Patent Application
20110100715
Reverse-circulation, center-discharge drilling (RCCD) through concentric tubing is a proven method for minimizing formation damage while drilling producing formations, such as tight gas sand and coal bed methane. Because RCCD drilling returns cuttings through the inner diameter of a double-wall drill pipe, it does not expose the formation to possible damage from drilling fluid and cuttings.
This technique is accomplished with a concentric rotary drill string and a center discharge drill bit. A vacuum may be applied at the surface to reduce the bit face pressure to a level below the formation pore pressure, to further reduce the potential for formation damage; however, the vacuum assist from this approach is limited.
The deployment of concentric jointed tubing represents significant additional time and cost for drilling the well to completion. Concentric coiled tubing (CCT) can speed the deployment time, and allows continuous drilling operations in the producing formation. Drilling operations using coiled tubing requires a motor to turn the drill bit. Rotary drilling motors capable of operating on dry gas with a center discharge are not available.
It is generally desirable to operate a drill motor on dry gas for completion drilling of water sensitive formations. Progressive cavity motors incorporate elastomeric stators that degrade rapidly when operated on dry gas. Turbodrills are capable of operation on gas, but these tools stall easily when operated on gas, and the motor speed is generally much too high for effective drilling. These motors also tend to be very long, which limits steerability. A previous attempt to develop a gas turbine motor for drilling application involved the use of a multi-stage planetary gear, to increase torque and reduce the speed, to drive a conventional roller cone drill bit. The relatively high cost and complexity of the multistage planetary gearbox prevented commercial acceptance of that design. Further, the transmission employed in that design was not suited for a center discharge passage.
It would be desirable to provide a compact, steerable gas turbine motor and a speed reduction transmission suitable for RCCD drilling, capable of providing the speed and torque required for drilling with conventional roller cone or polycrystalline diamond compact (PDC) bits.
This application specifically incorporates by reference the disclosures and drawings of each patent application and issued patent identified above as a related application.
A first aspect of the concepts disclosed herein is a drill tool including a compact, steerable gas turbine motor and a speed reduction transmission capable of providing the speed and torque required for drilling with conventional roller cone or polycrystalline diamond compact (PDC) bits. Significantly, the concepts disclosed herein combine a relatively high speed turbine with a relatively compact differential planetary gear transmission capable of providing a significant speed reduction ratio. High speed operation of the turbine section allows efficient mechanical power generation in a relatively short turbine. The differential planetary gear transmission offers high speed reduction ratio in a short package relative to multistage planetary gears. Thus, the concepts disclosed herein enable a compact drill tool to be provided. Compactness is important if one desires to steer the tool, as the turning radius increases as the tool lengthens. In an exemplary, but not limiting embodiment, a drill tool combining a gas turbine and compact differential planetary gear transmission will have a diameter of about 3.75″ and a length of about 48″, which allows the tool to be mounted on a bent housing for steering applications.
The transmission employs multistage differential planetary gears, configured to accommodate a center discharge passage along a central axis of the transmission, which is in fluid communication with a similar center discharge passage in the turbine, which couples in fluid communication with an inner tube in a concentric tubing drill string or coiled tube drill string. The transmission includes an upper sun gear coupled to an output shaft of the gas turbine motor, a lower sun gear coupled to the output shaft of the gas turbine motor, an upper spider assembly rotatably supporting a plurality of upper planet gears, a lower spider assembly rotatably supporting a plurality of lower planet gears, and a ring gear circumferentially engaging the planetary gears. The upper spider assembly is fixed in position (i.e., is fixedly attached to a housing of the tool), such that rotation of the upper sun gear results in the rotation of the ring gear at a reduced speed. A diameter of the lower sun gear is different than a diameter of the upper sun gear, and the diameters of the lower planetary gears are also different than the diameters of the upper planetary gears, such that the lower spider assembly rotates at a further reduced speed. In at least one embodiment, the transmission enables a speed reduction ratio and torque ratio of about 32:1 to be achieved.
A second aspect of the concepts disclosed herein is the incorporation of a flow restriction element in the drill tool defined above, the flow restriction element providing a mechanism to increase a density of the gas in the turbine section, which results in reducing a rotational speed of the turbine output shaft, providing an additional speed reduction capability. In an exemplary but not limiting embodiment, the flow restriction element is a port in an outer housing of the tool disposed below the turbine section, the port being coupled in fluid communication with the wellbore. In most cases of reverse circulation drilling the wellbore is sealed, so that the only flow path for the gas discharged from the flow restriction port is though the central passage in the bit, and upward through the central passage in the transmission and turbine. The flow restriction element can be sized to control the motor speed. If desired, the flow restriction may be ported to the bottom of the assembly to provide better bit cleaning. If the borehole is not sealed, the flow restriction port can be sealed. In an exemplary embodiment, the flow restriction port is reconfigurable, such that the tool (i.e., the tool comprising the turbine, the differential planetary transmission, and the flow restriction port) can be removed from the wellbore to modify the flow restriction port, enabling the drill speed achieved by the tool to be modified to suit a particular wellbore application.
A third aspect of the concepts disclosed herein is the incorporation of a venturi into the center discharge volume, to provide vacuum assist to reduce bottom hole pressure. Reducing bottom hole pressure below formation pressure, and vacuuming cuttings though the center of the bit, prevents fine cuttings from contacting the formation and prevents damage to wellbore permeability. In an exemplary, but not limiting embodiment, a center discharge drill bit coupled to the center discharge tool (i.e., the tool comprising the turbine, the differential planetary transmission and the venturi) is equipped with a skirt to direct flow entrained by the venturi around the cutters of the bit. In an exemplary, but not limiting embodiment, the venturi is implemented using a removable tubular venturi element fitted to the center discharge volume, such that the venturi can be reconfigured (or eliminated) by replacing or removing the tubular venturi element. Gas discharged from the turbine and routed around the differential planetary transmission is used to generate a Coanda-effect venturi capable of generating the desired pressure differential between the bit face and inlet to the inner return line of the concentric tubing (i.e., the center discharge volume). The venturi, in addition to generating the vacuum assist, also functions as a flow restriction element, increasing a gas density in the turbine and reducing turbine speed.
In a related embodiment, the central discharge volume in the tool is plugged, and turbine and differential planetary gear transmission discussed above are used to energize a non-center discharge drill bits, and cuttings are retrieved at the surface using the annulus between the tool and the borehole.
This Summary has been provided to introduce a few concepts in a simplified form that are further described in detail below in the Description. However, this Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
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:
Exemplary embodiments are illustrated in referenced Figures of the drawings. It is intended that the embodiments and Figures disclosed herein are to be considered illustrative rather than restrictive. No limitation on the scope of the technology and of the claims that follow is to be imputed to the examples shown in the drawings and discussed herein. Further, it should be understood that any feature of one embodiment disclosed herein can be combined with one or more features of any other embodiment that is disclosed, unless otherwise indicated.
The center discharge gas turbine tool of
The rotation of the turbine shaft is transmitted through differential planetary gear transmission B, which increases torque and slows the rotation rate to a level that is useful for drilling with a roller cone bit 24. Other bit types may also be used with the concepts disclosed herein. A distal portion of turbine shaft 9 extends into differential planetary gear transmission. An upper sun gear 17 and a lower sun gear 20 rotatingly engage the turbine shaft. Note that the hollow center of the turbine shaft (which forms part of the central discharge volume) enables gas diverted distal of the differential planetary gear transmission to flow from a distal portion of the housing to a proximal portion of the housing through the central discharge volume. Upper sun gear 17 engages upper planet gears 16 (which are rotatably supported by upper shafts 15 in upper spider 14; noting that upper spider 14 is a planet gear carrier, and in an exemplary embodiment, each upper planet gear is the same size), which in turn engage an outer ring gear 18. Upper sun gear 17 thus functions as an input (being drivingly rotated by the turbine shaft). Upper spider 14 is fixed to housing 5, so outer ring gear 18 rotates with a lower speed and greater torque relative to the input provided by the turbine shaft.
A further speed reduction and torque increase is provided by the lower portion of the differential planetary gear transmission. The lower portion of the differential planetary gear transmission includes a lower spider 21, which rotatingly supports lower planet gears 19 (via lower shafts 22; noting that lower spider 21 is a planet gear carrier, and in an exemplary embodiment, each lower planet gear is the same size), and lower sun gear 20 (which is drivingly rotated by the turbine shaft). Lower planet gears 19 engage both outer ring gear 18 and lower sun gear 20. Significantly, upper sun gear 17 and lower sun gear 20 have different diameters, as do upper planet gears 16 and lower planet gears 19. The differential sizes of the sun gears and the planet gears, and the motion of the lower planet gears due to the rotation of the turbine shaft and the outer ring gear, results in the rotation of lower spider 21 at a lower speed and greater torque relative to outer ring gear 18 (and to an even greater extent, the turbine shaft), providing the further speed reduction and torque increase. Those skilled in the art will recognize that the size and number of teeth on the gears may be selected so that the lower spider rotates at much lower speed and is driven at much higher torque than the turbine shaft. In an exemplary but not limiting embodiment, the differential planetary gear transmission provides a speed reduction ratio and torque ratio of about 32:1. Exemplary, but not limiting gear dimensions are provided in Table 1.
Further details of the differential planetary gear transmission B are shown in
Upper spider 14 and lower spider 21 are shown in
Referring once again to
Lower spider 21 (which provides the output of the differential planetary gear transmission) is fixed to a coupling 23, which is supported by radial bearings 25, so that coupling 23 is free to rotate relative to turbine housing 5. Roller cone drill bit 24 is attached to coupling 23, enabling the output of the differential planetary gear transmission to be used to drive the bit. Although a roller cone bit is shown in the Figures, those skilled in the art will recognize that other open-flow center-discharge bit types may be used. Note that coupling 23 also includes an axial volume 37 that is coupled in fluid communication with the hollow axial portion of the turbine shaft, extending the central discharge volume to the bit, which itself includes an axial volume 39, which in turn extends the central discharge volume to a bit face 36, enabling cuttings and debris from the bit face to be placed in fluid communication with return line 4 of the concentric tubing. Thus, it should be understood that the center discharge volume coupling bit face 36 to return line 4 of the concentric tubing includes the hollow turbine shaft, axial volume 37 in coupling 23, and axial volume 39 in bit 24.
Gas exhausted from turbine section A passes around the differential planetary gear transmission through turbine exhaust pressure passages 30. A portion of the exhaust gas may be exhausted into an annulus 35 between the housing and the borehole in which the tool is disposed through a flow restriction 26. The remaining exhaust gas flow is ported through passages 31 to an annular gap 32, between a bottom of turbine shaft 9 and coupling 23. Note that annular gap 32 also forms a flow restriction. The combined area of annular gap 32 and flow restriction 26 can be sized to increase the discharge pressure of the turbine, which increases the discharge gas density, and provides additional speed control over the turbine (i.e., speed control beyond that provided by the differential planetary gear transmission).
Significantly, annular gap 32 defines a primary jet of a Coanda-effect venturi capable of generating a pressure differential between the bit face and inner return line 4 of the concentric tubing. The annular primary gas jet entrains secondary gas and cuttings from bit face 36 though axial volume 37 in coupling 23. The primary and secondary flows are mixed in a mixing duct 33, imparting momentum to the flow. The mixed flow momentum is recovered in a diffuser section 34 to maintain pressure in return line 4 to pump gas and cuttings to surface. In an exemplary embodiment, mixing duct 33 and diffuser section 34 are formed by tubular inserts placed into a distal end of the hollow turbine shaft, although if desired they can be formed integrally into the turbine shaft. The use of inserts is somewhat preferred, as inserts can be removed and replaced to enable changes to the mixing and diffusing to be implemented. In an exemplary, but not limiting embodiment, a replaceable tube 41 is used to form the inner diameter of gap 32. Tubes of different diameters can be installed to adjust the flow area of gap 32. The gap dimension can be minimal, in which case, the venturi effect is eliminated.
The venturi feature provides a vacuum assist to reduce bottom hole pressure. By reducing bottom hole pressure below the formation pressure, and vacuuming cuttings though the center of the bit, fine cuttings are prevented from contacting the formation and possibly damaging wellbore permeability. In a preferred embodiment, center discharge roller cone drill bit 24 is equipped with a skirt 38 to direct flow entrained by the venturi around cutters 43 of the bit. In a sealed borehole, the area ratio between flow restriction 26 and annular gap 32 determines the ratio of entrained secondary gas to primary. If the borehole is not sealed, flow restriction 26 can be plugged. The venturi will entrain gas from the formation or from the wellhead to clean the cuttings from the face of the bit. To reiterate, the primary gas stream is exhaust gas from the turbine flowing in passages 30 through annular gap 32 into the center discharge volume. The secondary gas stream is from exhaust gas exiting flow restriction 26, moving around the bit, and up into the center discharge volume through the axial volumes in the bit and coupler.
In another embodiment of the concepts disclosed herein shown in
In another embodiment of the concepts disclosed herein shown in
In each embodiment, the relative sizes of flow restriction 26 and/or flow restriction 40 can be modified to change a magnitude of the speed reduction for the turbine. The larger the sum of the venturi and gas port flow area, the faster the turbine will run. The gas port (i.e., flow restriction 26 and/or flow restriction 40) allows independent adjustment of the flow capacity. The venturi is effective over a relatively narrow range of flow ratios (i.e., the secondary flow can only be about 10% to about 30% of the total before the venturi looses effectiveness). In some embodiments, the users can remove the tool from the bore hole and change the size of the flow restrictions in the field.
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 within the scope of the claims that follow. Accordingly, it is not intended that the scope of these concepts in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.
This application is based on a prior copending provisional application Ser. No. 61/256,211, filed on Oct. 29, 2009, the benefit of the filing date of which is hereby claimed under 35 U.S.C. §119(e).
| Number | Date | Country | |
|---|---|---|---|
| 61256211 | Oct 2009 | US |
