DOWNHOLE TOOL AND METHODS

Abstract

A tool (1) for cementing an annulus in a subsea oil or gas well, and methods for using the tool are provided. The tool includes a safety module (2) providing fluid communication between an umbilical and a perforation and circulation module (4) mounted below it. The safety module (2) includes a mechanical lock for connection to a well head. The perforation and circulation module includes upper and lower seals (6, 8) for sealing to the inner surface of a casing; an upper perforating device (14) mounted between the seals; a lower perforating device (16) mounted below the lower seal (8); and supply (S) and return (R) fluid flow paths for circulating fluid from the safety module (2). A diversion means (20) is provided in the supply fluid flow path, operable to redirect fluid supplied to the supply fluid flow path to a space defined between the upper and lower seals (6, 8).

Claims

1. A tool for cementing an annulus in a subsea oil or gas well, the tool comprising: a safety module and a perforation and circulation module; wherein the safety module provides fluid communication between an umbilical and the perforation and circulation module, and includes a mechanical lock for connection into engagement to a well head in use; and wherein the perforation and circulation module is mounted below the safety module, and comprises: i) upper and lower seals for sealing to the inner surface of a casing inside a wellbore; ii) at least one upper perforating device, mounted between the upper and lower seals, for perforating casing; iii) at least one lower perforating device, mounted below the lower seal, for perforating wellbore casing; iv) a supply fluid flow path to supply fluid from the safety module through the upper and lower seals to below the perforation and circulation module; v) a return fluid flow path from between the upper and lower seals to the safety module; and vi) a diversion means in the supply fluid flow path, operable to redirect fluid supplied to the supply fluid flow path to a space defined between the upper and lower seals.

2. The tool of claim 1 wherein the diversion means comprises a valve in the supply fluid flow path, operable to redirect fluid supplied to the supply fluid flow path to the space in-between the upper and lower seals.

3. The tool of claim 1 wherein the mechanical lock is formed and arranged to make a clamping engagement to an inside surface of a well head.

4. The tool of claim 1 wherein the mechanical lock is formed and arranged to make a clamping engagement to an outside surface of a well head.

5. The tool of claim 1 wherein the mechanical lock comprises a plurality of dogs, distributed circumferentially about a surface of the tool and operable, by means of an axially moving cam ring, to move outwards to engage an inner surface of a well head.

6. The tool of claim 1 wherein the mechanical lock comprises a plurality of dogs, distributed circumferentially about a surface of the tool and operable to move inwards by means of an axially moving cam ring to engage an outer surface of a well head.

7. The tool of claim 5 wherein the plurality of dogs is operated by an axially moving cam ring powered by a hydraulic system.

8. The tool of claim 7 further comprising a secondary unlocking arrangement for unlocking the mechanical lock, the secondary unlocking arrangement comprising a separate hydraulic system, or a separate part of a hydraulic system, from that normally employed to operate the axially moving cam ring.

9. The tool of claim 1 wherein the safety module includes a disconnection system for disconnection and reattachment of an umbilical to the tool without breakage of component parts.

10. The tool of claim 9 wherein the disconnection system of the safety module comprises a disconnection part and a base part; and wherein the disconnection part is for connection to an umbilical and has a quick release coupling for connecting to the base.

11. The tool of claim 10 wherein the quick release coupling is hydraulically operable via the umbilical.

12. The tool of claim 10 wherein the quick release coupling comprises a plurality of dogs, distributed circumferentially about a surface of the disconnection part and operable to move inwards to engage an outwards directed surface of the base.

13. The tool of claim 12 wherein the plurality of dogs of the quick release coupling are operated by an axially moving cam ring.

14. The tool of claim 13 wherein the axially moving cam ring and dogs are formed so that on axial movement of the cam ring to disengage the dogs, the cam ring engages with a hook feature on each of the dogs to positively hold the dogs in the disengaged state.

15. The tool of claim 10 wherein the quick release coupling comprises a plurality of dogs, distributed circumferentially about a surface of the disconnection part and operable to move outwards to engage an inwards directed surface of the base.

16. The tool of claim 15 wherein the plurality of dogs of the quick release coupling are operated by an axially moving cam ring.

17. The tool of claim 15 wherein the axially moving cam ring and dogs are formed so that on axial movement of the cam ring to disengage the dogs, the cam ring engages with a hook feature on each of the dogs to positively hold the dogs in the disengaged state.

18. The tool of claim 11 wherein the safety module includes one or more alignment features on at least one of the disconnection part and base part.

19. The tool of claim 13 wherein the safety module includes a secondary disconnection arrangement comprising a disconnection ring operable to move the axially moving cam ring of the quick release coupling.

20. The tool of claim 1 wherein the safety module comprises: a disconnection system for disconnection and reattachment of an umbilical to the tool without breakage of component parts, said disconnection system including a disconnection part for connection to an umbilical and a base part including the mechanical lock, wherein the disconnection part has a quick release coupling for connecting to the base; wherein the mechanical lock is formed and arranged to make a clamping engagement to an inside surface of a well head and comprises a plurality of dogs, distributed circumferentially about a surface of the tool and operable, by means of an axially moving cam ring, to move outwards to engage an inner surface of a well head; and wherein the tool further comprises a secondary unlocking arrangement for unlocking the mechanical lock, the secondary unlocking arrangement comprising a separate hydraulic system, or a separate part of a hydraulic system, from that normally employed to operate the axially moving cam ring.

21. The tool of claim 20 wherein the base part of the safety module includes a connection that allows fluid access to the interior of the tool, to operate the secondary unlocking arrangement when the disconnection part is separated from the base part.

22. The tool of claim 21 wherein the connection that allows fluid access accepts a drill pipe end for transmission of fluid to operate the secondary unlocking arrangement.

23. The tool of claim 20 wherein a bursting disc or other mechanism that opens under the application of excess pressure is used to provide access to the separate hydraulic system or part of a hydraulic system.

24. The tool of claim 20 wherein an axially moving unlocking ring powered by fluid pressure of the separate hydraulic system, or the separate part of a hydraulic system, from that normally employed to operate the axially moving cam ring is employed to unlock the mechanical lock by disengaging the plurality of dogs.

25. The tool of claim 1 wherein the lower seal is a passive seal, operable without control from above.

26. The tool of claim 25 wherein both the upper and lower seals are passive seals.

27. The tool of claim 1 wherein the lower seal is an expandable or inflatable seal.

28. The tool of claim 1 wherein the supply and return fluid flow paths comprise a pipe in pipe arrangement within a mandrel that mounts the upper seal.

29. The tool of claim 2 wherein the valve in the supply fluid flow path is a sliding sleeve valve.

30. The tool of claim 29 wherein the sliding sleeve valve is hydraulically operated.

31. The tool of claim 2 wherein on operation the valve redirects all of the fluid flowing in the supply fluid flow path to between the upper and lower seals.

32. The tool of claim 2 wherein on operation the valve redirects a portion of the fluid flowing in the supply fluid flow path to between the upper and lower seals.

33. The tool of claim 1 wherein the tool is provided with a cement wiper system comprising a wiper plug detachably mounted below the lower seal of the perforation and circulation module.

34. The tool of claim 33 wherein the wiper plug is attached to the tool by a frangible connection.

35. The tool of claim 33 wherein the wiper plug is attached to the tool by a releasable connection.

36. The tool of claim 33 wherein the wiper plug comprises a passage therethrough forming part of the supply fluid flow path.

37. The tool of claim 36 wherein the passage through the wiper plug includes a seat for a ball, whereby a ball dropped into the seat allows pressure from fluid pumped down the supply fluid flow path to cause detachment of the wiper plug from the perforation and circulation module.

38. A method for cementing an annulus in a subsea oil or gas well, the method comprising: a) providing a tool in accordance with claim 1: b) deploying the tool, attached to an umbilical, from a surface vessel or rig mounted on the seabed, into the central casing of the well via a wellhead; c) operating the mechanical lock to connect the tool into sealing engagement with the well head; d) operating the at least one lower perforation device, to perforate wellbore casing, thereby allowing fluid communication with an annulus; e) operating the at least one upper perforating device to perforate wellbore casing between the upper and lower seals, thereby allowing fluid communication with the annulus; f) cleaning the annulus by at least one of: passing fluid from, and back to, the surface vessel or rig through the umbilical, the supply fluid flow path, into the annulus and returning via the return fluid flow path and the umbilical, and passing fluid from, and back to, the surface vessel or rig through the umbilical, the return fluid flow path, into the annulus and returning via the supply fluid flow path and the umbilical; g) charging the annulus with cement by: passing a charge of cement slurry followed by a tail fluid from the surface vessel or rig through the umbilical, the supply fluid flow path and into the annulus; or by passing a charge of cement slurry followed by a tail fluid from the surface vessel or rig through the umbilical, the return fluid flow path and into the annulus; h) allowing the cement to set; i) cleaning between the upper and lower seals by operating the diversion means in the supply fluid flow path to redirect fluid supplied to the supply fluid flow path to a space defined between the upper and lower seals, and at least one of: passing cleaning fluid from, and back to, the surface vessel or rig through the umbilical, the supply fluid flow path, through the diversion means and returning via the return fluid flow path and the umbilical, and passing cleaning fluid from, and back to, the surface vessel or rig through the umbilical, the return fluid flow path, through the diversion means and returning via the supply fluid flow path and the umbilical; and j) unlocking and removing the tool from the well head.

39. The method of claim 38 wherein the diversion means comprises a valve in the supply fluid flow path, operable to redirect fluid supplied to the supply fluid flow path to the space in-between the upper and lower seals.

40. The method of claim 38 wherein at least one of the upper and lower seals of the tool is an expandable or inflatable seal and the method comprises inflating or expanding the seal or seals into sealing engagement with well bore casing on deployment of the tool.

41. The method of claim 38 wherein the tool comprises a cement wiper system comprising a wiper plug detachably mounted below the lower seal of the perforation and circulation module and step g) is carried out by charging the annulus with cement by: passing a charge of cement slurry followed by a tail fluid from the surface vessel or rig through the umbilical, the supply fluid flow path and into the annulus with the wiper plug detaching from the perforation and circulation module in advance of the charge of cement slurry.

42. The method of claim 38 further comprising cementing the central casing of the well.

43. The method of claim 42 wherein a further charge of cement slurry is passed into the central casing and followed by a tail fluid.

44. The method of claim 43, wherein the diversion means comprises a valve in the supply fluid flow path, operable to redirect fluid supplied to the supply fluid flow path to the space in between the upper and lower seals, the method further comprising redirecting the bulk of the fluid passing down the umbilical, in advance of the further charge of cement slurry, by operating the valve in the supply fluid flow path; wherein the redirection is to between the upper and lower seals, thereby allowing fluid to pass down the umbilical and be redirected back up the umbilical via the return fluid flow path.

45. The method of claim 42 further comprising removing the tool from the wellbore as the further charge of cement slurry is delivered.

46. The method of claim 40, when the lower seal is an expandable or inflatable seal, further comprising cleaning between the upper and lower seals by: deflating or collapsing the lower seal from sealing engagement with well bore casing; and passing cleaning fluid from, and back to, the surface vessel or rig through the umbilical, the supply fluid flow path, and returning via a return fluid flow path including passing upwards in the wellbore casing past the deflated or collapsed lower seal.

47. The method of claim 38 further comprising at least one pressure test selected from the group consisting of: pressure testing the annulus after the operating the at least one lower perforation device; pressure testing between the upper and lower seals; pressure testing after cement has set in the annulus, and pressure testing following cementing the central casing of the well.

48. A method for cementing two annuli in a subsea oil or gas well having a first, inner annulus and a second, outer annulus, the method comprising: a) providing a tool in accordance with claim 1, wherein the tool includes: at least a second upper perforating device, mounted between the upper and lower seals, for perforating casing or tubing after the first annulus is sealed by cement.; and at least a second lower perforating device, mounted below the lower seal, for perforating casing or tubing after the first annulus is sealed by cement; b) deploying the tool, attached to an umbilical, from a surface vessel or rig mounted on the seabed, into the central casing of the well via a wellhead; c) operating the mechanical lock to connect the tool into sealing engagement with the well head; d) operating the at least one lower perforation device, to perforate wellbore casing, thereby allowing fluid communication with the first annulus; e) operating the at least one upper perforating device to perforate wellbore casing between the upper and lower seals, thereby allowing fluid communication with the first annulus; f) cleaning the first annulus by at least one of: passing fluid from, and back to, the surface vessel or rig through the umbilical, the supply fluid flow path, into the annulus and returning via the return fluid flow path and the umbilical, and passing fluid from, and back to, the surface vessel or rig through the umbilical, the return fluid flow path, into the annulus and returning via the supply fluid flow path and the umbilical; g) charging the first annulus with cement by: passing a first charge of cement slurry followed by a tail fluid from the surface vessel or rig through the umbilical, the supply fluid flow path and into the first annulus; or by passing a first charge of cement slurry followed by a tail fluid from the surface vessel or rig through the umbilical, the return fluid flow path and into the first annulus; h) allowing the cement to set; i) cleaning between the upper and lower seals by operating the diversion means in the supply fluid flow path to redirect fluid supplied to the supply fluid flow path to a space defined between the upper and lower seals, and at least one of: passing cleaning fluid from, and back to, the surface vessel or rig through the umbilical, the supply fluid flow path, through the diversion means and returning via the return fluid flow path and the umbilical, and passing cleaning fluid from, and back to, the surface vessel or rig through the umbilical, the return fluid flow path, through the diversion means and returning via the supply fluid flow path and the umbilical; j) operating the at least one second lower perforation device, to perforate wellbore casing through to the second annulus, thereby allowing fluid communication with the second annulus; k) operating the at least one second upper perforating device to perforate wellbore casing between the upper and lower seals, thereby allowing fluid communication with the second annulus; l) cleaning the second annulus by at least one of: passing fluid from, and back to, the surface vessel or rig through the umbilical, the supply fluid flow path, into the second annulus and returning via the return fluid flow path and the umbilical, and passing fluid from, and back to, the surface vessel or rig through the umbilical, the return fluid flow path, into the second annulus and returning via the supply fluid flow path and the umbilical; m) charging the second annulus with cement by: passing a second charge of cement slurry followed by a tail fluid from the surface vessel or rig through the umbilical, the supply fluid flow path and into the second annulus; or by passing a second charge of cement slurry followed by a tail fluid from the surface vessel or rig through the umbilical, the return fluid flow path and into the second annulus; n) allowing the cement to set; and o) unlocking and removing the tool from the well head.

49. The method of claim 48 wherein the diversion means comprises a valve in the supply fluid flow path, operable to redirect fluid supplied to the supply fluid flow path to the space in-between the upper and lower seals.

50. The method of claim 48 wherein the second charge of cement slurry delivered at step m) is calculated to be sufficient to fill both the second annulus to the desired extent and to fill the production casing up to a desired level below the lower seal of the tool.

51. The method of claim 48 wherein the first charge of cement slurry is delivered at step g) by passing the first charge of cement slurry followed by a tail fluid from the surface vessel or rig through the umbilical, the return fluid flow path and into the first annulus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0107] FIG. 1 shows in schematic elevation a tool for cementing an annulus;

[0108] FIG. 2 shows in schematic cross section the interior of the safety module of the tool depicted in FIG. 1;

[0109] FIGS. 2a and 2b shows in schematic cross section details of the interior of the safety module of the tool depicted in FIG. 1;

[0110] FIG. 3 shows in partial schematic perspective the parts of a safety module;

[0111] FIGS. 4a and 4b show parts of a perforation and circulation module in schematic perspective views;

[0112] FIGS. 5a and 5b show in schematic cross section parts of a wiper plug arrangement;

[0113] FIGS. 6a to 6d show in schematic cross sections fluid flow paths in a well bore fitted with a cementing tool;

[0114] FIGS. 7a and 7b show in schematic cross sections parts of a safety module;

[0115] FIGS. 8a and 8b shows in schematic cross section parts of a wellhead connector section; and

[0116] FIG. 9 shows in schematic cross section parts of a wellhead fitted with a Blow Out Preventer, showing retrieval of a tool.

DETAILED DESCRIPTION OF THE DRAWINGS

[0117] A tool for cementing an annulus and for methods of cementing a subsea oil or gas well to provide a permanent barrier is shown in schematic elevation in FIG. 1. Umbilical connection from the top of the tool to a surface vessel or rig are not shown, for clarity. Breaks are shown between parts 2, 4 and the lower end of part 4, to allow showing the whole tool in one view.

[0118] The tool 1 includes a safety module 2 and a perforation and circulation module 4. The perforation and circulation module 4 includes upper and lower seals 6,8 each having two passive cup seal members 10,12 in this example. Upper perforating devices, in this example perforating guns 14 (only one visible in this view) are mounted between seals 6,8. Lower perforating devices in this example perforating guns 16 are suspended (at some distance) below the lower seal 8 by cables and/or hydraulic control lines 18.

[0119] A cross-over valve 20 (in this example a sliding sleeve valve) is mounted between the seals 6,8 and has apertures 21 that can allow passage of fluid when the valve is opened. Also visible in FIG. 1 is a ball holding unit 22 which is part of a cement wiper system that includes wiper plug 23.

[0120] In this example the safety module includes a disconnection part 26 joined to a base part 28. The base part 28 includes a wellhead connector section 29. The wellhead connector section 29 includes a mechanical lock whose retractable dogs 24 are visible in an expanded (locking) position in this figure.

[0121] The disconnection part 26 includes alignment projections 30 (only one visible in this view, typically at least two are used). The projection 30 sits in a groove of the base part 28.

[0122] FIG. 2 shows in schematic cross section the interior of the safety module 2 of the tool depicted in FIG. 1. Parts are numbered the same as in FIG. 1.

[0123] Visible in the interior of the safety module 2 is a bore 32 leading to a central bore 34 that passes down the centre of the tool and continues to the perforation and circulation module (not shown in this figure). These bores 32, 34 are part of the supply fluid flow path for supply of fluid, including cement slurry, from an umbilical, in use of the tool. The normal direction of flow is suggested by arrows S, but as described herein the flow direction may be reversed in some operations. Ball holding/releasing unit 22 is in communication with the supply fluid flow path so that a ball released from the unit 22 will pass down the supply fluid flow path.

[0124] Also shown is a bore 36 that constitutes part of a return fluid flow path, for return of fluids back up through an umbilical in use. The flow path from bore 36 continues as an outer pipe 38 surrounding an inner pipe 40 that is a continuation of bore 34. The normal direction of flow is indicated by arrows R, but as described herein the flow direction may be reversed in some operations.

[0125] Outer pipe 38 ends at upper seal 6 (FIG. 1) whilst inner pipe 40 extends to below lower seal 8 (FIG. 1).

[0126] As depicted in FIG. 2 the dogs 24 of a mechanical lock are engaging with corresponding grooves on the interior bore of a typical subsea well head housing 42, positively locking the tool 1 to the wellhead 42, in particular acting to prevent upwards or downwards (i.e. axial) movement. The dogs are operated by a hydraulically driven cam ring 44, and are releasable if the ring moves upwards.

[0127] In this figure the disconnection part 26 of the safety module is shown unlatched and ready for release from the base part 28. Dogs 46 are shown held away from engagement with circumferential groove 48 by hydraulically driven cam ring 50 which includes a projection 52 engaging with hook features 54 on the dogs 46. (See magnified part view FIG. 2a). On separation of the disconnection part 26 from the base part, appropriate fail-safe closed valves 55 will act to seal the fluid flow paths and non-return valves will act to seal hydraulic control lines where required. In this example, in the base disconnection part 26, two tandem fail-safe valves are provided in each flow-path.

[0128] In normal use the cam ring 50 is in a lowered position to force dogs 46 into groove 48, as shown in magnified detail of FIG. 2b.

[0129] FIG. 3 shows in schematic perspective the parts of disconnection part 26 and base part 28 separated from each other. In this view the projections 30 of disconnection part 26 and corresponding grooves 56 on base part 28 can be seen. On connection or reconnection of parts 26 and 28 the projections 30 and grooves 26 ensure fitting together in the correct orientation to allow alignment of parts such as the bores for fluids shown in FIG. 2. Pad eyes 58 are shown in this figure, for use in connecting lifting members, e.g. ropes constructed from wire or other materials, cables etc. for lifting and lowering the tool.

[0130] FIGS. 4a and 4b show in schematic perspective the major part of the perforation and circulation module 4 of the tool depicted in FIG. 1. The part in FIG. 4b is rotated with respect to the view of FIG. 4a to provide two viewing angles.

[0131] As can be seen in FIG. 4a two upper perforating guns 60 (the perforating devices employed in the example) are mounted between seals 6,8. The guns 60 include a number of apertures 62 for directing products and energy from detonation of explosives outwards to perforate casings downhole. As can be seen more easily in FIG. 4b a pipe section 64, which is a continuation of pipe 40 shown in cross section FIG. 2 passes by the upper perforating guns 60 and through the mandrel mounting lower seal 8. Pipe section 64 is part of the supply fluid flow path and is in fluid communication with outlets 21 of cross-over valve 20, when the valve is opened. Hollow studs 66 connect the upper and lower parts of the perforation and circulation module 4 and one or more can carry hydraulic lines. For example, for operating cross-over valve 20 and lower perforation guns (not shown in these figures). In general, the hollow studs 66 may carry hydraulic lines, electrical signal and/or power lines in some examples of the tool.

[0132] FIG. 5a shows in cross section schematic detail a wiper plug arrangement typically made of flexible, e.g. elastomeric, material fitted below the lower seal 8 of a perforation and circulation module. Wiper plug 23 is attached by a frangible connection 67 to the bottom end of the perforation and circulation module 4. Hydraulic control lines 68 and 70 (for firing lower perforation guns—see FIG. 1) pass through the body of the wiper plug 23. Only line 70 is fully shown in FIG. 5a but both are shown in FIG. 5b (see detail in FIG. 5b). The lines 68 and 70 include some “slack” 72 to accommodate axial travel of couplings 74 that will release when the wiper plug 23 detaches and descends away from the perforation and circulation module 4. Passing through the wiper plug 23 is a passage 76, part of the supply fluid flow path S, through which fluids, including cement slurry can pass. The passage 76 includes a seat 78 for a ball that is dropped from ball holding/releasing unit 22 (FIG. 1).

[0133] Detail view FIG. 5b shows a ball 80 that has been dropped from ball holding/releasing unit 22 situated in seat 78. The ball 80 seals passage 76. Therefore, when fluid flow S is supplied the pressure causes wiper plug 23 to detach and descend (direction D). The circumferential ribs 82 are flexible and sized to fit close to/in contact with the inner surface of a production casing. Thus, when the plug 23 descends in the casing driven by the cement slurry or other fluid above it, it sweeps debris out of the way and acts to avoid unwanted mixing between fluid being delivered and fluid already in the bore of a well.

[0134] FIGS. 6a, 6b, 6c, and 6d show schematically in cross sections flow paths for cementing operations and fluid circulating procedures. The well casing lengths are not to scale, but are greatly shortened to allow viewing of lower perforations in the figures.

[0135] In FIG. 6a a tool 1 (like that of FIG. 1) is located in a production casing 84 of a well with two further casings 86 and 88 creating two annuli, inner annulus 90 and outer annulus 92. One or more lower perforating guns 16 have made lower perforations 94 in casing 84. One or more upper perforating guns (not shown for clarity, but located between upper and lower seals 6,8) have made upper perforations 96 in casing 84. The perforations 94 and 96 allow fluid circulation. As indicated by arrows in FIG. 6a a supply fluid flow path allows fluid flow S (of cleaning fluid such as seawater) to be sent from an umbilical through the supply fluid flow path in the tool 1, including through wiper plug 23, down the production casing 84 and through lower perforations 94. The fluid returns in flow R as suggested by arrows from the lower perforations 94, up through inner annulus 90, through upper perforations 96 to between seals 6,8 and back toward the surface by the route through the upper part of tool 1 depicted in FIG. 2. When carrying out a cleaning operation the fluid flows depicted can be reversed.

[0136] In FIG. 6b the same fluid flow paths depicted in FIG. 6a are used to deliver cement to the inner annulus 90, which is filled with a cement slurry charge 98 to a maximum height level L below that of the upper perforations 96. The production casing 84 is substantially freed of cement by following cement charge 98 with a tail fluid (such as seawater). Cement charge 98 is allowed to set following which pressure testing can be undertaken. If the outer annulus 92 is not present, or if filling it with cement is not required, then cementing operations can continue by supplying a further charge of cement slurry followed by a tail fluid pumped down an umbilical and through the supply fluid flow path into the production casing that forms the central bore of the well.

[0137] As an alternative (the flow path used to fill the inner annulus can be the reverse of that depicted, with cement charge 98 supplied in the direction R.

[0138] In FIG. 6c the cementing of the second annulus 92 is depicted. After perforating through both the casings 86 and 88 with one or more second lower perforating gun(s) the perforations 100 allow access to the second annulus 92. Similarly, one or more second upper perforating gun(s) have made perforations 102 through to the second annulus between the seals 6,8. Cementing in the second annulus 92 is made, in this example, with the use of the cement wiper plug 23. After dropping a ball (FIG. 5b) to sit in the seat of the wiper plug 23 a second charge of cement slurry has been passed down the supply fluid flow path. This flow S has detached the wiper plug which is pushed to below perforations 100, cleaning the bore of the production casing 84. The second cement charge 104 fills second annulus 92 up to level L2 below upper perforations 102. Conveniently cementing the bore of production casing 84 is carried out by applying a cement 104 charge that also fills that bore, at least below the bottom seal 8 of the tool to maximum height level L3.

[0139] In FIG. 6d the use of the cross-over valve 20 in cleaning or flushing the production bore of casing 84 is illustrated. Operating cross-over valve 20 diverts some fluid flow back into the space between seals 6 and 8. In this example as well as diverted flow 106 from supply fluid flow S, the valve 20 also continues to allow flow 108 to below lower seal 8. This arrangement of flows cleans between seals 6 and 8 and prevents debris such as set/setting cement slurry obstructing removal of the tool upwards.

[0140] If the lower seal 8 in FIG. 6d is an expandable packer having an expandable seal element, the seal element can be relaxed to allow flow past the seal to wash out cement from below seal 8. Such an arrangement of flows can clean between seals 6 and 8 and below seal 8. This can prevent debris such as set/setting cement obstructing removal of the tool upwards.

[0141] FIG. 7a shows in schematic cross section the interior of a safety module 2, similar to that shown in FIGS. 2, 2a and 2b.

[0142] In FIG. 7a the dogs 46 are held in the locked position by axially moving cam ring 50. Therefore, the disconnection part 26 is in locking engagement with the base part 28. In this example the quick release coupling of the disconnection system also includes a secondary disconnection arrangement including disconnection ring 110 placed around the circumference of axially moving cam ring 50.

[0143] In the event that the movement of cam ring 50 fails due to e.g. failure of a hydraulic circuit, then disconnection ring 110 can be operated (by its own hydraulic system) to move axially in direction D. (Hydraulic fluid is pumped in below the ring 110.) The motion of ring 110 engages circumferential rib 112 driving cam ring 50 upwards and thereby causing dogs 46 to disengage to the unlocked position depicted in FIG. 7b. Thus disconnection of disconnection part 26 and the base part 28 can be achieved.

[0144] FIG. 8a shows in schematic cross section a view of the well head connector section 29 of a tool similar to that depicted in FIGS. 2, 2a and 2b and connected to a subsea well head housing 42. The dogs 24 of the mechanical lock are shown in locking engagement with subsea well head housing 42, held in place by axially moving cam ring 44. The dogs 24 overlap in the radial direction with the subsea wellhead housing 42, preventing movement in the axial direction.

[0145] In this example, the axially moving cam ring 44 is movable by hydraulic fluid pressure applied vie passage way 114 to below an inwards projecting rib 115. Fluid pressure applied via passage way 114 causes upwards motion (suggested by arrow U) of the cam ring 44, allowing the dogs 24 to disengage from subsea well head housing 42.

[0146] FIG. 8b shows the arrangement of FIG. 8a, but through a different cross section and with more of the base part 28 of the safety module depicted. This cross-section allows viewing of a separate hydraulic arrangement, that constitutes a secondary unlocking arrangement. Where valves 55 are closed, fluid pressure applied via supply fluid flow path S acts on bursting disc 117 (shown in FIG. 9) in hydraulic fluid passageway 116. Hydraulic fluid can then proceed down passageway 116 to lift axially moving cam ring 44 by pressure applied to beneath an outwards projecting rib 118.

[0147] FIG. 9 shows and arrangement where a Blow Out Preventer (BOP) 120 is fitted to a well head housing 42. A base part 28 and lower parts of a tool such as depicted in the preceding figures is still in the well bore.

[0148] As depicted in this figure a section of drill pipe 122 is screwed into fitting 124 provided in the base part 28 of the tool, after a disconnection has occurred, such as discussed above with reference to FIG. 7.

[0149] The dogs 24 of the mechanical lock are still engaged. To release the mechanical lock the secondary unlocking system as discussed above and with reference to FIGS. 8b and 9 is employed. Fluid pressure applied via drill pipe 122 proceeds through bursting disc 117 and enters passageway 116 (FIG. 8b) to cause lifting (U) of cam ring 44 allowing disengagement of dogs 24 as suggested by arrows D. At this time the tool can be withdrawn (W) from the well bore and the BOP 118.

[0150] As an alternative, the tool may be removed in a similar fashion in situations where a BOP has not been fitted. An ROV can be used to insert a stab into the central bore (instead of drill pipe 122 shown in FIG. 9). Fluid pressure applied via the stab can be used to operate the secondary unlocking system.

[0151] Some cementing operations making use of tools and methods described herein will now be described by way of example. Where perforating guns are referred to, it will be understood that other perforating devices may be employed.

[0152] 1. Single Annulus and Production Bore Cementing

[0153] 1.1. Perforate production casing with lower perforating gun

[0154] 1.2. Test and verify integrity of the casing string annulus

[0155] 1.3. Perforate production casing at upper elevation with upper perforating gun

[0156] 1.4. Circulate cleaning fluid through the annulus between production and intermediate casing strings to remove debris and to prepare for cement introduction, monitor returning fluid until clean. Cleaning can be by forward circulation, i.e. down supply fluid flow path and returning up the annulus and the return fluid flow path through the tool to surface. The reverse flow can also be used.

[0157] 1.5. Drop ball to seal off wiper plug

[0158] 1.6. Pump a volume of cement less than or equal to the volume within the casing string annulus over the distance between the upper and lower perforations. He

[0159] The wiper plug shears off from its connection at the bottom of the perforation and circulation module and descends into the well like a ‘piston’ to promote an optimal, undiluted, charge of cement slurry into the annulus. The rate of descent of the wiper plug can be controlled by varying the volume flow-rate of the return fluid and the wiper can only descend to the depth of the lower perforations because the well is sealed below, causing a hydrostatic lock. In one embodiment of the wiper plug this action simultaneously disconnects the lower perforating guns, which are suspended from the wiper plug, however other hydraulically actuated disconnection mechanisms could be used to release the lower perforating guns.

[0160] 1.7. Following the calculated volume of cement slurry, an additional volume of propulsive fluid would be pumped, referred to as tail fluid, comprising sea water and/or other non-setting fluid. The purpose of the ‘tail’ is to ensure no residual cement can remain in close proximity to the tool, which could set and seal the tool into the well. (Dilution between the tail and the end of the cement can be minimized by having a slug of a viscous fluid at the front of the tail.

[0161] 1.8. Open sliding sleeve cross-over valve (XOV) situated between the seals on the tool and circulate cleaning fluid to ensure cement is dispersed from close proximity to the tool.

[0162] 1.9. Wait for the cement in the annulus to set and pressure test.

[0163] 1.10. Verify pressure integrity

[0164] 1.11. Introduce cement into the bore of the production casing. The procedure may include the tool being withdrawn to allow the full desired volume of cement to be introduced whilst avoiding trapping the tool in cement.

[0165] 1.12. Pressure test the barrier that has been created across the annulus and the production bore.

[0166] 1.13. The well head may now be removed and the well abandoned.

[0167] Two Annuli and Production Bore Cementing

[0168] The tool s fitted with first and second lower perforating guns and first and second upper perforating guns. The first set to perforate production casing into an inner annulus and the second set to perforate casings through to the outer, second annulus.

[0169] 2.1 First/Inner Annulus

[0170] 2.1.1 Perforate the production casing with 1st lower perforating gun.

[0171] 2.1.2 Test and verify integrity of the inner annulus.

[0172] 2.1.3 Perforate the production casing at an upper position, between the seals on the perforating and circulation module with 1st upper perforating device.

[0173] 2.1.4 Circulate cleaning fluid in the annulus between production and intermediate casing strings to remove debris and to prepare for cement introduction, monitor returning fluid until clean. Cleaning can be by forward circulation, i.e. down supply fluid flow path and returning up the annulus and the return fluid flow path through the tool to surface. The reverse flow can also be used.

[0174] 2.1.5 Pump a volume of cement less than or equal to the volume within the casing string annulus over the distance between the upper and lower perforations. The preferred method is by reverse circulation during which the charge of cement slurry is held in position in the annulus by balancing with pressure adjustment in the production bore

[0175] 2.1.6 Open sliding sleeve of XOV and circulate cleaning fluid to ensure any lingering cement is washed out

[0176] 2.1.7 Wait for cement to set and pressure test

[0177] 2.1.8 Verify integrity of inner annulus cement plug

[0178] 2.1.9 Second/Outer Annulus

[0179] 2.1.10 Perforate production and intermediate casings with 2nd lower perforating gun

[0180] 2.1.11 Test and verify integrity

[0181] 2.1.12 Perforate production and intermediary casings at upper elevation with 2nd upper perforating gun

[0182] 2.1.13 Circulate the second annulus volume between production and intermediate casing strings to remove debris and to prepare for cement introduction, monitor returning fluid until clean. Cleaning can be by forward circulation, i.e. down supply fluid flow path and returning up the annulus and the return fluid flow path through the tool to surface. The reverse flow can also be used.

[0183] 2.1.14 Drop ball to seal off wiper plug

[0184] 2.1.15 Pump a volume of cement slurry less than or equal to the volume within the casing string annulus over the distance between the upper and lower perforations and to fill the production bore, from above the lower perforations and to below the tool. The wiper plug shears off from its fixation at bottom of tool and descends into the well like a ‘piston’ to promote an optimal (undiluted) slug of cement into the annulus. The rate of descent of the wiper can be controlled by varying the volume flow-rate of the return fluid and the wiper can only descend to the depth of the lower perforations because the well is sealed below, causing a hydrostatic lock. In one embodiment of the wiper plug this action simultaneously disconnects the lower perforating guns, which are suspended from the wiper, however other hydraulically actuated disconnection mechanisms could be used to release the lower perforating devices.

[0185] The level of cement in the 2nd annulus will tend to balance the length of cement within the production bore, thus establishing cement plugs in both annuli and the bore of approximately equal lengths all at the same elevation

[0186] 2.1.16 Open sliding sleeve cross-over valve (XOV) and circulate cleaning fluid to wash out as the tool is retrieved. Displaced wellbore liquid mixed with flushing flow would be returned to surface via the annular flow-path within the tool. This avoids the lower parts of the perforating and circulating module becoming stuck by residual lumps of cement slurry.

[0187] 2.1.17 In one alternative embodiment, the lower seal assembly comprises an expandable packer. In this embodiment the expandable packer incorporates an annular hydraulic piston which acts axially upon a toroidal elastomeric element to open and close an annular an annular space between the toroidal element surface and casing wall, through which liquids could flow. In its relaxed state, when no hydraulic control pressure is applied behind the piston, the element is smaller in diameter than the casing bore, which facilitates liquid flow over the elastomeric element surface. In the energized state, when hydraulic control pressure is applied behind the piston, the resulting axial travel compresses the element which constrains it to expand radially, thereby closing off the annular space. In this way an expandable packer is able to open and close a circulation path between the bottom outlet of the perforation and circulation module and the return flow path between the upper and lower seals. This flow path can be used for cleaning purposes. A flow of cleaning fluid such as seawater can wash out any remaining cement, thus preventing the lower part of the tool from becoming stuck while waiting for the final cement plug to set.

[0188] 2.1.18 Wait for cement to set and pressure test combined plug in the outer annulus and central bore.

[0189] 2.1.21 Verify integrity.

[0190] 2.1.22 Retrieve tooling

[0191] 2.1.23 The well head may now be removed and the well abandoned.