Steerable Gas Turbodrill

Abstract

A gas turbodrill with an adjustable bent housing for use in a spur lateral drilling application. The gas turbodrill includes a high-speed gas turbine, a gearbox assembly, a pivoting shaft connection point, a gimbal assembly comprising a hollow ball and socket joint, a bearing assembly and drill bit assembly. The gas turbodrill gimbal assembly enabling a bend through an angle of up to 5 degrees while drilling. Springs and the application of pressure will lock the bend in place once drilling commences to facilitate lateral drilling of the spur.

Description

BACKGROUND

1. Field of the Invention

The present invention relates to gas turbodrills for downhole drilling operations.

2. Description of the Related Art

It is generally desirable to operate a drill motor on dry gas for completion drilling of water sensitive formations. However, some types of drill motors are not suitable for this purpose. For example, progressive cavity motors incorporate elastomeric stators that can rapidly degrade when operated on dry gas. Turbodrills are capable of operation on dry gas, but these tools stall easily when operated on gas, and the motor speed is generally much too high for effective drilling. Typical turbodrill motors also tend to be very long, which limits the steer-ability of the drill string. In a paper entitled, “Downhole pneumatic turbine motor: testing and simulation results,” SPE Drilling Engineering, September pp 239-246, Lyons et al. describe the development and testing of a gas turbine motor for drilling. As described in this paper, the gas turbine motor included a single stage radial-flow turbine operating at extremely high rotary speed (i.e., at more than 100,000 rpm) and a multi-stage planetary transmission to reduce the speed and increase torque to the level needed to drive a conventional roller cone drill bit. There are technical challenges that arise when exiting an open hole during completion drilling, which include:

1. Orientation of the lateral bores in vertical, inclined, or horizontal wells;

2. The kickoff of the lateral;

3. Transport of cuttings away from the drill bit;

4. Hole stability; and

5. Trajectory control.

Coiled tubing drilling (“CTD”) systems capable of sidetracking and drilling multiple lateral bores are available. These systems have been used extensively in Alaska to access compartmentalized oil reservoirs. The cost of a CTD bottom hole assembly (“BHA”) including measurement-while-drilling and downhole bit face orientation tools is relatively high, as is the cost of the surface equipment required to support this apparatus. These systems offer full steer-ability and tracking and are capable of drilling at up to 50°/100 ft dogleg severity (“DLS”). [DLS is a normalized estimate of the overall curvature of a well path between two consecutive directional survey stations, according to the minimum curvature survey calculation method.] A conventional CTD system incorporates a positive displacement motor (“PDM”) designed to operate on drilling mud. This system develops significant torque and requires constant trajectory measurement using measurement while drilling tools and steering adjustment using a downhole orienter. These steering systems are complex and expensive and greatly increase the length of the BHA. Wire in coil systems can be required for operation on dry gas since mud pulse telemetry is not feasible when running dry gas.

It would be desirable to develop a steerable gas turbodrill (“SGTD”) that enables high-power, high-rotary speed drilling at a lower torque than a PDM system and which requires minimal steering, once the SGTD is properly oriented. This approach would eliminate the need for high-cost measurement while drilling and the need for bit face orientation systems in the bottomhole assembly. This tool should be relatively compact and capable of being readily steered, for example, at least through a 200 ft. lateral arc having a constant 120 ft. radius, i.e. a spur lateral.

It would further be desirable to employ a SGTD that uses dry nitrogen, and which includes a gear box, enabling operation at a high rotary speed, for efficient power conversion, and but achieving a lower rotational speed on the output of the gear box, than is possible for a gas turbine power section.

SUMMARY OF THE INVENTION

In accordance with the present invention, the problems discussed above are solved by a gas turbodrill that includes a drill-bit section, a bearing assembly, a gearbox assembly, a gimbal assembly, a high-speed gas turbine power section and a flexible tubing string are fed downhole at the end of a string of pipe for a spur lateral drilling application.

In one embodiment of the invention, the high-speed gas turbine power section in the upper section of the gas turbodrill rotates a flexible shaft that extends through a gimbal assembly. The lower section of the turbodrill then contains the gearbox assembly, bearing assembly and drill-bit section. The gimbal assembly serves as a flex joint for the entire gas turbodrill, which allows the drill to move at an angle away from the central wellbore, with a whipstock serving as a guide. In a preferred embodiment of the invention, the power section is located above the gearbox which is above the gimbal section and the flexible shaft passes through the gimbal and drives the bit.

As the gas turbodrill is lowered downhole on the end of a pipe string and the gas turbodrill reaches the whipstock, which has been pre-installed, the lower turbodrill section will change direction with the gimbal assembly providing a pivot point. As the gas turbodrill and drill string are lowered further into the wellbore, the flex joint bends until it reaches a preset bend angle limit. A highly compressed spring inside of the gimbal assembly locks the bend into position.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and attendant advantages of one or more exemplary embodiments and modifications thereto will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1A is a is a cross-sectional view of an exemplary openhole spur lateral drilling configuration showing the steerable gas turbodrill configured in accordance with the invention at the start of drilling the lateral;

FIG. 1B is a cross-sectional view of an exemplary openhole spur lateral drilling configuration showing the steerable gas turbodrill in accordance with FIG. 1A just after the spur lateral has started;

FIG. 1C is a cross-sectional view of an exemplary openhole spur lateral drilling configuration showing the steerable gas turbodrill in accordance with FIGS. 1A and 1B at the completion of spur lateral drilling;

FIG. 2A is a cross-sectional view of the exemplary steerable gas turbodrill pinch point of FIGS. 1A, 1B, and 1C.

FIG. 2B is a cross-sectional enlarged view of the pinch point of FIG. 2A.

FIG. 3 is an exemplary fixed cutter bit selection chart, which is published by Dimatec Inc., for use in selecting a suitable cutter bit that can be driven by the exemplary SGTD of FIG. 1;

FIG. 4 is an exemplary graph that can be used for a gas turbodrill circulation analysis, in connection with the SGTD discussed herein;

FIG. 5A is a cross-sectional view of an exemplary gas turbodrill configuration;

FIG. 5B is an enlarged cross-sectional view of the exemplary gas turbodrill configuration of FIG. 5A;

FIG. 5C is an enlarged cross-sectional view of a gimbal assembly of the exemplary steerable gas turbodrill configuration of FIG. 4A;

FIG. 5D is a side perspective view of the exemplary steerable gas turbodrill configuration of FIGS. 5A, 5B, and 5C;

FIG. 6A is a side-elevational view of the exemplary steerable gas turbodrill configuration; and

FIG. 6B is a cross-sectional view of the exemplary gas turbodrill configuration of FIG. 6A.

DETAILED DESCRIPTION

It is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted,” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings.

Air drilling systems have advantages for borehole completion applications because this technique leaves a dry, open borehole that requires no additional cleanout and avoids water contact with the formation.

An exemplary embodiment of an openhole spur lateral drilling configuration for a steerable gas turbodrill (“SGTD”) is shown in FIGS. 1A, 1B, and 1C. A wellbore 10 is shown with a liner 26 supporting a whipstock 30 using liner hanger. The liner 26 and whipstock 30 may be lowered into the well and oriented using a rotary drill rig or workover rig with rotary capability which is not shown but are well known to those skilled in the arts of drilling, well completion and well intervention. A drillstring 24 extends from above ground into the wellbore 10 through a liner 26 that also extends from above ground and into the wellbore. The liner 26 guides the drillstring 24 through a medial portion of the wellbore 10. The drillstring 24 and SGTD 28 are fed into the wellbore with the drill rig or workover rig using standard methods of handling jointed tubing. Alternatively the drillstring may be a continuous length of tubing that is fed into the well with a coiled tubing well service unit also well known in the art. The SGTD 28 is coupled to the drillstring 24 before insertion into the wellbore 10. Prior to the insertion of the SGTD 28, a whipstock 30 is run into the wellbore using liner 26. Alternatively, the whipstock 30 can be run-in separately from liner 26 and placed with an openhole packer, not shown. The whipstock 30 serves to guide the SGTD 28 and drillstring 24 at a desired angle to thereby allow access to oil and gas bearing formations that are not directly downhole from the initial wellbore 10. Once it is inserted, the whipstock 30 can be directionally aligned such that the whipstock 30 will guide the SGTD 28 in a specific radial direction downhole. For example a wireline azimuth measurement tool can be lowered into a wireline orientation shoe just above the whipstock 30 and the line can be rotated from surface to the desired azimuth of the whipstock 30. The SGTD 28 has a gimbal joint 32 that allows the SGTD 28 to bend and change direction as the SGTD 28 is guided by the whipstock 30. The gimbal joints 32 must allow the SGTD 28 to move through a pinch point 34 when the whipstock 30 begins changing the direction of the SGTD drill bit 36.

An embodiment of the pinch point 34 in FIGS. 1A-1C is shown in greater detail in FIGS. 2A and 2B. The whipstock 30 incorporates a whipstock ramp 38 that guides the SGTD 28 on a lateral spur into the formation. Stabilizer vanes 40 are located on the underside of the whipstock ramp 38 to hold the whipstock 30 in a position in the borehole adjacent the area intended for spur lateral drilling. The stabilizer vanes 40 are sized to slide readily into the open borehole and to allow easy rotation for azimuthal orientation. The vanes 40 prevent lateral motion of the whipstock 30, for example, in excess of half of an inch or a distance significant enough to prevent the drill bit 36 from kicking off from the borehole and into the formation. In an embodiment the whipstock 30 can further have a flat spring 44 connected to a ramp 46 that pushes sideways against the SGTD as it passes through the whipstock 30. The ramp 46 has gradually ramped surfaces that allow the drill bit 36 to travel up or down in the borehole and slides past the ramp 46. The spring force of the flat spring 44 is chosen to overcome the frictional bending resistance of the gimbal joint 32. In addition, the spring 44 has sufficient travel to allow the drill bit 36 to pass without excessively dragging on inward facing surfaces of the ramp 46. Although a flat spring 44 is shown, springs of other types could be substituted to achieve a similar effect.

This system can be designed for operation off a drilling or workover rig, which can includes the following steps:

• • 1. Run a whipstock into the well using a liner.
  • 2. Orient the whipstock using a wireline and hang in the slips, or with a casing hanger.
  • 3. Run the SGTD into the well until it reaches the whipstock. The SGTD will bend at the pinch point on the whipstock. Pressure applied downwardly on the SGTD by running it further into the borehole will push the turbodrill against the whipstock and cause the SGTD gimbal joints to bend. Additionally, in an embodiment including the pinch point, the spring and ramp of the pinch point will push against the gimbal joint of the of the steerable gas turbodrill as it travels past the pitch point thereby providing an additional force to bend the SGTD gimbal joint. Eventually the gimbal joint will lock at a substantially maximum angle of bend based on the configuration and internal structure of the gimbal joint.
  • 4. Drill a lateral at minimum weight on bit (WOB).
  • Cuttings are transported out of the well through the liner.

Air Compressor and Surface Equipment Pressure Capacity: For a well at which the SGTD will initially be employed, a current available air compressor capacity is 1200 psig (8 MPa) @2500 scfm (70 scmm). The maximum pressure, consistent with safe operation on air, is 2000 psig (14 MPa). These specifications are not intended to in any way be limiting on the use or functionality of the SGTD.

Exemplary Bit Design: The exemplary embodiment of the high-speed SGTD operates with minimal torque at high speed. The SGTD may be operated with a variety of fixed cutter or roller come bits. In a preferred embodiment of the invention surface set diamond bits are used. Those skilled in the art will recognize that the maximum bit speed is limited by thermal wear of the diamonds. The reactive torque from a surface set diamond bit operating at maximum rotary speed is related to the WOB, W, bit diameter, D_b, and friction, μ (about 0.4 for rock drilling) according to the following equation:

$\begin{array}{cc}M=\frac{{\mathrm{WD}}_b\mu }{3}+\frac{\delta \left(\mathrm{SA}-W\right)}{2\pi },& \left(1\right)\end{array}$

where S is the drilling strength—assumed to equal the confined compressive strength of the rock, A is the surface area of the bit, and δ is the depth of cut per revolution. The torque will increase with rate of penetration.

An important requirement for the SGTD is to maintain well trajectory without any additional steering input once the drill has exited the primary wellbore. Conventional PDM motors operating conventional fixed cutter bits generate enough torque to cause the drillstring to twist or wind up several revolutions so that it is not possible to predict the orientation of the SGTD bend while drilling. The present SGTD invention is designed to limit the drilling torque and therefore limit the windup angle to an acceptable error level. For example if the maximum windup can be limited to less than 45 degrees, the well azimuth can be predicted to within this angle. If the drilling torque is known, the windup can be predicted and accounted for when planning the well.

The windup of an example SGTD BHA and drillstring makeup is provided below in Table 2. The estimated torque while drilling with a 2-7/8″ surface set diamond bit at about 500 lbf WOB in the Marcellus shale (15,000 psi CS) is 35 ft-lbf. The analysis is shown for 3½ or 2⅞ heavy wall drill pipe. Using the larger diameter pipe cuts the windup in half and will provide more accurate azimuthal control, however the 2⅞″ drillstring may be required to accommodate return circulation. In these examples the total windup is 22 to 53 degrees. This amount of windup may be acceptable or compensated for by rotating the drillstring to the right by the windup angle once the lateral is spudded, or by orienting the whipstock to the right by the same amount.

TABLE 2
Drilling Parameters and windup angle estimates.
WOB	500	Lbf
Motor Speed	640	Rpm
ROP	50	ft/hr
Shale Compressive	15000	Psi
Strength
Reactive Torque	35	ft/lbf
Drillstring	3-1/2″ 12.95 #/ft	2-7/8″ 8.7 #/ft
Length	6000	Ft
OD	3.5	2.88	Inch
ID	2.75	2.259	Inch
windup	22	48	Degrees
SGTD Whip	1-1/2″ Type CS 2.9#/ft
Length	200	Ft
OD	1.9	Inch
ID	1.53	Inch
windup	5	Degrees
Total Windup	27	53	Degrees

Exemplary Steerable Gas Turbodrill: FIGS. 4A, 4B, 4C, and 4D show an exemplary embodiment of a steerable gas turbodrill (“SGTD”) 40 for a spur lateral drilling application as shown in FIG. 1. The gas turbodrill 40 includes a high-speed gas turbine power section 42 and a two stage planetary gearbox assembly 44 in the upper section. The two-stage gearbox assembly 44 reduces the speed of turbine power section 42 output by a factor of 12:1 and has an output shaft 46 that extends into a clamp coupling assembly 48. The gearbox assembly output shaft 46 connects to a flexible shaft 50 in the clamp coupling assembly 48. FIG. 4C shows an enlarged view of the gimbal assembly 54. When the SGTD 40 is drilling and begins to bend in the borehole, such as when coming into contact with a whipstock, the flexible shaft 50 bends through an arc within the gimbal assembly 54. The flexible shaft 50 extends through a gimbal joint 56 that enables the tool to bend through a fixed angle of up to five degrees. The precise angle and distance from the bend to the bit determines the radius of curvature of the spur lateral. The gimbal joint has a ball and socket. The application of internal pressure plus the force of one or more heavy springs 58, such as Belleville washer springs acting on a lock ring with a spherical seat that presses against the ball. Friction between the lock ring and ball and between the ball and socket holds the gimbal in the bent position, thereby allowing drilling of a fixed radius arc or spur lateral. The friction force is chosen to allow the gimbal joint to bend when subjected to side loads inside the whipstock. The flexible shaft 50 couples to a bottom drill assembly 60 through a flow coupling assembly 62 that extends into a bearing assembly section 64. The bottom drill assembly 60 rotates a drill bit 64.

An alternate configuration for this tool is shown in FIGS. 5A and 5B. The gas turbodrill 150 includes a high-speed gas turbine 152 and a clamp coupling assembly 154 in the upper section, with a gearbox 156 and a bearing assembly 158 located in the lower section of the gas turbodrill 150. A flexible shaft 160 connects to the turbine output in the clamp coupling assembly 154 and extends through a gimbal 162 that enables the tool to bend through an angle of up to about 5 degrees. The application of internal pressure plus the force applied by one or more springs in the gimbal assembly lock the gimbal in place once drilling commences. Drill bit assembly 164 couples to the gearbox output 156 and rotates a drill bit 166. Example turbine specifications are listed below, in Table 4. A circulating model of the turbine in a wellbore is provided in the graph shown in FIG. 3. Most of the pressure differential through the motor is developed through the bit nozzles. This approach reduces the turbine speed to a manageable level. The gearbox is a conventional two-stage planetary design. The output torque of the motor at maximum power is half the stall torque and this is the recommended operating condition. Operation at near the peak power will require WOB control to within 100 lbf. Over weighting the bit will cause it to stall, while underweighting will not enable it to drill. These characteristics are common to turbodrills, but the relatively light weight of the JTD discussed herein is unique to this tool.

TABLE 4
Gas Turbodrill Turbine specifications
	Diameter	2-3/8″
	Length	5.9	ft
	Turbine stages	20
	Gas Flow rate	2000	scfm
	Turbine Pressure differential	260	psi(1.8 MPa)
	Turbine Runaway speed	15000	rpm
	Two-stage planetary gear reduction	12:1
	Motor stall torque	70	ft-lbf
	Operating speed	625	rpm
	Drilling weight on bit range	300-600	lbf
	Operating torque	35	ft-lbf
	Maximum Bend	5	degrees

Although the concepts disclosed herein have been described in connection with the preferred form of practicing them and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made thereto. Accordingly, it is not intended that the scope of these concepts in any way be limited by the above description.

Claims

1. A gas turbodrill system including a gas turbodrill assembly, the gas turbodrill assembly comprising: a gas turbine assembly configured to couple to a source of pressurized fluid and comprising a drive shaft;a gearbox assembly configured to couple to the drive shaft of the gas turbine and comprising an output shaft, the gearbox assembly being configured to rotate the output shaft at a lower rate than the drive shaft of the gas turbine;a gimbal assembly that comprises a hollow ball and socket joint that extends around a flexible shaft, the flexible shaft configured to couple to the output shaft of the gearbox assembly, the flexible shaft extending through the hollow ball and socket joint; anda drill-bit assembly connected to the gimbal assembly, the drill-bit assembly having a shaft connected to the flexible shaft.

2. The gimbal assembly of claim 1, further comprising at least one spring that presses a lock ring with a spherical seat against the ball of the ball and socket joint, the lock ring positioned to lock the position of the ball and socket joint when a pre-determined force is applied and configured to retain the locked position of the ball and socket joint when force is removed.

3. The gas turbodrill assembly of claim 1, further comprising a preloaded spring section in the gimbal assembly that presses against the ball and socket joint and is positioned to press against the ball and socket joint when a force is applied and assist in retaining the position of the ball and socket joint when the force is removed

4. The gas turbodrill assembly of claim 1, further comprising a clamp coupling assembly enclosing the connection of the gearbox assembly output shaft and the flexible gimbal assembly shaft.

5. The gas turbodrill assembly of claim 1, further comprising a spring support section in the gimbal assembly positioned to tension against the flexible gimbal assembly shaft.

6. The gas turbodrill assembly of claim 1, further comprising a drill-bit assembly having one or more fixed cutters.

7. The gas turbodrill system of claim 1, further comprising a drill-bit assembly having a plurality of surface set diamond bits.

8. The gearbox assembly of claim 1, further comprising a planetary gear transmission.

9. The gas turbodrill system of claim 1, further comprising: a whipstock positioned to guide the gas turbodrill assembly on a lateral exit path, the whipstock having a ramp connected to a spring, the ramp and spring positioned to exert force against the gas turbodrill assembly at a pinch point near an upper end of the whipstock, thereby providing additional force to rotate the ball and socket joint of the gimbal assembly and bend the flexible shaft.

10. A gas turbodrill system including a gas turbodrill assembly, the gas turbodrill assembly comprising: a gas turbine assembly configured to couple to a source of pressurized fluid;a gimbal assembly that comprises a hollow ball and socket joint that extends around a flexible shaft, the flexible shaft configured to couple to the output shaft of the gas turbine, the flexible shaft extending through the hollow ball and socket joint;a gearbox assembly configured to couple to the flexible shaft, the gearbox assembly having an output shaft, the gearbox assembly being configured to rotate the output shaft at a lower rate than the flexible shaft; anda drill-bit assembly connected to the gearbox assembly, the drill-bit assembly having a shaft connected to the gearbox assembly output shaft.

11. The gas turbodrill assembly of claim 10, further comprising a clamp coupling assembly enclosing the connection of the gas turbine output shaft and the flexible shaft.

12. The gimbal assembly of claim 10, further comprising at least one spring that presses a lock ring with a spherical seat against the ball of the ball and socket joint, the lock ring positioned to lock the position of the ball and socket joint when a pre-determined force is applied and configured to retain the locked position of the ball and socket joint when force is removed

13. The gas turbodrill assembly of claim 10, further comprising a preloaded spring section in the gimbal assembly that presses against the ball and socket joint and is positioned to press against the ball and socket joint when a force is applied and assist in retaining the position of the ball and socket joint when the force is removed.

14. The gas turbodrill assembly of claim 10, further comprising a drill-bit assembly having one or more fixed cutters.

15. The gas turbodrill assembly of claim 10, further comprising a drill-bit assembly having a plurality of surface set diamond cutters.

16. The gearbox assembly of claim 10, further comprising a planetary gear transmission.

17. The gas turbodrill system of claim 10, further comprising: a whipstock positioned to guide the gas turbodrill assembly on a lateral exit path, the whipstock having a ramp connected to a spring, the ramp and spring positioned to exert force against the gas turbodrill assembly at a pinch point near an upper end of the whipstock, thereby providing additional force to rotate the ball and socket joint of the gimbal assembly and bend the flexible shaft.

18. A method of drilling a spur lateral well with a gas turbodrill assembly, comprising the steps of: running a whipstock into the well;positioning the whipstock lateral exit path using a wireline;running a steerable gas turbodrill assembly into the well until it reaches the whipstock;applying force downwardly on the steerable gas turbodrill causing the gas turbodrill to press against the whipstock, thereby rotating a ball and socket joint and bending a flexible shaft housed in a gimbal assembly of the steerable gas turbodrill.

19. The method of drilling a spur lateral well as described in claim 18, further comprising the step of: applying force downwardly on the steerable gas turbodrill with sufficient force to cause the gimbal assembly flexible shaft to bend and substantially lock in place.

20. The method of drilling a spur lateral well as described in claim 18, further comprising the steps of: applying force downwardly on the steerable gas turbodrill causing a ramp and spring of the whipstock to exert force against the gas turbodrill assembly near a pinch point on an upper end of the whipstock, thereby providing additional force to rotate the ball and socket joint of the gimbal assembly and bend the flexible shaft.