GB2044824A - Drilling boreholes - Google Patents

Abstract

In a method and apparatus for drilling bore holes for recovering substances such as petroleum from subsurface earth formations a well is drilled whereby the bore hole does not curve away from the vertical along a pattern defined by an arc of a circle but rather follows a curvature which is defined by the catenary function. The well may be extended from point D in a generally horizontal direction or along the bedding plane of the formation of interest. <IMAGE>

Description

SPECIFICATION Drilling boreholes This invention relates to methods and apparatus for drilling boreholes.

There is a dramatic need for new methods and apparatus to recover existing oil, coal and gas reserves. There is also a critical need to develop new methods to dispose of radioactive wastes to locations that are safe to the environment and for the time required for radioactive decay.

Approximately sixty per cent of all oil that has been discovered is still in the ground and cannot be recovered because there is no known technique to recover it effectively at this time. A great deal of time and money have been expended in what are known as tertiary recovery methods such as steam flooding, electrolysis, hot air or in situ burning, but such methods have not proved sufficiently effective in all cases. Secondary recovery methods such as sand fracing and hydrofracing have not fared much better. Hence, the fact remains that even with existing secondary and tertiary techniques, vast quantities of known oil and gas remain in the ground and the reservoir pressure is depleted to the point that such reserves cannot be produced.

As an example, the U.S. Geological Survey has stated that there may be as much as one hundred trillion cubic feet of natural gas in a 120,000 square mile area of the Great Plains.

However, these vast quantities of natural gas are trapped in low permeability type reservoirs at depths generally less than 4,000 feet. Since the recoverable domestic natural gas reserves are estimated to be about 208 trillion cubic feet, it can be seen that this one potential source represents about fifty per cent of the total potential gas reserve in the United States.

It is therefore the purpose of the herein described invention to provide a new drilling method and new and novel associated downhole tools, to achieve a more economically viable development of low permeability reservoirs such as shallow gas sands and the like, and also the eastern Devonian gas bearing shales, as well as existing depleted oil reserves which are estimated to contain the equivalent of 60% of the existing oil reserves, but which are not actually available to develop at the present time.

It is well known that if producing oil and gas formations could be penetrated along a longitudinal axis down through the middle of the seam rather than along a transverse axis perpendicular to the seam, then considerably more of the formation would be exposed to the well bore resulting in a more efficient recovery of the reserve. Thus, the significant advantages of such techniques are that there is provided a reduced flow path for the oil or gas in the formation to the borehole resulting in a much greater flow rate for a given drive pressure.

It is obvious that if a tight formation, shale, limestone or sandstone formation, has been drilled along its longitudinal axis for several hundred feet, and if sufficient hydraulic horse power is applied to frac this formation, then the permeability of the formation will be greatly improved in the vicinity of the well bore. It is also apparent then that an acidizing operation applied to a tight limestone formation which has been drilled in this manner will greatly increase the formation exposure to the acid, and will therefore improve the permeability with resulting greater production.

However, directional drilling techniques currently in use for effecting such longitudinal drilling have not proved to be altogether satisfactory. This is for the reason that various "stuck" points are encountered in the drilling pattern resulting in the impossibility of removal of the drill pipe string from the well bore with resultant abandonment of the well.

Such "stuck" points or dog legs in the borehole result from changes in the dip angle of drilling in attempting to reach the point of maximum vertical depth and the predetermined point of maxium horizontal displacement, as well as from differential pressures acting across the drill string in the area of the collars. Thus, in a typical well of 5,1 00 feet total vertical depth and 1,700 feet total horizontal displacement, there may be typically three or more changes in the dip angle. Each of these dip angle changes conventionally follow a curve corresponding to the segment of a circle and thus provide a potential "stuck" point whereby any attempt to pull on the string of pipe within the well bore will result in sections of pipe being forced against the wall of the hole in the areas of the "stuck" points.Such "stuck"points are by far the most common cause of lost holes which are sought to be drilled along the bedding plane of the formation.

According to one aspect of the present invention a method of drilling a borehole into a subsurface earth formation, comprising: drilling a borehole along a catenary curve into said formation.

In one embodiment of the invention the borehole is extended further through said formation along a substantially horizontal path.

In another embodiment of this invention, a well is drilled to follow the curve of a catenary until the bedding plane of the formation of interest is intersected and thereafter the borehole departs from the catenary curve to follow the bedding plane of the formation so as to produce a borehole which extends along the bedding plane of the formation to a maximum extent.

According to another aspect of the invention we provide a driving means for a drill bit comprising: A method a hydraulic jack having first and second telescoping members, means defining an orifice in one end of said first member, and with the other end of said first member being located within said second member, and means for delivering a flow of mud to one end of said second member.

According to yet another aspect of the invention we provide an anti-friction stabiliser, comprising: a solid body member having an annular flow passageway therethrough.

inlet and outlet connector means at each end of said body member respectively, at least one raceway in the exterior surface of said solid body member, and a plurality of spherical elements located in said raceway and being free to roll and rotate therein.

Referring to the accompanying drawings: Figure 1 is a graph showing a comparison between the drill pattern of a well drilled along the arcs of a circle versus one drilled along a catenary curve in accordance with the present insvention .

Figure 2 is a simplified pictorial representation of a tool for driving a drill bit once the borehole has departed from vertical to an extent that drill collars no longer provide a driving force.

Figure 3 is a simplified pictorial representation of a combination anti-friction stabilizer useful in the lower portion of a catenary borehole to minimize buckling and drag forces.

Figure 4 is a simplified pictorial representation of the tool of Fig. 3 taken along line 4-4 thereof.

Referring now to Fig. 1 there is depicted therein a graph showing a comparison between a conventional drill pattern for a well drilled along arcs of a circle, and a drill pattern for a well drilled in accordance with the concepts of the present invention and which follows a catenary curve. In either case, the object is to begin drilling at point X on the surface and to a point D in a formation which is approximately 1,700 feet horizontally displaced from point X and some 5,000 feet in depth.

For a typical well drilled in accordance with prior art techniques and as seen in Fig. 1, the well starts vertically from the platform at point X and is drilled to 500 feet in a vertical plane. At this point conductor pipe is set and drilling is continued at a dip angle of two degrees per hundred feet for an additional 1,500 feet or until a maximum angle of thirty degrees to the vertical has been attained. This is shown by that portion of the curve in Fig. 1 between points A and B.

Another 2,000 feet are drilled with the dip angle remaining at thirty degrees. This step is illustrated in Fig. 1 by that straight portion of the curve between points B and C. At point C, however, drilling continues at a dip angle of two degrees per hundred feet for an additional 1,500 feet with the result that an inclination angle of zero degrees is again reached and the well bore returns to the vertical at point D. This step is shown by section C-D of the curve of Fig. 1.

It should be noted, and with reference to Fig. 1, that portions A-B and C-D trace a curvature of a circle and the calculations necessary to produce this curve are based on well known and conventional industry accepted methods ranging from what is known as "the radius of curvature method" to the older and practically obsolete "tangential method".

In any event, observance of Fig. 1 should make it apparent that any tendency to pull on drill pipe in a well bore drilled in this fashion will result in the whole section of pipe in area C-D being forced against the wall of the borehole in the area of points B and C. Thus, the greater is the pulling force applied to the drill string at point X, then the greater will be the force applied to the wall of the hole at points B and C forcing the string thereagainst, with the result that little or no force will be applied to any portion of the string below point C. In other words, not only do points B and C provide unwanted potential "stuck points" in the system, but they effectively prevent the application of an upward pulling force on the portion of the string in the area C-D.

This is by far the most common cause of lost holes in recent times, and particularly in the Gulf of Mexico, with the result that wells drilled in accordance with such patterns are abandoned because even through point D was successfully reached, nevertheless points B and C rendered it impossible to remove the drill string from the completed borehole.

As a viable alternative to drill pattern A, B, C, D, the present invention proposes a pattern of drilling that effectively eliminates potential stuck points B and C and which otherwise follows a pattern defined by a catenary function. Thus, a directional well is drilled angularly from point X to point D on a catenary instead of along a radius of curvature.

A catenary curve can best be defined by the following mathematical equations:

These are general mathematical functions for catenaries, and in order to adapt these formulations to a drilling pattern for a particular proposed borehole, the following expressions must also be considered in arriving at a predesigned and predetermined catenary for the borehole in question, thus: T0=WY (3) T, = Wa (4)

where:: a is a distance along the Y axis and a specific value for any particular catenary; + is the angle of orginal entry into the borehole; To is the maximum tension load or pull to be exerted along the longitudinal axis of the drill string in the event of a stuck point; S is the linear length of borehole from the surface to the point of intersection with the formation bedding plane; h is the vertical depth of the drill bit or the true vertical depth; W is the weight of drill pipe per foot of length; TH is the force created by the stuck point and is the horizontal component of the maximum tension load To; X is the horizontal offset of the drill bit; and Tv is the vertical component of the maximum tension load T,.

Since it is not the purpose of this invention to instruct the art in mathematics or computer techniques, it is merely stated that, given the above mathematical expressions, values and relationships, a catenary curve such as depicted in Fig. 1 can be more than readily designed and arrived at, and adapted to any desired set of circumstances, than is the case with the prior art curve shown in Fig. 1.

In any event, in an exemplary catenary curve designed for a drill pipe pull of (for example) 120,372 pounds at the surface (To); using 1 9.5 pounds per foot drill pipe (W); a vertical depth of 5,593 feet (h); a horizontal pull of 50,936 pounds (Th); and given an angle at the surface of 64.97 degrees (i; it will be apparent that the drill collars stick near the bottom (D) of the catenary well, and if a pull is applied to the top of the drill string which exceeds the design pull of 120,372 pounds, the drill string will begin to pull away from the low side of the well bore in a radial manner and incrementally with depth as the tension increases.This action literally "peels" the drill collar radially away from the well bore to eliminate the prior art stuck points B and C, and thereby frees the pipe string for further operation in the drilling sequence.

Thus, as hereinbefore noted, the unique advantages of a catenary curve are twofold. First, the catenary curve is smooth and continuous and without radical departures in dip angle along its linear path. Thus, the problem of "stuck" points B and C inherent in the prior art are avoided.

Second, a particular inherent and unique feature of a catenary curve is the fact that a pull (To) exerted at the top of the drill string and which exceeds the design pull, will start to pull away the drill string from the low side of the well bore in a radial manner and incrementally with depth as tension increases. This action "peels" the drill collar radially away from the well bore thereby freeing the "stuck" pipe, if any. Thus, the application of tension results in the characteristic curve of the catenary changing but changing in a manner that pulls pipe away from the wall of the borehole with the resultl that "stuck'' points B and C are eliminated for all practical purposes in a well drilled along the curve of a catenary.

After having completed the drilling of the catenary well of Figure 1, and assuming that the subsurface formation of interest residues at point D, then the well may be produced at that point. However, it is contemplated herein that the catenary well may be continued to be drilled from point D and further into and further along either the bedding plane of the formation of interest or in a generally horizontal direction. Thus, if another segment of borehole were continued for another 1 ,000 to 2,000 feet horizontally past point D, then a substantial linear portion of the borehole would be exposed to the formation of interest and with the result that production will be substantially enhanced.Thus in the event that secondary or tertiary recovery methods are required, the effectiveness of the drill hole would be greatly multiplied by virtue of tthe increased ratio of borehole exposure to the formation area of interest.

However, as has been previously noted, if a directional well is drilled approaching the horizontal, a problem arises due to the fact that the driving force on the drill bit diminishes as the bore well approaches horizontal. This is for the reason that dead-weight drilling bit driving forces, such as that provided by drill collars, begin to diminish as the drill pattern approaches horizontal. It is therefore required that some alternate means of driving the bit be provided when horizontal drilling is contemplated. Since the herein described method of catenary drilling is adaptable to approach horizontal drilling, means are provided herein to assure that a driving force is maintained on the drill bit during the horizontal or near horizontal phase of the drilling pattern. Such means take the form of a hydraulic jack which is shown in Fig. 2.

Referring now to Fig. 2, there is depicted and illustrates therein a downhole tool comprising a hydraulic jack assembly adapted to be inserted into the drill string upstream of the drill bit when horizontal drilling is to be commenced. Thus, it is contemplated herein that the drill string will be removed at point D of the catenary borehole of Fig. 1, and that the drill collars will be eliminated and that the jack of Fig. 2 substituted in their place, and drilling commenced again along the horizontal.

In any event, a male spline member 10 is provided and including a threaded section 11 adapted to be connected to the drill bit (not shown). One end of spline member 10 includes an orifice 1 2 which communicates with the mud line of the bit, and the other end of which communicates with drill pipe 1 3 which extends back up to the surface of the earth.

Section 1 3 of drill pipe includes inner and outer threads 14 and 15, and with a female spline member 1 6 connected to threads 1 5 of drill pipe 1 3. A wash pipe 1 7 is screw-threadedly attached to the threads 1 4 of pipe 13, and provides the interconnection for mud flow through pipe 1 3 and onto the drill bit which is noted above is downstream of orifice 1 2.

The female spline member 1 6 includes in its interior a series of guide slots 1 8 and 19, which co-operate with a corresponding series of ribs 20 and 21 on the male spline 10. Such construction provides for the telescoping and reciprocating action of male spline member 10 with respect to female sline member 16. Shoulders 22 and 23 on the wash pipe 1 7 and male spline member 10, respectively, prevent the male spline member 10 from being ejected off of the wash pipe 17.

A suitable sealing means 24 is included on male spline 10 and provides for the movement of the male spline member 10 telescopically between the female spline 1 6 and wash pipe 1 7. The driving force for the male spline 16 and wash pipe 17. The driving force for the male spline 10 can of course be realised when it is considered that because of the presence of the orifice 12, that the pressure P2 downstream of orifice 1 2 will be less than the pressure P, upstream thereof, and that the resultant pressure differential will effectively act upon the exposed surface area 27 of the end of wash pipe 17, with the result that male spline member 10 will tend to eject itself out of female spline member 1 6 along wash pipe 17, thereby providing a thrust to the drill bit which is downstream therefrom.

Thrust (f) acting on the drill bit should be sufficient to enable the bit to continue to bore through the formation, and given any set of circumstances can be increased or decreased by varying the size of the orifice 12, the surface area 27 of the wash pipe 17, or the mud pressure being employed. In any event, the jack of Fig. 2 will provide enough driving force (F) for the drill bit when drilling approaches horizontal and wherein standard drill collar techniques are no longer effective to drive the bit through the formation.

While the orifice 1 2 in Fig. 2 is seen to be manually installed, it is possible and is contemplated herein that orifice 1 2 could be inserted on a wire line and retrieved on a wire line, and it would be desirable that this orifice 1 2 be replaced from the surface without having to pull the drilling assembly. At any rate, the orifice 1 2 is an absolute necessity to regulate the pressure exposed to the area 27 which in turn in the available force F acting on the bit. An equal and opposite force is thereby applied to the drill string 13 and puts the drill string 1 3 in compression, but since the curve is a catenary, the horizontal component of that compression force is absorbed in a push against the bottom section of the hole.As the entire drilling string is suspended from the point where differential sticking occurs when the tension applied to the drill string exceeds the value designed into the catenary, then too the entire drill string, from the surface to the lower limit of the catenary, is supported along the bottom side of the well bore as the compressive load is applied from the jack shown in Fig. 2.

While the hydraulic jack 2 of Fig. 2 may be seen to resemble in some respects what are known in the art as "bumper subs", there is a fundamental difference between the two. Thus, bumper subs have been used in offshore drilling in order to compensate for the heave in the vessel or floating platform which results in the drill bit coming off the bottom of the hole. Such bumper subs were not and have not been intended to transmit a positive driving force to the bit, however. In fact, such offshore bumper subs have included drill collars thereabove to prevent this. Other, more complex and expensive devices have been designed with internal hydraulic systems so as to be pressure balanced and therefore exert no jacking force.

In other words, bumper subs include a telescopic joint adjacent the drill bit whereby when the drilling vessel heaved upwards, the joint would extend downwardly to maintain the bit on the bottom of the hole, and when the vessel heaved downwardly then the telescopic joint of the bumper sub would collapse so as to maintain an equal or constant weight on the drill bit. Such bumper subs were designed to function strictly as a function of the rise and fall of the vessel, and in cases where mud pressure tended to open the bumper sub, drill collars equivalent in weight to the bumper sub opening hydraulic force, were provided to prevent such opening of the sub except in response to the wave action rise and fall of the vessel.

The fundamental concept of the jack of Fig. 2, however, is that such construction is to be used downhole and without the presence of drill collars since it is the function of the jack of Fig.

2 to extend in response to mud pressure in order to create driving force F to propel the bit through the formation of interest and only during that phase of the drilling operation where the hole angle approaches the horizontal plane and the dead weight from conventional drill collars is no longer effective for driving the bit into the formation. Thus, a fundamental functional difference should be seen to exist between the jack of Fig. 2 and the offshore bumper subs of the prior art.

Referring now to Figs. 3 and 4, there will be seen a further perfecting feature of the present invention and which comprises a downhole tool for use in the catenary drilling method set forth herein and which comprises an antifriction stabilizer for use in the lower section of the well bore where the catenary curve is the least or in the horizontal section where advantages from the catenary are minimized. In such section of the borehole it has been found that drag forces increase more than is ordinary or to be expected, and that there further exists buckling forces acting on the drill string. It is contemplated to neutralize such drag and buckling forces in the lower section of the catenary borehole by the installation in the drill string, at appropriate intervals as for example at approximately twenty foot intervals above the drill bit and jack of Fig.

2, stabilizers as seen in Figs. 3 and 4.

Referring again to Figs. 3 and 4, there will be seen therein stabilizer 34 comprising a solid body member 30 having a plurality of continuous raceways 32 circumferentially thereabout.

While four raceways 32 are illustrated, this is exemplary only and obviously three or five or more or less raceways 32 could be included therein.

In any event, stabilizer body portion 30 includes inlet and outlet connections 29 and 28 respectively for interconnection into the drill string. A plurality of hard rubber balls 33 are located in each of the raceways 32 and are free to roll and rotate therein. Passageway 31 is provided internally of solid body 30 and provides the flow communication between ends 28 and 29 of the stabilizer 34. The rubber balls are preferably of hard construction and any suitable heat and friction resistant composition may be used in construction such as Bakelite, for example, or conventional rubber compositions as SBR, neoprene, butyl, nitrile, polybutadiene, polyisoprene, ethylene-propylene, and natural rubbers. However, with such stabilizers 34 in the lower catenary bore it will be seen that drag forces and tendencies of buckling will be resisted or at least minimized by the presence therein of the free rolling rubber ball construction of the unit of Figs. 3 and 4.

Claims (22)

1. A method of drilling a borehole into a subsurface earth formation, comprising: drilling a borehole along a catenary curve into said formation.

2. A method according to claim 1 in which said borehole is used for withdrawing substances from said formation.

3. A method according to claim 1 in which said borehole is used for depositing waste materials.

4. A method according to any of claims 1 to 3 in which said borehole is extended further through said formation along a substantially horizontal path.

5. A method according to any of claims 1 to 3 in which said borehole is drilled until the bedding plane of the formation is intersected, and said borehole is extended along the bedding plane of the formation to a maximum extent within the bedding plane.

6. A method according to claims 4 or 5 in which said borehole is extended in a substantially horizontal path or along said bedding plane by a drill bit driven by a driving force generated in response to mud pressure.

7. Driving means for a drill bit comprising: A method a hydraulic jack having first and second telescoping members, means defining an orifice in one end of said first member, and with the other end of said first member being located within said second member, and means for delivering a flow of mud to one end of said second member.

8. Driving means according to Claim 7 including a tubular wash conduit located within said second member, one end of said wash conduit being in flow communication with said delivering means and the other end of said wash conduit being located interiorly of said other end of said first member.

9. Driving means according to Claim 7 or 8 including at least one guide slot extending along the interior of said second member, and at least one rib member on the exterior of said first member and in engagement with said guide slot of said second member.

10. Driving means according to Claim 8 or 9 including an enlarged shoulder on said other end of said wash conduit, and a corresponding shoulder on said other end of said first member, said shoulders being adapted for abutting engagement one with the other to limit the forward travel of said first member within said second member.

11. Driving means according to any of Claim 7 to 10, including vent means in said one end of said second member.

1 2. Driving means according to any of Claims 8 to 11 including sealing means being interposed between said first member and said wash conduit.

1 3. Driving means according to any of Claim 7 to 12 wherein means are provided on the said one end of the said first member for connection of said first member to said drill bit.

14. Driving means according to claim 7 substantially as herein described with reference to and as shown in the drawings.

1 5. An anti-friction stabilizer, comprising: a solid body member having an annular flow passageway therethrough, inlet and outlet connector means at each end of said body member respectively, at least one raceway in the exterior surface of said solid body member, and a plurality of spherical elements located in said raceway and being free to roll and rotate therein.

16. A stabilizer according to Claim 1 5 wherein said spherical elements comprise balls constructed of a heat and friction resistant material.

1 7. A stabilizer according to Claim 1 5 wherein said balls are constructed of an elastometic material.

1 8. A stabilizer according to claim 1 5 substantially as herein described with reference to and as shown in the drawings.

19. A method according to any of claims 1 to 6 using a driving means for driving the drill bit as claimed in any of claims 7 to 14.

20. A method according to claim 1 9 in which a stabilizer as claimed in any of claims 15 to 1 8 is located upstream of said driving means.

21. A method of recovering substances from a subsurface earth formation substantially as herein described with reference to and as shown in the accompanying drawings.

22. A method of disposing of waste materials, such as radioactive materials, substantially as herein described with reference to the accompanying drawings.