers.

Different fiber reinforcement designs and fabric weights are acceptable. For the 1O illustrated embodiment, the bladder can withstand inflation pressure up to 1500 psi.

This inflation strength is achieved with a 400 denier 4-tow by 4-tow basket weave Kevlar 29 fabric. The design includes consideration for fatigue by a maximum strain criterion of 25% of the maximum elongation of the fibers. It has been experimentally determined that a minimum thickness of 2.3mm (0.090 inches) of rubber is required on 15 the inner surface to assure pressure integrity.

For both the non-flextoe and flextoe embodiments, the packerfeet are preferably positioned near the extreme ends of the EST, to enhance the tool's ability to traverse underground voids. The packerfeet are preferably about lm (39 inches) long. The metallic parts of the packerfeet are preferably made of CuBe alloy, but other non 20 magnetic materials can be used.

During use, the packerfeet (all of the above-described embodiments, i.e., Figures 60 and 65) can desirably grip an open or cased borehole so as to prevent slippage at high longitudinal and torsional loads. In other words, the normal force of the borehole against each packerfoot must be high enough to prevent slippage, giving due 25 consideration to the coefficient of friction (typically about 0.3). The normal force depends upon the surface area of contact between the packerfoot and the borehole and the pressure inside the packerfoot bladder, which will normally be between 3.4 x 106 l.1 x 107Nm 2 (500-1600 psi), and can be as high as 1. 65 x 107Nm 2 (2400 psi). When inflated, the surface area of contact between each packerfoot and the borehole is 30 preferably at least 39cm2 (6 in2), more preferably at least 58cmZ (9 in2), even more preferably at least 84cm2 (13 in2), and most preferably at least 116cm2 (18 in2).

Those in the art will understand that fluid seals are preferably provided throughout the EST, to prevent drilling fluid leakage that could render the tool inoperable. For example, the propulsion cylinders and packerfeet are preferably sealed to prevent leakage to annulus 40. Annular pistons 140, 142, 144, and 146 are preferably 5 sealed to prevent fluid flow between the rear and front chambers of the propulsion cylinders. The interfaces between the various housings of control assembly 102 and the flanges of shafts 118 and 124 are preferably sealed to prevent leakage. Compensation piston 248 is sealed to fluidly separate the oil in electronics housing 130 and motor housing 132 from drilling fluid in annulus 40. Various other seals are also provided 10 throughout the tractor. Suitable seals include rubber O-rings, tee seals, or specialized elastomeric seals. Suitable seal materials include AFLAS or NBR rubber.

Sensors As mentioned above, the control algorithm for controlling motorized valves 154, 15 156, and 158 is preferably based at least in part upon (1) pressure signals from pressure transducers 182, 184, 186, 188, and 190 (Figures 3 and 4A-F), (2) position signals from displacement sensors 192 and 194 (Fi res 4A-) on the annular pistons inside the aft and forward propulsion cylinders, or (3) both.

The pressure transducers measure differential pressure between the various fluid 20 passages and annulus 40. When pressure information from the above-listed pressure transducers is combined with the differential pressure across the differential pressure sub for the downhole motor, the speed can be controlled between 0.08-610m (0.25-2000 feet) per hour. That is, the tractor can maintain speeds of 0.08m (0.25 feet) per hour, 610m (2000 feet) per hour, and intermediate speeds as well. In a preferred embodiment, 25 such speeds can be maintained for as long as required and, essentially, indefinitely so long as the tractor does not encounter an obstruction which will not permit the tractor to move at such speeds. Differential pressure information is especially useful for control of relatively higher speeds such as those used while tripping into and out of a borehole (80-305rn per hour) (250-1000 feet per hour), fast controlled drilling (1.5-46m per hour) 30 (5-150 feet per hour), and short trips (9-305m per hour) (30-1000 feet per hour). The EST can sustain speeds within all of these ranges. Suitable pressure transducers for the

EST are Product No. 095A20iA, manufactured and sold by Industrial Sensors and Instruments Incorporated, located in Roundrock, Texas. These pressure transducers are rated for lxlO8Nm 2 (15000 psi) operating pressure and 1. 7 x 107Nm 2 (2500 psid) differential pressure. 5 The position of the annular pistons of the propulsion cylinders can be

measured using any of a variety of suitable sensors, including Hall Effect transducers, MIDIM (mirror image differential induction-amplitude magnetometer, sold by Dinsmore Instrument Co., Flint, MI) devices, conventional magnetometers, Wiegand sensors, and other magnetic and distance-sensitive devices. If magnetic displacement sensors are 10 used, then the components of the EDST are preferably constructed of nonmagnetic materials which will not interfere with sensor pefionnance. Suitable materials are CuBe and Stabaloy. Magnetic materials can be used if non-magnetic sensors are utilized.

For example, displacement of aft piston 142 can be measured by locating a MIDIM in connector 122 and a small magnetic source in piston 142. The MIDIM 15 transmits an electrical signal to logic component 224 which is inversely proportional to the distance between the NIIDIM and the magnetic source. As piston 142 moves toward the lMIDIM, the signal increases, thus providing an indication of the relative longitudinal positions of piston 142 and the MIDIM. Of course, this provides an indication of the relative longitudinal positions of aft pacl erfoot 104 and the tractor 20 body, i.e., the shafts and control assembly 102. In addition, displacement information is easily converted into speed information by measuring displacement at different time intervals. Another type of displacement sensor which can be used is a Wiegand sensor. In one embodiment, a wheel is provided on one of the annular pistons in a manner such 25 that the wheel rotates as the piston moves axially within one of the propulsion cylinders.

The wheel includes two small oppositely charged magnets positioned on opposite sides of the wheel's outer circumference. In other words, the magnets are separated by 180 .

The Wiegand sensor senses reversals in polarity of the two magnets, which occurs every time the wheel rotates 1 SO . For every reversal in polarity, the sensor sends an electric 30 pulse signal to logic component 224. When piston 142 moves axially within cylinder 110, causing the wheel to rotate, the Wiegand sensor transmits a stream of electric

N pulses for every 180 rotation of the wneei. The position of the piston i42 with respect to the propulsion cylinder can be determined by monitoring the number of pulses and the direction of piston travel. The position can be calculated from the wheel diameter, since each pulse corresponds to one half of the wheel circumference.

5 Figures 77A-C illustrate one embodiment of a Wiegand sensor assembly. As shown, annular piston 142 includes recesses 574 and 576 in its outer surface. Recess 574 is sized and configured to receive a wheel assembly 560, shown in Figures 77A and 77B. Wheel assembly 560 comprises a piston attachment member 562, amls 564, a wheel holding member 572, axle 570, and wheel 566. Wheel 566 rotates on axle 570 10 which is received within holes 569 in wheel holding member 572. NIernbers 562 and 572 have holes for receiving arms 564. Wheel assembly 560 can be secured within recess 574 via a screw received within a hole in piston attachment member 562. Arrxls 564 are preferably somewhat flexible to bias wheel 566 against the inner surface of propulsion cylinder 110, so that the wheel rotates as piston 142 moves within cylinder 15 110. Wheel 566 has oppositely charged magnets 568 separated by 180 about the center of the wheel. Recess 576 is sized and configured to receive a Wiegand sensor 578 which senses reversals of polarity of rl a ets 568, as described above. The figures do not show the electric wires through which the electric signals flow. Preferably, the wires are twisted to prevent electrical interference from the motors or other components 20 of the EST.

Those skilled in the art will understand that the relevant displacement information can be obtained by measuring the displacement of any desired location on the EST body (shafts 118, 124, control assembly 102) with respect to each of the packerfeet 104 and 106. A convenient method is to measure the displacement of the 25 annular pistons (which are fixed to shafts 118 and 124) with respect to the propulsion cylinders or connectors (which are fixed with respect to the packerfeet). In one embodiment, the displacement of piston 142 is measured with respect to connector 122.

Alternatively, the displacement of piston 142 can be measured with respect to an internal wall of propulsion cylinder 110 or to control assembly 102. The same 30 information is obtained by measuring the displacement of piston 140. Those skilled in the art will understand that it is sufficient to measure the position of only one of pistons

i40 and 142, and only one of pistons i44 and i4o, relative to paclcerreel 10- and luo, respectively. Electronics Configuration 5 Figures 69 illustrates one embodiment of the electronic configuration of the EST. All of the wires shown reside within wire passages described above. As shown, five wires extend uphole to the surface, including two 30 volt power wires 502, art RS 232 bus wire 504, and two 1553 bus wires 506 (MILSTD-1553). Wires 502 provide power to the EST for controlling the motors, and electrically communicate with a 10-pin 10 connector that plugs into electronics package 224 of electronics housing 130. Wire 504 also communicates with electronics package 224. Desired EST parameters, such as speed, thrust, position, etc., may be sent from the surface to the EST via wire 504.

Wires 506 transmit signals downhole to the bottom hole assembly. Commands can be sent from the surface to the bottom hole assembly via wires 506, such as commands to 15 the motor controlling the drill bit.

A pair of wires 508 permits electrical communication between electronics package 224 and the aft displacement sensor, such as a Wiegand sensor as shown.

Similarly, a pair of wires 510 permits communication between package 224 and the forward displacement sensor as well. Wires 508 and 510 transmit position signals from 20 the sensors to package 224. Another RS 232 bus 512 extends from package 224 downhole to communicate with the bottom hole assembly. Wire 512 transmits signals from downhole sensors, such as weight on bit and differential pressure across the drill bit, to package 224. Another pair of 30 volt wires 514 extend from package 224 downhole to communicate with and provide power to the bottom hole assembly.

25 A 29-pin connector 213 is provided for communication between electronics package 224 and the motors and pressure transducers of control assembly 102. The signals from the five pressure transducers may be calibrated by calibration resistors 515.

Alternatively, the calibration resistors may be omitted. Wires 516 and 518 and wire pairs 520, 522, 524, 526, and 528 are provided for reading electronic pressure signals 30 from the pressure transducers, in a manner known in the art. Wires 516 and 518 extend to each of the resistors 515, each of which is connected via four wires to one pressure

transducer. Mire pairs 52u, 5Z:z, 5z,:26, and 528 extend to the resistors; 15 and pressure transducers.

Wire foursomes 530, 532, and 534 extend to motors 164, 162, and 160, respectively, which are controlled in a manner known to those skilled in the art. Three 5 wires 536 and a wire 538 extend to the rotary accelerometers 531 of the motors for transmitting motor feedback to electronics package 224 in a manner known to those skilled in the art. In particular, each wire 536 extends to one accelerometer, for a positive signal. Wire 538 is a common ground and is connected to all of the accelerometers. In an alternative embodiment, potentiometers may be provided in place 10 of the rotary accelerometers. Note that potentiometers measure the rotary displacement of the motor output.

EST Performance A particular advantage of the EST is that it can sustain both high and low 15 speeds. Thus, the EST can be used for a variety of different activities, such as drilling, milling into a casing, tripping into a hole, and tagging bottom (all described below).

The EST can sustain any speed preferably within a range of 0.08-510m (Q. 25-200Q feet) per hour, more preferably within a range of 3-230m (10-750 feet) per hour, and even more preferably within a range of 11-213m (35700 feet) per hour. More importantly, 20 the EST can sustain both fast and slow speeds, desirably less than 0.08m (0.25 feet) per hour and more than 610m (2000 feet) per hour. The table below lists pairs of speeds, wherein a single EST or a "string" of connected ESTs (any number of which may be operating) can desirably sustain speeds less than the smaller speed of the pair and can desirably sustain speeds greater than the larger speed of the pair.

Less than Greater than Metres per hour/feet per hour Metres per hour/feet per hour 0.08 0.25 610 2000

0.08 0.25 230 750

0.08 0.25 76 250

0.08 0.25 46 150

l 0.05 0.20 3u 100 0.08 0.25 23 75

0.08 0.25 15 50

0.15 0.5 23 75

0.6 2 460 1500

0.6 2 610 2000

4.6 15 23 75

4.6 15 30 100

0.8 25 23 75

0.8 25 30 100

1.5 5 30 100

1.5 5 76 250

1.5 5 150 500

1.5 5 230 750

1.5 5 460 1500

1.5 5 610 2000

3 10 30 100

3 10 38 125

3 10 76 250

3 10 150 500

3 10 230 750

3 10 460 1500

3 10 610 2000

9 30 30 100

9 30 76 250

9 30 230 750

9 30 460 1500

9 30 610 2000

15 50 30 100

15 50 76 250

15 50 150 500

l5 flu 230 7 0 15 50 460 1500

15 50 610 2000

Movement of a tractor into and out of an open hole (non-cased section) at high speeds is referred to in the art as "tripping" into the hole. Tripping speeds tend to have a significant effect on the overall costs of the drill process. Faster speeds result in less 5 operational time and less costs. Tripping speeds generally depend upon the amount of load that the tractor carries. The higher the load, the slower the maximum speed of the tractor. For example, one embodiment of an EST has a diameter of 8. 6cm (3.375 inches) and, while carrying a 4050kg (9,000 pound) load, can travel up to speeds preferably within a range of 0-55m (0-180 feet) per hour, and more preferably within a 10 range of 37-46m (120-150 feet) per hour. While carrying a 1665kg (3,700 pound) load the same EST can travel up to speeds preferably within a range of 137-175m (450-575 feet) per hour, and more preferably within a range of 152-160m (500-525 feet) per hour.

These speeds constitute a significant improvement over prior art tractors.

As mentioned abo v e, a string of multiple tractors can be cormected end to end to 15 provide greater overall capability. For example, one tractor may be more suited for tripping, another for drilling, and another for milling. Any number and combination of tractors may be provided. Any number of the tractors may be operating, while others are deactivated. In one embodiment, a set of tractors includes a first tractor configured to move at speeds within 180-61 Om (600-2000 feet) per hour, a second tractor 20 configured to move at speeds within 3-76m (10-250 feet)- per hour, and a third tractor configured to move at speeds within 0.3-3m (1- lO feet) per hour. On the other hand, by providing multiple processors or a processor capable of processing the motors in parallel, a single tractor of the illustrated EST can move at speeds roughly between 3 230m (10-750 feet) per hour.

25 Figure 70 shows the speed performance envelope, as a function of load, of one embodiment of the EST, having a diameter of 8.6cm (3.375 inches). Curve B indicates the performance limits imposed by failsafe valve 150, and curve A indicates the performance limits imposed by relief valve 152. Failsafe valve 150 sets a minimum

supply pressure, and hence speed, for tractor operation. Relief valve 1 2 sets a maximum supply pressure, and hence speed.

The EST is capable of moving continuously, due to having independently controllable propulsion cylinders and independently inflatable packerfeet.

5 When drilling a hole, it is desirable to drill continuously as opposed to periodically. Continuous drilling increases bit life and maximizes drilling penetration rates, thus lower drilling costs. It is also desirable to maintain a constant load on the bit.

However, the physical mechanics of the drilling process make it difficult to maintain a constant load on the bit. The drill string (coiled tubing) behind the tractor tends to get 10 caught against the hole wall in some portions of the well and then suddenly release, causing large fluctuations in load. Also, the bit may encounter variations in the hardness of the formation through which it is drilling. These and other factors may contribute to create a time-varying load on the tractor. Prior art tractors are not

equipped to respond effectively to such load variations, often causing the drill bit to 15 become damaged. This is partly because prior art tractors have their control systems

located at the surface. Thus, sensor signals must travel from the tool up to the surface to be processed, and CGDilG1 signals must travel frolic the surface back down to the tool.

For example, suppose a prior art drilling tool is located 4600m (15,000 feet)

underground. While drilling, the tool may encounter a load variation due to a downhole 20 obstruction such as a hard rock. In order to prevent damage to the drill bit, the tool needs to reduce drilling thrust to an acceptable level or perhaps stop entirely. With the tool control system at the surface, the time required for the tool to communicate the load variation to the control system and for the control system to process the load variation and transmit tool command signals back to the tool would likely be too long to prevent 25 damage to the drill bit.

In contrast, the unique design of the EST permits the tractor to respond very quickly to load variations. This is partly because the EST includes electronic logic components on the tool instead of at the surface, reducing communication time between the logic, sensors, and valves. Thus, the feedback control loop is considerably faster 30 than in prior art tools. The EST can respond to a change of weight on the bit of 45kg

(100 pounds) preferably within 2 seconds, more preferably within 1 second, even more

/ preferably within 0.5 seconds, even more preferably within 0.2 seconds, and most preferably within 0 1 seconds. That is, the weight on the drill bit can preferably be changed at a rate of 45kg (100 pounds) within 0.1 seconds. If that change is insufficient, the EST can continue to change the weight on the bit at a rate of 100 5 pounds per 0.1 seconds until a desired control setting is achieved (the differential pressure from the drilling motor is reduced, thus preventing a motor stall). For example, if the weight on the drill bit suddenly surges from 900kg (2000 lbs) to 1350kg (3000 lbs) due to external conditions, the EST can compensate, i.e. reduce the load on the bit from 1350kg (3000 lbs) to 900kg (2000 lbs), in one second 10 Typically, the drilling process involves placing casings in boreholes It is often desirable to mill a hole in the casing to initiate a borehole having a horizontal component. It is also desirable to mill at extremely slow speeds, such as 08-1.2rn (0.25-4 feet) per hour, to prevent sharp ends from forming in the milled casing which can damage drill string components or cause the string to get caught in the milled hole.

15 The unique design of propulsion valves 156 and 158 coupled with the use of displacement sensors allows a single EST to mill at speeds less than 30cm (l foot) per hour, and mote preferably as low as or even less than O.OSm (0.25 feetJ per hour. us, appropriate milling ranges for an EST are 0.08-8m (0.25-25 feet) per hour, 0.08-3m (0.25-10 feet) per hour, and 0.08-1.8m (0.25-6 feet) per hour with appropriate non-barite 20 drilling fluids.

After milling a hole in the casing, it is frequently desirable to exit the hole at a high angle turn. The EST is equipped with flexible connectors 12O, 122,126, and 128 between the packerfeet and the propulsion cylinders, and flexible shafts 118 and 124.

These components have a smaller diameter than the packerfeet, propulsion cylinders, 25 and control assembly, and are formed from a flexible material such as CuBe. Desirably, the connectors and shafts are formed from a material having a modulus of elasticity of preferably at least 2xlOiiNm-2 (29,000,000 psi), and more preferably at least 1.3xl Oi iNm 2 (19,000,000 psi). This results in higher flexibility regions of the EST that act as hinges to allow the tractor to perform high angle sums. In one embodiment, the 30 EST can turn at an angle up to 60 per 30m (100 feet) of drilled arc, and can then traverse horizontal distances of up 760015000m (25,000-50,000 feet).

The tractor design naiances such 1exibiiity against the des a'vili,y of hang relatively long propulsion cylinders and packerfeet. It is desirable to have longer propulsion cylinders so that the stroke length of the pistons is greater. The stroke length of pistons of an EST having a diameter of 8.57cm (3.375 inches) is preferably at least 5 25cm (10 inches) and preferably from 25-50cm (10-20 inches), and more preferably at least 30cm (12 inches). In other embodiments, the stroke length can be as high as 1.5m (60 inches). It is also desirable to have packerfeet of an appropriate length so that the tool can more effectively engage the inner surface of the borehole. The length of each packerfoot is preferably at least 38cm (15 inches), and more preferably at least lm (40 10 inches) depending upon design type. As the length of the propulsion cylinders and packerfeet increase, the ability of the tool to turn at high angles decreases. The EST achieves the above-described turning capability in a design in which the total length of the propulsion chambers, control assembly, and packerfeet comprises preferably at least 50% of the total length of the EST and, in other design variations, 50%- 80%, and more 15 preferably at least 80% of the total length of the EST. Despite such flexibility, a 8.57cm (3.375 inch) diameter EST is sufficiently strong to push or pull longitudinal loads preferably as high as 4725kg (10,500 pounds'.

Advantageously, one aspect of the invention is that a single EST can generate a thrust to push and/or pull various loads. The desired thrust capabilities of various sizes 20 of the EST are summarized in the following table: EST Diameter Preferred Thrust Desired Thrust inches cm pounds N pounds N 2.125 5.4 1,000 4,400 2,000 8,900

3.375 8.6 5,250 23,000 10,500 47,000

4.75 12.1 13,000 58,000 26,000 120,000

6.0 15.2 22,500 100,000 4S,000 200,000

Additionally, the EST resists torsional compliance, i.e. twisting, about its longitudinal axis. During drilling, the formation exerts a reaction torque through the 25drill bit and into the EST body. When the aft packerfoot is engaged with the borehole

and the forward packerfoot is retracted, the portion of the body forward of the aft packerfoot twists slightly. Subsequently, when the forward packerfoot becomes engaged with the borehole and the aft packerfoot is deflated, the portion of the body to the aft of the forward packerfoot tends to untwist. This causes the drill string to 5 gradually become twisted. This also causes the body to gradually rotate about its longitudinal axis. The tool direction sensors must continuously account for such rotation. Compared to prior art tractors, the EST body is advantageously configured to

significantly limit such twisting. Preferably, the shaft diameter is at least 4.4cm (1.75 inches) and the control assembly diameter is at least 8. 57cm (3.375 inches), for this 10 configuration. When such an EST is subjected to a torsional load as high as 670Nm (500 ft-lbs) about its longitudinal axis, the shafts and control assembly twist preferably less than 5 per step of the tractor. Advantageously, the above-mentioned problems are substantially prevented or minimized. Further, the EST design includes a non-rotational engagement of the packerfeet and shafts, via rotation restraints 364 (Figure 49A). This 15 prevents torque from being transferred to the drill string, which would cause the drill string to rotate. Also, the flextoe packerfeet of the EST provide improved transmission of torque to the borehole wall, via the flextoes.

When initiating further drilling at the bottom of a borehole, it is desirable to "tag bottom," before drilling. Tagging bottom involves moving at an extremely slow speed 20 when approaching the end of the borehole, and reducing the speed to zero at the moment the drill bit reaches the end of the formation. This facilitates smooth starting of the drill bit, resulting in longer bit life, fewer trips to replace the bit, and hence lower drilling costs. The EST has superior speed control and can reverse direction to allow efficient tagging of the bottom and starting the bit. Typically, the EST will move at near 25 maximum speed up to the last 1 5m (50 feet) before the bottom of the hole. Once within 15m (50 feet), the EST speed is desirably reduced to about 3.7m (12 feet) per hour until within about 3m (10 feet) of the bottom. Then the speed is reduced to minimum. The tractor is then reversed and moved backward 30- 60cm (1-2 feet), and then slowly moved forward.

30 When drilling horizontal holes, the cuttings from the bit can settle ore the bottom of the hole. Such cuttings must be periodically be swept out by circulating drilling fluid

close to the cutting beds. The EST has the capability of reversing direction and walking backward, dragging the bit whose nozzles sweep the cuttings back out.

As fluid moves through a hole, the hole wall tends to deteriorate and become larger. The EST's packerfeet are designed to traverse holes up to 10% larger than the 5 drill bit without losing traction.

Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications thereof.

10 Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.

Claims

1. tractor for moving within a borehole, comprising: an elongated body configured to pull equipment within said borehole, 5 said equipment exerting a longitudinal load on said body; a gripper longitudinally movably engaged with said body, said gripper having an actuated position in which said gripper limits movement between said gripper and an inner surface of said borehole, and a retracted position in which said gripper permits substantially free relative movement between said gripper 10 and said inner surface; and a propulsion system on said body for propelling said body through said borehole while said gripper is in said actuated position; wherein said body is suff1cientiy ftexibie such cleat said tractor can Audi up to 30 per 30m (100 feet) of travel, while said longitudinal load is at least 15 23kN (5250 pounds).

2. The tractor of Claim 1, wherein said body is sufficiently flexible such that said

tractor can turn up to 45 per 30m (100 feet) of travel, while said longitudinal load is at least 23kN (5250 pounds).

3. The tractor of Claim l, wherein said body is sufficiently flexible such that said tractor can turn up to 60 per 30m (100 feet) of travel, while said longitudinal load is at least 23kN (5250 pounds).

25 4. The tractor of Claim 1, 2 or 3, wherein said tractor has large diameter segments and small diameter segments, said large diameter segments including one or more of a valve housing having valves configured to control the flow of fluid to components of said propulsion system, a motor housing having motors configured to control said valves, an electronics housing having logic componentry configured to control said 30 motors, one or more propulsion chambers configured to receive fluid to propel said body, pistons axially movable within said propulsion chambers, and said gripper, said

large diameter segments having a diameter of at least 7.9cm (3.125 inches) , substantially all of the bending of said tractor occurring in said small diameter segments.

5. The tractor of Claim l, 2, 3 or 4, wherein said body has a diameter of at least S 2.5cm (1 inch) 6. The Factor of Claim 5, wherein said tractor has a length of at least 3m (10 feet).

7. The tractor of Claim 4, wherein said pistons are fixed to said body and movable l O along a stroke length inside said propulsion chambers, said stroke length being at least 25cm (10 inches).

8. The tractor of Claim 4 or 7, wherein the total length of said large diameter segments comprises at least 50% of the length of said tractor.

9. The tractor of any preceding Claim, wherein the flexibility of said tractor is such that said tractor can bend so as to tum up to 60 per 30m (100 feet) oftravel, while said longitudinal load is as high as 47kN (10, 500 pounds).

20 10. The tractor of any preceding Claim, wherein said body has longitudinal boreholes for transporting fluid through said body, said boreholes being gun-drilled.

11. The tractor of any preceding Claim, wherein said body comprises an outer cylinder and an inner cylinder diffusion-bonded together, one of said cylinders having 25 longitudinal interruptions so that longitudinal passages are formed between said cylinders, said longitudinal passages being configured to transport fluid through said body. 12. The tractor of Claim 4, wherein said large diameter segments include said 30 propulsion chambers and said pistons inside said propulsion chambers, said pistons being fixed to said body, each of said pistons surrounding a segment of said body and having an annular surface generally perpendicular to said body, said annular surface being configured to receive thrust from a fluid flowing to said piston, said pistons having a diameter of at least 3.8cm (1.5 inches).

13. The tractor of any preceding Claim, wherein said gripper has a length of at least 25cm (10 inches).

5 14. A tractor for moving within a borehole, comprising: an elongated body defining a longitudinal axis and being configured to transmit torque through said body, said body configured so that when said body is subjected to a torque about said longitudinal axis of as high as 680Nm (500 fit lbs), twisting of said body is limited to 5 per step of the tractor; 10 a first gripper axially movably engaged with said body, said first gripper having an actuated position in which said first gripper limits movement of said first gripper relative to an inner surface of said borehole and a retracted position in which said nrst gripper permits substantially Wee relative rrlovernent be w-een said first gripper and said inner surface, said first gripper being rotationally fixed 15 with respect to said body so that said first gripper resists rotation of said body with respect to said borehole when said first gripper is in said actuated position; a second gripper axially movably engaged with said body, said second gripper having an actuated position in which said second gripper limits movement of said second gripper relative to an inner surface of said borehole 20 and a retracted position in which said second gripper permits substantially free relative movement between said second gripper and said inner surface, said second gripper being rotationally fixed with respect to said body so that said second gripper resists rotation of said body with respect to said borehole when said second gripper is in said actuated position; and 25 a propulsion system on said body, for propelling said body when at least one of said grippers is in said actuated position; wherein said body is sufficiently flexible such that said tractor can turn up to 60 per 30m (100 feet) oftravel.

30 15. The tractor of Claim 14, said tractor having large diameter segments and small diameter segments, said large diameter segments including one or more of a valve housing having valves configured to control the flow of fluid to components of said

propulsion system, a motor housing having motors configured to control said valves, an electronics housing having logic componentry configured to control said motors, one or more propulsion chambers configured to receive fluid to propel said body, pistons axially movable N;vithin said propulsion chambers, and said grippers, said large diameter 5 segments having a diameter of at least 7.9cm (3.125 inches), said small diameter segments having a diameter of 5.2cm (2.05 inches) or less, substantially all of the bending of said tractor occurring in said small diameter segments.

16. The tractor of Claim 14 or 15, wherein said body has a diameter of at least 10 2.5cm (1 inch).

17. The tractor of Claim 14, 15 or 16, wherein said grippers are configured to transmit a torque of up to 680Nm (500 h-lbs) to said borehole.

15 18. The tractor of Claim 14, 15, 16 or 17, wherein each of said grippers comprises: a generally tubular mandrel concentrically slidably engaged on said body, said mandrel having one or more longitudinal grooves engaged on splines on the outer surface of said body so that said mandrel is prevented from rotating with respect to said body; and 20 an inflatable bladder fixed to said mandrel.

l9. The tractor of Claim 18, wherein each of said grippers further comprises one or more flexible beams extending longitudinally across said bladder, said beam or beams configured to flex and grip said inner surface of said borehole when said bladder is 25 inflated, said beam or beams being configured to transmit torque from said body to said inner surface of said borehole.

20. A tractor according to Claim 1, substantially herein described with reference to and as illustrated in the accompanying drawings.