GB2316427A - Intermediate radius steerable tool - Google Patents

Abstract

A single jointed rotatable and steerable drilling tool (10) having a flexible motor stator housing (21), and optionally a flexible bearing housing (19). The flexible portions can have a reduced outside diameter or increased inside diameter, and they can be made from a relatively high yield strength, low modulus of elasticity material. Also disclosed is a method for using the tool (10) to drill all sections of a deviated wellbore from kickoff to the desired inclination angle in a target reservoir. The tool (10) has but one angle (A), such as an angled sub (14), making it rotatable to drill a straight hole and to increase the radius of curvature of a curved hole. The tool (10) also has a fluid driven motor (12), making it operable in the sliding mode of operation for drilling a short radius of curvature. The flexibility enables the drilling tool (10) to be used around tighter curves, and the rotatability enables the tool (10) to drill in both sliding and rotating drilling modes of operation to complete all sections of the lateral wellbore with a single tool.

Description

INTERERMEDIATE RADIUMS STEERABLE TOOL This invention is in the field of equipment and methods for drilling a wellbore deviated from an original wellbore. Specifically, the present invention is in the field of steerable drilling tools, and their use for drilling a wellbore deviated to a desired angle of inclination.

Most oil or gas wells have been drilled essentially vertically into the earth to reach a subterranean reservoir of hydrocarbons. Many, if not most, subterranean reservoirs are disposed more or less horizontally, having a limited thickness but covering a large area.

Very often, an original wellbore passing through a reservoir only has a limited portion of its length situated in the reservoir. This limits the production capacity of the well, and it can even mean that the overall percentage of hydrocarbon recoverable from the reservoir is limited, unless numerous wells are drilled into the reservoir. The drilling of numerous wells significantly increases the cost of producing the hydrocarbons for consumption.

It has become increasingly economically beneficial to drill more or less horizontal lateral welibores extending outwardly from an original wellbore, in order to position a greater length of the wellbore in the reservoir, and to therefore increase the production capacity of the well. This in turn reduces the need for drilling numerous welibores from the surface into the reservoir. A single original wellbore can have several lateral wellbores extending or emanating in various directions, thereby draining a much larger portion of the reservoir. Further, the lateral wellbores extending from a single original wellbore can be placed at various depths to reach several different reservoirs found at different depths.

Equipment and methods are currently available for deviating from an original wellbore to drill a lateral wellbore, at a selected depth, and extending in a selected direction. One factor which limits the economic benefit of drilling lateral welibores is that currently known equipment and methods require the repeated pulling of one drilling tool from the wellbore to replace it with another tool to accomplish the drilling of another section of the wellbore. This happens because some currently known tools are suited for drilling the initial kickoff section from the original wellbore, others are better suited for more rapidly building inclination angle after kickoff and drilling a curved section having the desired radius, to reach the desired inclination angle, still others are better suited for gradually transitioning from the curved section to achieve a "soft landing" at the desired depth with a horizontal wellbore, while still others are better suited for drilling the horizontal section of the lateral wellbore and maintaining a desired depth or a desired dip angle.

The curved section can have a radius that is considered a "long" radius, "medium" radius, "intermediate" radius, or "short" radius, listed in order of declining radius. Radius of the curve is usually given in meters or feet. The definition of a "long", "medium", "intermediate" or "short" radius depends upon the type and diameter of drill pipe used in the wellbore. A "long" radius curve is said to have a low Build Up Rate (BUR), degrees of inclination per 100 feet of hole. Conversely, a "short" radius curve is said to have a high BUR.

The type of tool best suited for performing the kickoff from an original wellbore is a so-called "stiff' tool, differentiating it from an articulated tool having several swivel joints. A typical "stiff' tool has a fluid driven motor, through which drilling fluid is pumped to rotate the drill bit without rotation of the drill string. This is called the "sliding" mode of operation. The "stiff' tool usually has either one or two angled subs, set at predetermined angles, to orient the axis of the drill bit at an angle relative to the longitudinal axis of the drill string. Angled subs can be located above the drill motor, below the drill motor, or both. Sometimes the angle of the angled sub is permanent, and sometimes it is adjustable. If only one angled sub is used, the drilling tool can be thought of as a "single jointed" tool, while if two angled subs are used, the drilling tool can be thought of as a "double jointed" tool.

This "stiff' tool is lowered into the original wellbore, elastically flexed into a slightly straightened condition, causing the drill bit to press against the side of the original wellbore, and placed under compressive stress by the weight of drill pipe above.. When the drill bit is rotated by the drill motor, the tool cuts out of the original wellbore to "kick off' from the original bore, returning the tool to its predetermined curvature. The "stiff' tool can then be used to drill further, by building angle of inclination, but because of its stiffness and its set angle, it is not always the best choice for building angle at the desired Build Up Rate (BUR).

It is often necessary to build angle quickly, for instance, if the target reservoir lies immediately under another stratum of relatively hard material. Where the desired BUR is between (1.856 x pipe yield strength -: pipe diameter) and (3.052 x pipe yield strength - pipe diameter), this is often called a "short radius" section, with BUR being calculated in degrees per 100 ft. of hole, pipe yield strength given in kpsi, and pipe diameter given in nominal inches. Pipe bending stresses in a "short radius" section typically range from about 53 kpsi to about 87 kpsi. Rotation of the pipe in a short radius section is not allowed, since fatigue failure would occur within tens or at least hundreds of rotations, or in other words, within minutes or certainly less than an hour.

Where the desired BUR is between (0.878 x pipe yield strength - pipe diameter) and (1.489 x pipe yield strength -: pipe diameter), this is often called an "intermediate radius" section, where, as above, BUR is calculated in degrees per 100 ft. of hole, pipe yield strength is given in kpsi, and pipe diameter is nominal inches. Pipe bending stresses in an "intermediate radius" section typically range from about 25 kpsi to about 43 kpsi.

Fatigue damage of the drill pipe will accumulate in an intermediate radius section of borehole, if the pipe is rotated. However, this cumulative fatigue damage can be managed through effective planning, to insure that failure of the drill pipe will not occur during drilling of a given well. When operating in the rotating mode of operation, the number of revolutions of a given joint of pipe in an intermediate radius section of hole can be tracked, in order to insure that the joint of pipe is taken out of service before a fatigue failure is likely to occur. API specifications currently do not support the rotation of pipe in short or intermediate radius holes.

An articulated drilling tool is better suited than a stiff tool for quickly building angle, with several swivel or universal joints making the body of the tool better able to pass through a tight curve. The articulated tool can not be rotated with the drill string, but it has a fluid driven motor for rotating the bit. So, the "stiff' tool is often pulled out of the wellbore and replaced with the articulated tool to drill an intermediate radius curved section of the lateral wellbore. This is the second trip in and out of the hole required for the drilling of the lateral wellbore.

Then, when the angle of inclination has nearly reached the desired angle, it is often necessary to transition into the final angle with a section of wellbore which has a larger radius of curvature, to allow the subsequent use of a longer, stiffer tool. The articulated tool does not have the capability to selectively drill a larger radius curve, since it is not rotatable with the drill string. Similarly, since it is less subject to orientation control, the articulated tool is not as reliable in changing azimuth. To drill this transition section and achieve a so-called "soft landing", a third tool is often used. The second, articulated, tool is pulled out of the hole and replaced with the third tool. This third tool is usually another "stiff' tool designed for a slightly larger radius than the articulated tool. This is the third trip in and out of the hole required for the drilling of a lateral wellbore incorporating an intermediate radius section.

Finally, it is often desired to continue drilling the lateral wellbore, relatively straight for some distance, at the desired angle of inclination. This requires the use of a steerable "stiff' type of tool having only a single angled sub and therefore being capable of rotation along with the drill string. Where the hole includes no intermediate radius sections, this type of drilling tool can be selectively rotated along with the drill string to drill a relatively straight hole, in the "rotating" mode. Also, the downhole drill motor can be operated, without rotation of the drill string, to drill a curved hole, in the "sliding" mode. This selective use of the two modes of drilling is used to keep the tool in the target reservoir and to change azimuth angle as desired. Unfortunately, however, use of this fourth tool requires the third "soft landing" tool to be pulled out of the hole, followed by running the steerable tool into the hole. This is the fourth trip in and out of the hole that may be required for the drilling of the lateral wellbore. With currently known equipment, the rotating mode of operation can not be accomplished with the drilling tool in an intermediate radius section of borehole.

Each trip in and out of the hole uses valuable rig time and adds considerably to the cost of drilling the well. It is an object of the present invention to provide a method and apparatus which will allow the drilling of all four sections of a lateral wellbore with a single drilling tool, even where the hole includes an intermediate radius section.

The present invention is a "single jointed" drilling tool having a drill motor and only a single angled sub, wherein both components can be built from materials having a relatively high yield strength, but a relatively low modulus of elasticity. Such a material is titanium, specifically Beta-C titanium, having a yield strength of 125,000 to 170,000 psi, and a modulus of elasticity of no greater than 17,000,000 psi. Another example of such a material is copper beryllium. Both of these materials are non-magnetic, thereby facilitating the use of magnetic sensing instrumentation. Reference is made herein to an "angled sub" incorporated in the drilling tool, but it should be understood that this term is intended to include a tool in which the angle is built into the stator housing or the bearing housing, rather than constituting a separate component threaded to the other components.

It has been found beneficial to build the stator housing of titanium, and the bearing housing or other mating parts of copper beryllium, to prevent galling of the threads.

Further, it has been found helpful to make the drilling tool more flexible by reducing the outer diameter, or increasing the inner diameter, and reducing the cross section of both the stator housing and the bearing housing. More specifically, it is helpful to make the outer diameter of the drilling tool, over a significant portion of its length, substantially equal to, or less than the outer diameter of the drill pipe with which the drilling tool will be used. Similarly, it is helpful to make the outer diameter of the joints of the drilling tool substantially equal to, or less than the outer diameter of the tool joints on the drill pipe with which the drilling tool will be used. This is true of either the stator housing or the bearing housing section of the drilling tool. Flexibility can be further enhanced by utilizing shorter, higher strength, thread forms in the connectors. Where possible, the length of the bearing stack within the bearing housing can be minimized, to enhance the flexibility of the remainder of the length of the bearing housing, thereby making a given length of bearing housing as flexible as possible.

Still further, it has been found helpful to shorten the overall length of the tool by such means as placing the angled joint in the stator housing or bearing housing of the motor, rather than using a separate angled sub. A rotatable low side wear pad or wear ring, of an abrasion resistant material such as tungsten carbide, can be mounted to the tool at or near the angle, to improve the durability of the tool during rotation of the drill string.

The angle and the wear ring will be placed under bending stress and side force by the forces imposed upon the tool as it passes through the borehole. The bending stress and side force can be reduced by making the stator housing and bearing housing more flexible via the methods listed above.

There is only one angle of the aforementioned predetermined magnitude built into the drilling tool, making the drilling tool suitable for rotation along with the drill string, as well as providing rotation of the drill bit by the drill motor. This is superior to a "double jointed" tool, because double jointed tools can not be typically rotated with the drill string. The present invention is also superior to an articulated tool, because articulated tools can not be rotated with the drill string, without losing control of the tool heading or damaging the tool from wobbling in the hole. There can be a knuckle joint, with a maximum angle more or less than the angle of the angled sub, above the drilling tool, merely to act as a "moment disconnect" or "moment release", allowing the top end of the drilling tool to more closely follow the outer radius of a curved section when weight is placed on the drill bit, rather than being forced against the inner radius of the hole. This allows drilling of a tighter radius hole than is otherwise possible, and it reduces the bending stress on the tool.

Being a "stiff' type of tool, the tool of the present invention is suitable for accomplishing the kickoff from the original wellbore, using the sliding mode of drilling operation. Second, since the tool has greater flexibility than the typical "stiff' type of drilling tool, because of the low modulus of elasticity and the reduced cross sections, the present tool is also suitable for rotation during drilling of a curved section having an intermediate radius. Third, since the tool is a "stiff' type of tool, it can be selectively and intermittently operated in the rotating mode of operation and in the sliding mode of operation to achieve a slower BUR, and a longer radius curve, to accomplish the "soft landing" at the desired angle of inclination. Finally, since the tool is operable in both the sliding mode of operation and in the rotating mode of operation, it is steerable enough to drill an appreciable amount of the lateral section while holding the desired inclination angle, or to turn the hole to a selected azimuth angle.

Therefore, the tool of the present invention can be used to drill all four sections of the lateral wellbore, even when an intermediate radius section is included, thereby saving at least three trips in and out of the wellbore.

The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, given by way of example only, in which similar reference characters refer to similar parts, and in which: Figure 1 is a schematic of a typical well profile with an original bore and a lateral bore; Figure 2 is a graph of angle of inclination versus drilled depth; Figure 3 is an elevation view of a single jointed drilling tool according to the present invention; Figure 4 is an elevation view of a second embodiment of a single jointed drilling tool according to the present invention, with the angled sub being of the double tilted type; Figure 5 is an elevation view of the single jointed drilling tool of Figure 3 shown in an original wellbore; Figure 6 is an elevation view of the tool of Figure 3 shown at the kickoff point; Figure 7 is an elevation view of the tool of Figure 3 shown drilling in the sliding mode of operation; Figure 8 is an elevation view of the tool of Figure 3 shown drilling in the rotating mode of operation; Figure 9 is an elevation view of the tool of Figure 3 shown kicking off from an original wellbore; Figure 10 is an elevation view of the tool of Figure 9, drilling the high build rate section of the lateral wellbore; Figure 11 is an elevation view of the tool of Figure 9, drilling the soft landing section of the lateral wellbore; and Figure 12 is an elevation view of the tool of Figure 9, drilling the lateral bore at a desired angle of inclination.

Figure 1 shows a typical well site 1, where a drilling rig 2 has been placed and used to drill a bore hole 3. The typical wellbore with a lateral bore begins by drilling an initial section 5, which is often lined with a casing and with cement 4. It may be desired to pass quickly through a first upper stratum 6 in the formation, and then to produce hydrocarbons from a second lower stratum 7. At a selected depth, a kickoff section 8 is drilled to exit the original wellbore 5. When using currently known equipment, the kickoff section 8 is drilled with a "stiff" drilling tool, without swivel joints, but with one or more angled subs and a fluid driven drill motor, as is known in the art. The stiff tool is elastically flexed slightly to fit through the original section 5, and its reaction to this flexing gives it the force to kickoff the wellbore. Once kickoff has been achieved, the stiff tool returns to its normal curvature, and it can drill a curved hole.

Often, the currently known stiff tool can not drill a short enough radius hole to land in the target reservoir at the lower stratum 7, so the stiff tool is pulled out of the hole, and an articulated tool, with several swivel joints, is lowered into the hole. The articulated tool is used to drill a higher build rate section 9, wherein the angle of inclination more rapidly approaches the desired target angle, which in the well shown is approximately 90 degrees. When the angle of inclination approaches the desired angle, say at approximately 85 degrees, the articulated tool is typically pulled out of the hole, because it is not capable of building angle at a lower, controlled rate to achieve a soft landing at the desired angle and the desired depth.

Another stiff tool, as currently known in the art, is lowered into the hole and used to drill the soft landing section 11 to transition from the higher build rate section to the desired inclination angle. The tool used for the soft landing section 11 is typically a stiff type of tool designed for a longer radius than the articulated tool, but it has a relatively short length, to allow operation of the drill motor in the curved section of the bore hole.

This short length drilling tool is usually not best suited for drilling straight hole, so it is often pulled out of the hole when the soft landing section 11 is finished.

Then, a fourth drilling tool is often lowered into the hole, this one being a relatively long single jointed steerable tool capable of drilling a substantially straight hole by being rotated with the drill string, called the rotating mode of drilling. This tool is also capable of drilling a curved hole by operation of the drill motor, without rotation of the drill string, called the sliding mode of drilling. The rotating mode can also involve operation of the drill motor to achieve the desired bit speed and rate of penetration. The two modes of drilling can be used in an alternating fashion, to keep the tool in the target reservoir, or to change azimuth.

It should be understood that the typical practice described above for tools currently known in the art can have many variations. Many different types of tools are obviously available, requiring variations in the procedure described. Some currently known rotatable tools can theoretically be used to build angle at differing rates, by repeatedly switching from the rotating mode to the sliding mode, for example. Of the currently known tools which are capable of operating in both modes, the actual stiffness of the tool severely limits the range of the radius which can be achieved without excessive switching from the rotating mode to the sliding mode, and it severely limits the radius through which the tool can pass. This adds to the overall cost of drilling the well.

Figure 2 illustrates the performance of these different currently known tools by showing a plot of the inclination angle versus the drilled depth of the bore hole. The inclination stays relatively constant and small, in the initial section 5 of the bore hole.

Then, using the first "stiff' tool with an angled sub, the inclination angle begins to build gradually as the drill bit makes the kickoff section 8. Atter kickoff, the articulated second tool is often used to achieve a higher build rate, as the inclination angle increases sharply at a constant rate, in the higher build section 9 of the bore hole. As the target angle is approached, the "stiff' type third tool is used to transition from the higher build rate to the selected inclination angle, in the soft landing section 11. Finally, the steerable fourth tool is used to drill the horizontal section 13. The tool and method of the present invention will drill the same profile shown in Figure 1, and the plot of angle versus depth will be the same as shown in Figure 2, but only one tool will be required, instead of four.

Figure 3 shows the drilling tool 10 of the present invention, which is a single jointed "stiff' type of rotatable and steerable tool. A fluid driven drilling motor 12 is attachable at its upper end to a drill string. The lower end of the stator housing 21 of the drill motor 12 is coupled to only a single angled sub 14, which is coupled to a drill bit 16, via a bearing housing 19. The bearing housing 19 houses a drive shaft from the motor 12 to the drill bit 16, and a bearing stack. Rather than utilizing a separate angled sub, the angle can be incorporated into the lower end of the stator housing 21 and the upper end of the bearing housing 19, by having an angled box in the stator housing 21 and an angled pin in the bearing housing 19. This helps to reduce the overall length of the drilling tool.

The amount of angle is determined by the degree of makeup of the box thread and the pin thread, with the necessary shims 13 being placed between the box and pin to set the degree of makeup.

A similar, but thicker shim is also placed between the box and pin, in the form of a "low side" wear pad or ring 15. This wear ring 15 resists the wear caused by rubbing against the bore hole wall during rotation of the drill string. It is placed in this connector, because the angle tends to rub against the bore hole wall more than any other location.

This wear ring 15 can have abrasion resistant material such as tungsten carbide built into its wear surface, for instance, in the form of inserts. If the wear ring 15 becomes too worn on the "low side", the threads can be loosened, and the wear ring 15 rotated to present a fresh low side wear resistant surface.

The term "single jointed tool" refers to the fact that the tool 10 has but one predetermined angle, such as the angled sub 14. An upper stabilizer 18 can be mounted to the upper end often stator housing 21 of the drill motor 12, and a lower stabilizer 20 can be mounted to the bearing housing 19 below the angle or below the angled sub 14.

The angled sub 14 exhibits a single predetermined bend, or angle A between the longitudinal axis ofthe drill motor 12 and the longitudinal axis ofthe drill bit 16.

Figure 3 also illustrates that the stator housing 21 ofthe drill motor 12 can have a reduced diameter to improve flexibility. Since the stiffness of the stator housing 21 is proportional to the fourth power of its outside diameter, a reduction in the outside diameter brings about an increase in the flexibility of the stator housing 21. This helps to concentrate the flexing of the drilling tool in the area of the stator housing 21, thereby reducing the stress on other portions of the tool, such as the connectors which can also be built with a reduced outside diameter, as compared to conventional tools. The connectors can also be designed with high strength thread forms to resist any bending stress that results from flexing of the tool. Further, utilization of short threads in the connectors can reduce the relatively stiffer length of the tool, and reduce the overall length of the tool, both of which facilitate the rotation of the tool in a curved section of bore hole.

Drilling of a curved section with a tool according to the present invention can be enhanced by use of a relatively small diameter, therefore flexible, Measure While Drilling (MWD) tool. Where MWD probe stabilizers are used, they should be a clamp-on type rubber stabilizer, rather than a bow spring stabilizer. Bow spring stabilizers would have a tendency to vibrate excessively while passing through the curved section of the hole, while rubber stabilizers will not.

Figure 4 shows a second embodiment of the single jointed tool 10' of the present invention, this one still having but one predetermined angle at sub 14'. In this embodiment, however, the angled sub 14' exhibits a double bend. The first bent segment angles in a first direction relative to the axis of the drill motor 12. The second bent segment angles in the opposite direction, leaving the axis of the drill bit 16 at an angle A equivalent to the angle A achieved by the single bend seen in Figure 3. The advantage of the double bend or "double tilted" type of angled sub is that the tool will drill a hole with a radius of curvature equal to the radius achievable by the tool of Figure 3, but the "double tilted" tool will make a smaller straight hole when the drilling tool 10' is rotated by the drill string. Either embodiment can have a permanent bend, or the predetermined angle A can be adjustable, as is known in the art, with adjustable kickoff tools. It should be understood any of the tools according to the present invention could utilize the reduced diameter as shown in Figure 3 to promote flexibility, as well as the rotatable wear ring 15, the built-in angle sub, and the shims 13.

Either of these embodiments of the tool of the present invention has advantages over currently known tools. Tools somewhat similar in appearance to those shown are currently known, with one important difference which is not evident from the Figures.

That is, in addition to utilizing a reduced diameter, or in the alternative to utilizing a reduced diameter, the tool of the present invention can be made more flexible than the currently known tools of the same type, by the use of a material having a high yield strength, and a low modulus of elasticity.

Although generically referred to as a "stiff' type of tool, the tool of the present invention is more flexible than currently known single jointed tools. This can be achieved in two ways. First, the drill motor 12 stator housing 21 and the bearing housing 19 can have a reduced outer diameter, or an increased inner diameter. Second, they can be constructed of a material which has a high yield strength similar to the yield strength of drill pipe steel, but which has a low modulus of elasticity approximately half the modulus of drill pipe steel. The yield strength is at least 125,000 psi, and preferably 170,000 psi.

The modulus of elasticity is no greater than 17,000,000 psi, and preferably 15,000,000 psi. An example of a material having these properties is Beta-C Titanium, with the cold worked variety having the higher strength rating. Another example, which also can be threaded to the Titanium components without galling, is Copper Beryllium. By comparison, S-135 high strength drill pipe steel has a yield strength of 135,000 psi, and a modulus of elasticity of 30,000,000 psi.

This means that the drilling tool of the present invention will flex elastically, without entering the plastic range, to a stress level comparable to, and preferably higher than a tool made from a material like the S-135 steel. Also, the drilling tool of the present invention will exhibit a given level of strain, or flexing, at a stress level which is approximately half that of a drilling tool made from a material like the S-135 steel. In other words, while the tool of the present invention will withstand just as much stress as conventional tools, it will flex much more easily and still return to its original shape.

A non-rotatable drilling tool made of this high yield, low modulus material is known, as evidenced by U.S. Pat. No. 5,368,109 to Pittard. However, the tool disclosed by the '109 patent is different from the present invention in one major respect, in that it is a double jointed tool, having a predetermined angle in a sub above the drill motor and another predetermined angle in a sub below the drill motor. Therefore, this currently known tool can not be rotated with the drill string and can not drill a straight hole. Th angle A, coupled with the lengthening of other segments of the tool. As shown in Figure 7, the tool 10 will drill a shorter radius curve in the high build rate section 9 of the bore hole than will conventional "stiff' tools, because the conventional tools are indeed less flexible than the tool 10. This is the sliding mode of drilling, in which the drill string is not rotated. The lack of flexibility of the conventional tool causes it to drill a longer radius curve for a given angle A, simply because the housing of the tool will not bend around a shorter radius curve as readily as will the tool 10 of the present invention. The tool disclosed in the '109 patent will also drill a short radius curve in the high build rate section 9, because of its flexible material of construction, and because it is used in a sliding mode.

Figure 8 shows the tool 10 drilling a straight hole during rotation of the drill string, with the diameter of the hole being determined by the predetermined angle A of the tool, the placement of the stabilizers, segment lengths and formation properties. This is the rotating mode of drilling, so the tool shown in the '109 patent can not be used to drill straight hole as shown here. When it is desired to use the tool 10 to drill a curve having a larger radius of curvature than that which results from the sliding mode of drilling, the tool 10 is simply operated intermittently in the rotating mode, to achieve the desired radius of curvature. Downhole instrumentation is used to monitor the inclination and azimuth. It is this transition from the shorter radius drilling to the longer radius of curvature that other tools are not easily capable of duplicating. Because of its flexibility, the tool 10 can achieve a very short radius of curvature, and it can be rotated in the intermediate radius section of borehole; therefore, the same tool 10 can achieve a longer radius of curvature. No other known tool can readily do both.

A similar advantage occurs upon changing from the large radius of curvature to the straight drilling. Because of its flexibility, the tool 10 can be built with a relatively long drill motor and still operate the drill motor in the tighter radius sections of the bore hole. Nevertheless, use of a short motor is still advantageous in passing through the curved section. Once through the curved sections of the bore hole, the longer tool 10 has the advantage of being capable of drilling a straighter hole than a shorter drilling tool and providing more drilling power. Currently known tools can not duplicate this advantage, because if they are long enough to drill the same straight hole, they are too stiff to operate the drill motor through the curved sections. Put another way, if the conventional tool is short enough to operate the drill motor in the curved sections, it is too short to drill a very straight hole in the rotating mode with acceptable drilling efficiency. Further, even where rotation of a known tool is possible, excessive switching between the rotating mode and the sliding mode may be required to drill a long radius curve. The tool of the '109 patent can operate the drill motor through the short radius sections, but it can not rotate with the drill string, so it can not drill straight hole at all.

Figures 9 through 12 show the tool 10 of the present invention drilling a well profile as shown in Figure 1. In Figure 9, the tool 10 has begun the kickoff, with both stabilizers against the outside of the hole, and the angle A is at its unconstrained value.

This will result in the drilling of a curved hole as shown in Figure 10, with the flexibility of the drilling tool 10 allowing the tool 10 to operate through the tight radius of curvature.

It can be helpful to have a "knuckle joint" 22 above the tool 10 of the present invention, to act as a "moment release" or "moment disconnect", allowing the top of the tool 10 to follow the outside of the curved section, rather than being forced against the inside of the curve. In Figure 11, the operator has begun to alternate between the sliding mode and the rotating mode of drilling, to open up the curve to a longer radius of curvature, and achieve a "soft landing". In Figure 12, the operator has begun to primarily operate in the rotating mode, to drill essentially straight hole. Occasional operation in the sliding mode can be used to keep the tool 10 in the target formation, or to change azimuth. All of this drilling, from kickoff to horizontal, is achieved with a single tool, on a single trip, requiring no unnecessary trips in and out of the hole.

While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.

Claims (24)

CLAIMS:

1. A method of deviating from a wellbore at an intermediate radius, using a single steerable tool in a single trip, said method comprising: attaching a single steerable drilling tool to a rotatable drill string, said single steerable tool comprising a fluid driven drill motor, an angled sub, a bearing housing, and a drill bit, and the longitudinal axis of said drill bit being oriented at a selected angle from the longitudinal axis of said drill string; lowering said steerable tool into a wellbore to a selected depth; orienting said drill bit in a selected azimuthal direction; pumping fluid through said drill motor, thereby operating said drill motor to rotate the bit for drilling in a sliding mode of operation, to drill a kickoff section of a deviated wellbore, until said angled sub exits the original wellbore; continuing to pump fluid through said drill motor to drill a buildup section of the deviated wellbore, to approach a desired inclination of the deviated wellbore; selectively intermittently rotating the drill string, thereby intermittently operating said drilling tool in a rotating mode of operation, to drill a soft landing section of the deviated wellbore, to gradually transition from said buildup section to said desired inclination of the deviated wellbore; and continuously rotating the drill string to cause said drilling tool to follow a substantially straight path at said desired inclination of the deviated wellbore.

2. A method of deviating from a wellbore using a single steerable tool in a single trip, as recited in claim 1, said method further comprising: constraining said steerable tool under compressive force within the original wellbore to flex said tool from its originally selected angled configuration and urging said drill bit against one side of the original wellbore; allowing said drill bit to return to said originally selected angle for drilling of said buildup section at a predetermined buildup rate once said drill bit has drilled an initial portion of the deviated wellbore emanating from the original wellbore.

3. A method of deviating from a wellbore at an intermediate radius using a single steerable tool in a single trip, as recited in claim 1 or 2, said rretfl fmrt avisw intermittently rotating the drill string during said drilling of said buildup section, thereby operating said steerable tool in the rotating mode of operation to reduce the buildup rate.

4. A steerable tool for one-trip drilling of the kickoff, buildup, soft landing, and lateral sections of an intermediate radius deviated wellbore, comprising: a fluid operated non-articulated drill motor having a flexible stator housing, said drill motor being attachable to a rotatable drill string; a sub presenting a predetermined angle of deviation from the longitudinal axis of said drill motor below said drill motor; and a bearing housing below said sub; said tool being free of other subs presenting a predetermined angle of deviation equal to or greater than the angle of said angled sub; and wherein said steerable tool is operable in a sliding mode of operation in said kickoff section, operable in both sliding and rotating modes of operation in said buildup and soft landing sections, and operable in a rotating mode of operation in said lateral section, to enable drilling of all said sections of said deviated wellbore in a single trip.

5. A steerable tool as recited in claim 4, for use with a carbon steel drill pipe, wherein said drill motor stator housing is sufficiently flexible to drill said sections of said deviated wellbore, with said buildup section having a buildup rate between 0.878 x pipe yield strength - pipe diameter and 1.489 x pipe yield strength + pipe diameter, where buildup rate is in degrees per 100 ft. of hole, pipe yield strength is given in kpsi, and pipe diameter is nominal inches.

6. A steerable tool as recited in claim 4 or 5, wherein said drill motor stator housing is sufficiently flexible to drill said sections of said deviated wellbore, with said buildup section having a buildup rate sufficiently high to generate pipe bending stress between 25 kpsi and 43 kpsi in high strength grade steel drill pipe.

7. A steerable tool as recited in claim 4, 5 or 6, wherein said stator housing is flexible by having an outside diameter over a significant portion of its length substantially equal to or less than the outside diameter of the drill pipe to which it is attached, and by having an outside diameter at its tool joints substantially equal to or less than the outside diameter of the drill pipe tool joints.

8. A steerable tool as recited in any of claims 4 to 7, wherein said stator housing is made flexible by having an inside diameter over a significant portion of its length greater than the inside diameter of the drill pipe to which it is attached.

9. A steerable tool as recited in iy of claims 4 t 8, wherein said stator housing is made flexible by being constructed of a material having a yield strength at least as g@ as the drill pipe to which it is attached, and a modulus of elasticity significantly lower than the drill pipe to which it is attached.

10. A steerable tool as recited in claim 9, for use with high strength steel drill pipe, wherein said stator housing material has a yield strength of at least 135 x 1 10 pounds per square inch, and a modulus of elasticity of no greater than 17 x 106 pounds per square inch.

11. A steerable tool as recited in claim 9 or 10, wherein said stator said material is selected from the group including Beta-C Titanium and Copper Beryllium.

12. A steerable tool as recited in any of dlains 4 to 11, in said bearing housing is also flexible.

13. A steerable tool as recited in claim 12, wherein said bearing housing is made flexible by having an outside diameter over a significant portion of its length substantially equal to or less than the outside diameter of the drill pipe to which said drill motor is attached, and by having an outside diameter at its tool joints substantially equal to or less than the outside diameter of the drill pipe tool joints.

14. A steerable tool as recited in claim 12, or 13, wherein said bearing housing is made flexible by having an inside diameter over a significant portion of its length greater than the inside diameter of the drill pipe to which it is attached.

15. A steerable tool as recited in claim 12, 13 or 14, wherein said bearing housing is made flexible by being constructed of a material having a yield strength at least as great as the drill pipe to which said drill motor is attached, and a modulus of elasticity significantly lower than the drill pipe to which said drill motor is attached.

16. A steerable tool as recited in claim 15, for use with high strength steel drill pipe, wherein said bearing housing material has a yield strength of at least 135 x 103 pounds per square inch, and a modulus of elasticity of no greater than 17 x 106 pounds per square inch.

17. A steerable tool as recited in claim 15 Cr 16, wherein mid bearing bit material is selected from the group including Beta-C Titanium and Copper Beryllium.

18. A steerable tool as recited in any of claim 4 to 17, wherein: said stator housing is constructed of a first material; said angled sub is constructed of a second material different from said first material; and said second material resists galling when threaded to said first material.

19. A steerable tool as recited in claim 18, wherein: said first material is cold worked Beta-C Titanium; and said second material is Copper Beryllium.

20. A steerable tool as recited in any of claims 4 to 19, wherein said sub comprises two segments, a first said segment being angled relative to said stator housing, and a second said segment being angled relative to said first segment.

21. A steerable tool as recited in any of claims 4 to 20, wherein said sub is adjustable to achieve a selected angle of deviation.

22. A steerable tool as recited in any of claims 4 to 21, further comprising a wear ring of abrasion resistant material on said tool at the upper end of said sub.

23. Methods of deviating a wellbore at an intermediate radius substantially as hereinbefore described with reference to Figs. 3-12 of the accompanying drawings.

24. Steerable tools substantially as hereinbefore described with reference to Figs. 3-12 of the accompanying drawings.