Pump down cement retaining device

A wall casing cement plug assembly includes a receptacle with an axial passage. The receptacle is pumped to a lower end of the casing string and locked in place. The receptacle has a casing seal that engages the string of casing and a retainer mechanism on its exterior that engages a profile in the string of casing. Cement is pumped through the receptacle by rupturing a blocking device in the axial passage of the receptacle. A wiper plug is pumped down the string casing. The wiper plug has a prong on its lower end that stabs into the axial passage of the receptacle. A latch located in the lower portion of the receptacle locks the wiper plug to the body.

FIELD OF THE INVENTION

This invention relates in general to cementing a casing string within a wellbore, and in particular to a pump down cement retaining device that prevents backflow of cement.

BACKGROUND OF THE INVENTION

Most oil and gas wells are drilled with a drill string comprised of drill pipe. After reaching a certain depth, the drill string is removed and casing is lowered into the wellbore. A cement valve, is normally attached to the lower end of the casing. The cement valve allows cement to be pumped down through the casing and up the annulus surrounding the casing, and prevents backflow of cement from the annulus back into the casing. Another type of casing string, referred to as a liner, may be installed in a similar manner. A casing string extends all the way back to the upper end of the well, while a liner string is hung off at the lower end of a preceding string of casing.

In another drilling technique, the casing is used as part or all of the drill string. The bit may be attached to the lower end of the casing string permanently, in which case it is cemented in place. Alternatively, it may be retrieved after reaching desired depth, such as by using a wireline, drill pipe, or pumping the bit assembly back up the casing. While drilling, the casing string may be rotated by a gripping mechanism and a top drive of the drilling rig. With liner drilling, the liner string serves as the lower end of the drill string, and a string of drill pipe is attached to upper end of the liner string.

In casing and liner drilling, if the bottom hole assembly, which includes a drill bit and optionally measuring instruments and steering devices, is to be retrieved before cementing, the operator will install a cement valve at the lower end of the liner after retrieval of the bottom hole assembly. The cement valve may be lowered into place on a wire line or a string of drill pipe and locked to a profile at the lower end depth of the liner string. Also, it is has been proposed to pump the cement valve down the casing, rather than convey it on a wire line. The cement valve may have a flapper valve to prevent back flow of cement. It may also have a frangible barrier to allow the cement valve to be pumped down the casing string. Once in place, increased fluid pressure causes the barrier to break and the fluid to flow out the lower end of the cement valve.

It has also been proposed to pump a receptacle down the casing string and latch it into a profile at the lower end prior to cementing. The receptacle has a passage that allows the downward flow of cement, but does not have a valve to prevent backflow. At the conclusion of cementing, a wiper plug or prong is pumped down into engagement with the receptacle. The prong stabs into the upper end of the receptacle to form a seal and retain the plug to prevent backflow of cement.

After the cement is cured, if the operator intends to drill the well deeper, the drill string must drill through the receptacle and wiper plug. It is thus desirable to make the receptacle and wiper plug of easily drillable materials. These materials must meet the requested specifications of the tools.

SUMMARY OF INVENTION

The method of this invention utilizes a receptacle that is positioned at the lower end of the casing string. A wiper plug is pumped down the string of casing following the pumping of cement. The wiper plug has a prong on its end with a seal that seals within a lower portion of the receptacle. The positioning of the seal places the receptacle under a compressive force when a pressure differential exists due to uncured cement in the annulus. Since the force is compressive, many of the components of the receptacle can be made of more easily drillable materials, such as plastic and resin composites, than in the prior art design. The prior art design had to accommodate at least some tensile forces.

In the preferred embodiment, the lower end of the prong is substantially flush with a lower end of the axial passage through the receptacle once locked in place. Preferably, the seal is also located at the lower end of the axial passage. The latching members of the prong and receptacle may comprise a ratchet sleeve and a grooved profile.

DETAILED DESCRIPTION OF INVENTION

Referring toFIG. 1, a string of casing11comprises tubular members secured together by threads for installation in a wellbore. The term “casing” is used broadly herein to include also a liner string, which is normally constructed the same as casing but does not extend fully to the surface, rather its upper end is hung off near the lower end of the preceding string of casing.

A lower or profile sub13is attached to the lower end and forms part of the string of casing11. Profile sub13has number of internal grooves that in this embodiment were used previously to secure a bottom hole assembly (not shown) for drilling. Profile sub13also has an annular recess15located therein that has a larger inner diameter than the inner diameter of the remaining portion of the string of casing11. Recess15is defined by an upper shoulder17and a lower shoulder19.

A cement plug receptacle21is shown latched into profile sub13. Cement plug receptacle21has a body23with an axial passage25extending through it. Body23has at least one and optionally a plurality of circumferential grooves27on its exterior. In this embodiment, grooves27are configured in a triangular fashion, resulting in a downward-facing conical flank29intersecting an upward-facing conical flank31. When viewed in cross-section, flanks29of grooves27are parallel to each other and flanks31are parallel to each other.

An outward-biased collar33surrounds body23at grooves27. Collar33is of a resilient material and is split so as to radially expand and contract. Collar33has at least one and optionally a plurality of internal grooves35for mating with grooves27of body23. The resiliency of collar35causes it to spring outward from grooves27when it reaches profile sub recess15. As receptacle21moves down casing11, prior to reaching recess15, the outer diameter of collar33will slidingly engage the inner diameter of casing11. Anti-rotation keys37, one at the upper end and one at the lower end of body23, engage collar33to prevent collar33from rotating relatively to body23. Grooves35have same configuration as grooves27, but body23is capable of axial movement from a lower position relative to collar33, shown inFIG. 4, to an upper position, shown inFIG. 5. In the lower position, downward-facing flanks29of body grooves27are engagement with collar grooves35but upward-facing flanks31are not in engagement with collar grooves35. In the upper position ofFIG. 5, upward-facing flanks31are engagement with grooves35, but downward-facing flanks29are not in engagement with grooves35.

Referring still toFIG. 1, body23has a lower body extension39that has a threaded neck41that secures it to the lower end of body23. Lower body extension39could optionally be integrally formed with body23. Axial passage25extends through lower body extension39. A latch member sleeve43with internal grooves is mounted within lower body extension39.

A lower seal45is attached to the lower end of lower body extension39by a threaded neck47. Lower seal45is illustrated as a cup seal, having a downward-facing concave interior; but it could be other types. Pressure acting on the lower side of lower seal45pushes seal45outward and upward into sealing engagement with profile sub13. A cylindrical seal member48is preferably located in the portion of axial passage25that extends through lower seal45.

An upper seal49is mounted to the upper end of body23by a threaded neck51in this example. Upper seal49may have the same general shape as lower seal45. Axial passage25extends through upper seal49but it is initially closed by a frangible barrier, which comprises a burst disk53in this example. Burst disk53closes axial passage25until the differential pressure acting on it exceeds a selected level, at which time it breaks or ruptures to allow flow through axial passage25. Burst disk53is secured to upper seal49by a shear cylinder retainer55.FIG. 1shows burst disk53as initially installed andFIG. 2shows burst disk53after being ruptured. Rather than the barrier device being a rigid frangible member, burst disk53could be a flexible elastomeric member or diaphragm that ruptures, or other types of devices.

FIG. 3shows a wiper plug57being pumped down following the dispensing of cement. Wiper plug57has flexible ribs59on its outer side that seal against the inner diameter of casing11as it moves downward. A prong61is mounted to the lower end of wiper plug57and protrudes downward. Prong61comprises a rod located on the axis of wiper plug57. A plurality of transverse ports67optionally may be formed along its length. A nose69is attached to the lower end of prong61. Referring toFIG. 7, nose69has one or more seal71that extends around it. Seals71seal against seal sleeve48located within lower seal45. A latch member comprising a ratchet sleeve73is mounted just above nose69. Ratchet sleeve73is a split cylindrical sleeve that is biased outward due to its internal resiliency. Ratchet sleeve73has grooves75on its exterior that will mate with the grooves in latch sleeve43. Grooves75and the mating grooves in latch sleeve43are configured to allow downward movement of prong61but not upward movement. During downward movement, the saw-tooth shape of grooves75in ratchet sleeve73cause ratchet sleeve73to retract and expand.

An annular retainer77located below ratchet sleeve73on the upper end of nose69has a tapered surface79on its upper end that faces upward and outward for urging ratchet sleeve73outward into tighter engagement due to internal pressure acting against nose seals71.

Preferably, most, if not all the components of cement plug receptacle21and wiper plug57are constructed of easily drillable materials to allow the operator to readily drill out the assembly after the cementing operation is over and the cement is secured. These materials may include composite materials, such as resin reinforced fiber as well as plastic materials. They may also include metallic materials such as aluminium.

In operation, after drilling to a desired depth and retrieving the bottom hole assembly (not shown), the operator places cement plug receptacle21into the upper end of the string of casing11and applies fluid pressure to casing11to pump it downward, typically with water. When cement plug receptacle21reaches recess15, the outward-biased collar33springs outward and secures cement plug receptacle21to profile sub13, as shown inFIG. 1. Once in engagement, downward movement is prevented by upward-facing shoulder19and upward movement is prevented by downward-facing shoulder17.

Continued fluid pressure after cement plug receptacle21has landed shears burst disk53, as shown inFIG. 2. Once burst disk53ruptures, the operator may pump cement through casing11, which flows through axial passage25and up the annulus surrounding casing11. When the desired quantity of cement has been dispensed, the operator places wiper plug57in casing string11, as shown inFIG. 3, and pumps wiper plug57downward, normally with water. Wiper plug57pushes the cement in casing string11downward through axial passage25. Eventually, prong61will stab into axial passage25, as shown inFIG. 4, and wiper plug57will land on retainer55. At this point, the tip of wiper plug nose69will be located substantially flush with the lower end of axial passage25. Seals71on nose69will be sealing engagement with seal sleeve48(FIG. 7). Ratchet sleeve73will be in locking engagement with latch sleeve43. Downward-facing flanks29on body23will be in engagement with grooves35in collar33. Most, if not all, of ribs59of wiper plug57will be located above receptacle21and do not perform any latching function or any sealing function against upward acting pressure.

The operator may then release the fluid pressure from above wiper plug57. The weight of the cement in the casing annulus tends to cause it to flow back upward into casing string11. Wiper plug57and body23will initially move upward slightly in unison due to the differential pressure force as shown inFIG. 5. This upward movement will stop once upward-facing flanks31on body23engage grooves35in collar33, as shown inFIG. 5. The load path due to the pressure of the cement in the annulus passes through lower seal45, lower body extension39and body23into collar33, which transfers the load to profile sub13through upper shoulder17. The load path also passes from nose69through latch sleeve43into lower body extension39. Lower body extension39, body23, nose69and collar33will be in compression. No components of receptacle21or wiper plug57will be in tension as a result of the upward acting pressure.

After the cement has cured, the operator may run a new drill string, which could comprise drill pipe or a smaller diameter string of casing. A drill bit on the lower end will drill out cement plug receptacle21, leaving only profile sub13.

An alternate embodiment is shown inFIGS. 8 and 9. Referring toFIG. 8, prong81differs from the first embodiment in that is does not have holes such as ports67(FIG. 2) extending through it perpendicular to its axis. Also, its internal cavity82is deeper than the internal cavity of prong61(FIG. 6). Nose83is longer than nose69of the first embodiment; however, seals85are positioned about the same distance from the lower end as seals71on nose69of the first embodiment. Nose83may have an axially extending internal cavity84, as shown. A split ratchet ring87is attached near the lower end of prong81as in the first embodiment. Wiper plug89on the upper end of prong81has seal ribs91that protrude radially less distance from the body of wiper plug89than seal ribs59of the first embodiment.

Referring toFIG. 9, receptacle93is shown anchored in a profile sub95that may the same as lower sub13of the first embodiment. Receptacle has a lower cup seal97that differs from lower seal45(FIG. 1) in that it is carried on a tubular cup mandrel99of a more rigid material than the material of seal97. An annular load ring101encircles cup mandrel99for transmitting upward compressive force from lower seal97to a tubular extension member103. The first embodiment does not have a load ring. The upper end of cup mandrel99is secured to extension member103, and the lower end of cup mandrel99extends below load ring101into lower seal97. Ratchet or internally grooved sleeve105is mounted within extension member103for engagement with ratchet ring87on prong81as in the first embodiment.

Body107is attached to the upper end of extension member103and may be constructed the same as body23of the first embodiment. A collar109encircles body107and springs outward into a recess111of profile sub95as in the first embodiment. An upper cup seal113similar to upper seal49(FIG. 1) is mounted on top of body107. A seat115containing a burst disc117is mounted within upper seal113. The operation of the embodiment ofFIGS. 8 and 9is the same as the operation of the first embodiment.

While the invention has been shown in only two of its forms, it should be apparent to those skilled in the art that is not so limited, but is susceptible to various changes without departing from the scope of the invention.