Downhole anchoring device

Abstract

There is disclosed an apparatus for axially anchoring a tool downhole in a well casing, the apparatus comprising: an anchor configured to be disposed in, and actuatably and rotatably engaged with, the casing; and a rotatable tool being configured to be disposed within the casing; whereby the anchor, when rotatably engaged with the casing, prevents movement of the rotatable tool in the axial direction A whilst allowing for rotation of the tool about the axial direction. A corresponding method of anchoring a tool downhole is also disclosed.

Claims

1. An apparatus for axially anchoring a tool downhole in a well casing, the apparatus comprising: an anchor configured to be disposed in, and actuatably and rotatably engaged with, the casing; and a rotatable tool being configured to be disposed within the casing; whereby the anchor, when rotatably engaged with the casing, prevents movement of the rotatable tool in the axial direction whilst allowing for rotation of the tool about the axial direction.

2. The apparatus of claim 1, wherein the rotatable tool is connected below the anchor in the axial direction.

3. The apparatus of claim 1 or 2, wherein the anchor and the rotatable tool are rotationally locked to one another.

4. The apparatus of any preceding claim, wherein the anchor is arranged to be actuated into rotatable engagement with the casing by a fluid, preferably drilling mud.

5. The apparatus of any preceding claim, wherein the anchor comprises a substantially hollow body.

6. The apparatus of any preceding claim, wherein the anchor comprises a plurality of extendable casing engagement blocks.

7. The apparatus of claim 6, wherein the blocks are actuatable from a retracted position in which the blocks are substantially housed within the body to an actuated position in which the blocks extend radially from the body.

8. The apparatus of claim 6 or 7, wherein the plurality of blocks are disposed circumferentially around the anchor in at least one plane perpendicular to the axial direction, each plane comprising at least two blocks.

9. The apparatus of any of claims 6 to 8, wherein the plurality of blocks are disposed in more than one plane perpendicular to the axial direction, and preferably in at least two planes perpendicular to the axial direction.

10. The apparatus of any of claims 7 to 9, wherein the blocks are biased towards the retracted position.

11. The apparatus of any of claims 7 to 10, wherein the body comprises at least one opening through which the plurality of blocks extends through in the actuated position.

12. The apparatus of claim 11, wherein a plurality of openings is provided, and wherein there is preferably one opening for every block of the plurality of blocks.

13. The apparatus of any of claims 6 to 12, wherein the blocks comprise at least one engagement wheel rotatably mounted thereto in a plane perpendicular to the axial direction, the at least one wheel being configured to rotatably engage with the inner surface of the casing.

14. The apparatus of claim 13, wherein each of the at least one wheels on each of the plurality of blocks has a knife-edge circumference arranged to cut into the surface of the casing.

15. The apparatus of claim 13 or 14, wherein each of the at least one wheels on each of the plurality of blocks has a circular outer circumference.

16. The apparatus of claims 13 to 15, wherein each of the wheels is formed of high carbon steel or tungsten carbide.

17. The apparatus of any of claims 13 to 16, wherein the engagement of the wheels with the casing forms a groove in the casing, whereby the wheel may freely rotate against the casing in a plane of the groove whilst preventing movement of the wheels in the axial direction.

18. The apparatus of any of claims 13 to 17, wherein a wheel on each of the plurality of blocks is axially aligned with at least one other wheel on another block.

19. The apparatus of any preceding claim, wherein the rotatable tool is a casing cutter for cutting through the casing.

20. The apparatus of claim 19, wherein the casing cutter comprises a plurality of blades, the blades being hydraulically actuatable into contact with the casing.

21. The apparatus of claim 20, wherein the blades comprise high carbon steel, or tungsten carbide.

22. A drillstring comprising the apparatus of any preceding claim, wherein the drillstring extends downwardly from a rig or vessel, and wherein rotation of the drillstring drives the apparatus into simultaneous rotation.

23. The drillstring of claim 22, configured to allow fluid to pass down therethrough to engage the apparatus.

24. The drillstring of claim 22 or 23, further comprising a length compensating device (LCD) disposed above the apparatus in the axial direction.

25. The drillstring of claim 24, wherein the LCD is a bumper sub.

26. The drillstring of claim 24 or 25, further comprising a controller associated with the LCD, the controller being configured to alter the length of the LCD.

27. The drillstring of any of claims 22 to 26, wherein the drillstring is connected to a surface heave compensator disposed at the rig or vessel.

28. A method of anchoring a tool downhole within a casing of a subsea oil well comprising the steps of: providing: a drillstring extending downwardly from a rig or vessel; an actuatable anchor rotatably mounted on the drillstring; and a rotatable tool disposed on the drillstring; locating the tool in a desired position downhole within the casing; actuating the anchor into rotatable engagement with the casing; and actuating the tool; whereby the anchor prevents axial movement of the actuated tool.

29. The method as claimed in claim 28, wherein the tool is a casing cutter.

30. The method as claimed in claim 29, further comprising extending the casing cutter into rotational contact with the casing whilst allowing the casing cutter to rotate about the axial direction so as to cut through a depth of the casing.

31. A method of cutting a casing of a subsea oil well comprising the steps of: providing the drillstring of any of claims 22 to 27; lowering the drillstring from the rig or vessel to align the anchor and the casing cutter in a desired position within the casing; and passing a fluid down the drillstring to activate the anchor and the casing cutter, wherein, the fluid actuates the anchor into rotatable engagement with the casing and the casing cutter into contact with the casing such that the anchor prevents movement of the casing cutter in the axial direction whilst allowing the casing cutter to rotate about the axial direction to cut through a depth of the casing in a radial plane perpendicular to the axial direction.

32. The method of claim 31, further comprising the use of a length compensation device controlled by a controller.

33. An apparatus for cutting a casing of a subsea oil well, the apparatus comprising: a drillstring extending downwardly from a rig or vessel; an anchor rotatably mounted on the drillstring and configured to be disposed in, and rotatably and actuatably engaged with, the casing; and a rotatable casing cutter disposed on the drillstring and configured to be disposed within and cut through the casing; whereby the anchor, when rotatably engaged with the casing, prevents movement of the casing cutter in the axial direction whilst allowing for rotation of the casing cutter about the axial direction.

Description

[0040] Certain embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

[0041] FIG. 1 is a schematic view of a rig or vessel carrying out an exemplary downhole operation in a subsea well;

[0042] FIG. 2 is an enlarged schematic view of a distal end portion of a drillstring, disposed within a casing of a subsea well, that incorporates an anchor of a first embodiment used in a P&A or slot recovery operation;

[0043] FIG. 3 shows a close up view of an embodiment of an anchor disposed on the drillstring;

[0044] FIG. 4 shows a close up view of another embodiment of the anchor disposed on the drillstring;

[0045] FIGS. 5A-5D are sequential enlarged schematic views of a distal end portion of a drillstring during the various stages involved in cutting of a casing in a P&A or slot recovery operation as is known from the prior art.

[0046] FIG. 1 illustrates a floating rig or vessel 3 carrying out a downhole operation. Specifically, FIG. 1 depicts a cutting process on a casing 5 of a subsea well that is typically done in a P&A or slot recovery operation. The rig or vessel 3 is positioned at sea level 1 in a position that is approximately axially aligned with the casing 5 extending from the seabed 2. A drillstring 4 extends down from the rig or vessel 3 to the casing 5 and is partly received within a casing 5 of the subsea well, such that the distal end portion 10 is disposed hundreds of metres downhole within the casing 5 (see FIG. 2). The distal end portion 10 is the portion of the drillstring predominantly responsible for cutting the casing 5 and will be described further below with reference to FIGS. 2-4.

[0047] It can clearly be seen in FIG. 1 how the effects of heave at the rig or vessel 3 due to surface waves would lead to an axial displacement of the drillstring 4. In FIG. 1 the surface of the sea 1 is depicted as level and, at the particular moment in time depicted in FIG. 1, there is no requirement for any length compensation to be provided to the distal end portion 10 of the drillstring 4. However, a surface heave compensator (not shown) is provided at the rig or vessel 3 to counteract some of the potential axial displacement of the drillstring 4 that may be caused by the heave of the rig or vessel 3. The remainder of the length compensation that is required to counteract the potential effects of heave felt by the drillstring 4 is provided by flexing or stretching of the drillstring 4 and/or elements contained in the distal end portion 10, which are described in more detail below.

[0048] Turning now to FIG. 2, the distal end portion 10 of the drillstring 4 of FIG. 1 is shown. The distal end portion 10 is disposed downhole within a section of the casing 5. The distal end portion 10 comprises a casing cutter 11, an anchor 12 and a length-compensating device (LCD) 13.

[0049] The LCD 13 is located above the anchor 12 and the casing cutter 11 on the drillstring 4 and it provides length compensation to both the anchor 12 and the casing cutter 11 such that when the drillstring 4 is caused to move upward in an axial direction A due to the effects of heave felt by the rig or vessel 3 at the surface, the LCD 13 extends in length along the axial direction A so that the anchor 12 and the casing cutter 11 experience no, or very little, displacement in the axial direction A. Following any upward motion of the rig or vessel 3 along the axial direction due to heave, there will be a consequential downward motion of comparable magnitude of the rig or vessel 3. During said downward motion the LCD 13 will contract from its extended length to ensure that the anchor 12 and casing cutter 11 again experience little, or no, displacement along the axial direction A. Thus, the LCD 13 provides some of the required length compensation to ensure that, regardless of the heave of the rig or vessel 3, the anchor 12 and the casing cutter 11 are maintained approximately, or precisely, in the same position relative to the axial direction A.

[0050] In the embodiments depicted herein the LCD 13 is a bumper sub. The bumper sub not only compensates for movement of the drillstring 4 in an axial direction A due to the upward and downward heave caused by typical surface waves, it also compensates for downward movement of the drillstring 4 due to a sudden downward displacement of the rig or vessel 3 below the resting sea level, for instance when heavy cargo is placed on the rig or vessel 3. This additional length compensation is achieved by the ability of the bumper sub to contract from its resting length along the axial direction A. This contraction in length of the bumper sub can accommodate for some, or all, of the downward displacement of the drillstring in the axial direction A caused by a downward displacement of the rig or vessel 3. Thus, as well as providing length compensation to the anchor 12 and the casing cutter 11 when the drillstring 4 is displaced in the axial direction A due to the effects of heave, the bumper sub also limits or prevents displacement of both the anchor 12 and the casing cutter 11 along the axial direction A when the drillstring 4 is displaced suddenly downward in the axial direction A.

[0051] As can also be seen in FIG. 2, the casing cutter 11 on the drillstring 4 is positioned below both the LCD 13 and the anchor 12. In the embodiment of FIG. 2, the cutter 11 has a plurality of blades 16, the blades 16 being formed from a material that is harder than the casing 5, e.g. a high carbon steel or tungsten carbide. The casing cutter 11 of the depicted embodiments is a hydraulically actuated cutter. It is configured such that when drilling mud is passed down through the drillstring 4 toward the distal end portion 10, the relatively high fluid pressure within the drillstring 4 forces the blades 16 of the casing cutter 11 radially outward from the drillstring 4 and into contact with an inner circumference of the casing 5. Thus, when the casing cutter is driven into rotation due to rotation of the drillstring 4, the engagement of the casing cutter 11 with the casing 5 enables the blades 16 to cut through the casing 5.

[0052] The anchor 12 is positioned on the drillstring 4 in a position below the LCD 13 but above the casing cutter 11. As can be seen in more detail in FIG. 3, the anchor 12 comprises a predominantly hollow body 21 that houses a plurality of extendable blocks 22. Hollow body 21 is in fluid communication with the drillstring 4 disposed above the anchor 12 and is also in fluid communication with the casing cutter 11 disposed below the anchor 12. Thus, when drilling mud flows down the drillstring 4, towards the distal end portion 10, the drilling mud hydraulically engages both the anchor 12 and the casing cutter 11 simultaneously. Hence, the anchor 12 and the casing cutter 11 are hydraulically actuated simultaneously.

[0053] The blocks 22 are movable between a retracted position, in which the blocks 22 are predominantly housed within the body 21, and an actuated position in which the blocks 22 are extended radially from the body 21 through openings 23 as is shown in FIG. 3. The blocks 22 are configured such that their movement is limited between the retracted position and the actuated position, and they are biased toward the retracted position. The blocks 22 are actuated by drilling mud that is passed down the drillstring 4 which causes the blocks 22 to extend against their bias into their actuated position.

[0054] In the embodiment depicted in FIG. 3, two blocks 22 are shown disposed diametrically opposed to one another about the anchor 12 to form a single axial row of blocks 22. Each block 22 is aligned with an opening 23 in the body 21 that allows for the blocks 22 to move from the retracted position to the actuated position.

[0055] As shown in FIG. 3, each block 22 has mounted thereon a set 25 of wheels 24, each set 25 comprising two wheels 24. Each wheel 24 is disposed in a plane perpendicular to the axial direction A and, in the present embodiment, each wheel 24 is axially aligned with one other wheel 24 on the block 22 diametrically opposed to the block 22 on which it is disposed. Those wheels 24 that share a common plane are said to form a plane of wheels 26.

[0056] The wheels 24 are rotatably supported on the blocks 22 in a manner that allows for rotation of each wheel 24 about its axis. The ability of the wheels 24 to rotate about their own axis means that the wheels 24 provide bearing support to the anchor 12 when disposed downhole within the casing 5 in the manner of roller bearings. The wheels 24 are formed from a material that is harder than the steel of the casing 5, for instance tungsten carbide or high carbon steel and their outer circumference is a sharp knife-edge. Thus, when the blocks 22 are hydraulically actuated into engagement with the casing 5 they score a groove into the inner circumference of the casing 5. These grooves provide a track for wheels 24 that allows the wheels 24 to freely rotate around the inner circumference of the casing 5 in their own plane that is perpendicular to the axial direction A whilst inhibiting any movement of the wheel 24 in the axial direction A.

[0057] All of the components in the distal end portion 10, i.e. the LCD 13, anchor 12 and cutter 11, are rotationally locked to the drillstring 4. Thus, when the drillstring 4 is driven into rotation at the rig or vessel 3, the casing cutter 11, the anchor 12 and the LCD 13 are also driven into rotation about the axial direction A.

[0058] The process of cutting the casing 5 begins with alignment of the casing cutter 11 into a desired position along the axial direction A in the casing 5. This is achieved by lowering the drillstring 4 from the rig or vessel 3 until the casing cutter 11 is positioned as desired, downhole within the casing.

[0059] Once the cutter 11 is placed in the desired downhole position, the drillstring 4 is driven into rotation, which in turn drives rotation of the distal end portion 10. Jointly, or shortly afterwards, drilling mud is passed down the drillstring 4 toward the distal end portion 10. The drilling mud causes the blocks 22 of the anchor to extend from their retracted position through openings 23 and into their actuated position, thereby forcing the wheels 24 disposed on each block into engagement with the casing. Simultaneously, the drilling mud causes the blades 16 of the casing cutter 11 to extend radially outwards and into contact with the inner circumference of the casing 5.

[0060] As the anchor 12 rotates, the wheels 24 move about their own axes around the inner circumference of the casing 5 (i.e. they rotate counter-synchronously to the anchor 12). This provides the necessary bearing support to the anchor 12 within the casing 5 such that the rotation of the anchor 12 and the casing cutter 11 about the axial direction A occurs relatively freely. As it does so, each plane of wheels 26 scores a circumferential groove in the casing 5 in which each plane of wheels 26 is free to run. The depth of the groove that each plane of wheels 26 forms in the casing 5 depends upon the drilling mud pressure and the relative dimensions of the anchor 12 and casing 5. It will be appreciated that the engagement of each plane of wheels 26 with each groove allows for the anchor 12 and the casing cutter 11 to freely rotate about the axial direction A whilst also preventing movement of the anchor 12 in the axial direction A.

[0061] As a result, the blades 16 are maintained in a constant position in the axial direction relative to the casing during the cutting process. Cutting may therefore proceed until a section of the casing 5 is separated from the remainder thereof.

[0062] The cutting process may then be terminated by stopping rotation of the drillstring 4 and stopping the flow of drilling mud to the distal end portion 10 of the drillstring 4. As such, the blades 16 retract radially inward under the bias force and the blocks 22 also withdraw into their retracted position within the body 21 such that there is no longer any engagement of the wheels 24 or blades 16 with the casing 5. The drillstring 4 may then be removed upwardly along the axial direction A from the casing 5 and be recovered and the portion of the casing 5 that has been cut from the remainder of the casing 5 may be removed.

[0063] During the cutting process, if the rig or vessel 3 is caused to move in an axial direction A due to the effects of heave, the interaction of the wheels 24 with the grooves that are formed in the inner circumference of casing 5, in combination with the length compensation provided by the surface heave compensator, the internal flexibility of the drillstring 4 and/or the LCD 13, ensures that the casing cutter 11 remains in a constant position along the axial direction A. For instance, when the rig or vessel 3 undergoes upward heave while the anchor 12 is actuated into engagement with an inner circumference of casing 5, the distal end of the drillstring 4 is held in place by anchor 12 whilst the remainder of drillstring 4 will extend in length by means of an extension in the LCD 13 and/or by means of the surface heave compensator, along with a certain amount of extension of drillstring 4 due to its inherent flexibility. Thus, the casing cutter 11 is kept in a constant axial position for the duration of the upward heave of the rig or vessel 3 and cutting of the casing 5 can be maintained in a single plane even during upward heave of the rig or vessel 3.

[0064] Subsequently, after said upward heave, the rig or vessel 3 will undergo a corresponding downward heave, which is accommodated in a similar manner except that the relevant components contract. As such, the casing cutter 11 is kept in a constant axial position for the duration of the heave cycle of the rig or vessel 3.

[0065] In cases where the axial forces due to heave may be larger, it is beneficial to provide additional wheels 24 that allow for the anchor 12 to tolerate larger axial loads whilst maintaining a constant axial position. An embodiment having additional wheels is depicted in FIG. 4. This embodiment is the same as the first embodiment, except that it comprises four blocks 22 arranged in two rows. This provides additional wheels 24 that interact with the casing 5 and thereby increases the axial resistance of the anchor 12 when engaged with the casing 5.

[0066] As the anchor 12 and the cutter 11 are only ever actuated simultaneously or almost simultaneously by the drilling mud that is passed through the drillstring 4, if the rig or vessel 3 experiences an amount of heave that displaces the drillstring 4 along the axial direction A by an amount that falls outside of the limits that the surface heave compensator, the inherent flexibility of the drillstring 4 and/or the LCD 13 can tolerate, temporarily terminating the provision of drilling mud to the distal end portion 10 allows the anchor to disengage from the casing 5 and the cutter 11 will retract and stop cutting. This prevents damage to the drillstring 4, anchor 12 and cutter 11, whilst also ensuring that the quality of the cut in the casing 5 is maintained in a single plane. Once the axial displacement of the drillstring 4 is back within tolerable limits of length compensation, the process of cutting the casing 5 as outlined above may be resumed.