GB2193741A

GB2193741A - Cementing of boreholes

Info

Publication number: GB2193741A
Application number: GB08719301A
Authority: GB
Inventors: Peter John Michael Hogarth
Original assignee: EASFIND Ltd
Current assignee: EASFIND Ltd
Priority date: 1986-08-16
Filing date: 1987-08-14
Publication date: 1988-02-17
Also published as: NO873447D0; GB8719301D0; GB8620004D0; NO873447L

Abstract

In a two-stage operation to cement the annulus between the wall of a borehole and a casing string (10), a stage collar (14) is inserted as a section within the casing string to enable cement to be pumped out through ports (18, 16) in the stage collar. The stage collar (14) includes a slidable sleeve (17) provided with an internal seat (19) which is engageable by lugs (54) of a pressure pack-off tool assembly (28). The tool assembly (28) is movable within the casing string to control both the opening and the closing of the stage collar. After the second stage cementing operation through the stage collar (14), an overpull is exerted on the drill string which, via the tool assembly (28), first closes and locks closed the stage collar and then breaks off the seat (19) which is lifted to the surface with the drill string. This leaves the borehole clear for the next drill bit without the need to drill out a collar, without any danger of leakage through the locked-shut stage collar (14). <IMAGE>

Description

SPECIFICATION Cementing of boreholes This invention relates to the cementing of boreholes, and is particularly concerned with an improved two-stage cementing process and with borehole cementing apparatus and equipment appropriate to carry out such a twostage cementing process.

The invention is concerned with a process in which cement is pumped down a relatively narrow drill pipe positioned within a much larger diameter borehole casing formed as a string of casing sections. The drill pipe is equipped with anciilary fittings, thereby to constitute a drill string which can be run in the casing.

In the conventional inner string cementing operation, the cement is pumped down the drill pipe from the drilling rig floor down to a collar or shoe which is located in the casing string, and this collar or shoe directs the cement to its proper location around the casing string between the casing string and the wall of the borehole itself.

In many circumstances it is desirable to be able to carry out a secondary cementing operation. In other words, one wishes to be able to direct a flow of cement from the central drill pipe, out through the casing string, and into the annular space between the casing string and the borehole wall. This secondary cementing process may be in order to direct a flow of cement down on to the top of the cement positioned in the first stage of the operation, or one may wish to direct cement to a particular site somewhere down the length of the borehole. This may be for example to strengthen weak formations. It is known to provide multiple stage cementers in the casing string at one or several positions down the length of the casing string in order to be able to direct cement out from the drill pipe through the casing string at these selected sites.

It is an object of the present invention to provide a stage collar for inclusion in a casing string which is appropriate for a secondary cementing operation and which in particular is locked closed after the secondary cementing operation has been completed.

It is a further object of the present invention to provide borehole cementing apparatus and a method of cementing a borehole in which the stage collar can be both opened and closed using a pack-off tool assembly adapted to be locked into place on the stage collar.

It is a further object of the present invention to provide a method of and apparatus for cementing a borehole in which at least some of the parts of the drill string are retrievable and re-usable.

It is yet another object of the present invention to provide a method of and apparatus for cementing a borehole in which, after the two stages of the cementing operation, the borehole is left clear for the next drill bit, without any need to drill out a collar.

In accordance with the invention there is provided a stage collar insertable as a section within a borehole casing string, the collar comprising an outer case provided with first cementing ports, and a sleeve slidable within the outer case and provided with second cementing ports, the collar being open when the first and second ports are in alignment and closed when the said ports are out of alignment, wherein the sleeve is provided with internal seating means engageable by a tool movable within the casing string to control both the opening and the closing of the collar, and wherein the sleeve is arranged to be locked irreversibly in a position where the collar is closed when the sleeve slides from a position where the collar is open to a position where the collar is closed.

Also in accordance with the invention there is provided, in combination with the stage collar, a drill string comprising a drill pipe and a pack-off tool assembly which can be latched to and unlatched from the drill pipe and which includes a pressure pack-off cup arranged to contact the internal wall of the casing string, wherein the tool assembly also includes latching means by which it can be locked into engagement with the stage collar whereby the tool assembly can be used both to open and close the stage collar with the tool assembly locked in place.

Also in accordance with the invention there is provided a method of carrying out a twostage cementing operation in a borehole fitted with a casing string incorporating a stage collar which comprises the steps of: a) making up a drill string comprising a drill pipe and a pack-off tool assembly which can be latched to and unlatched from the drill pipe and which includes a pressure pack-off cup arranged to contact the internal wall of the casing string; b) locating the lower end of the drill string in a collar or shoe within the casing string and pumping first stage cement down the drill pipe through the collar or shoe; c) releasing the drill string from the collar or shoe;; d) unlatching the tool assembly from the drill string above the stage collar raising the rest of the drill string, latching the tool assembly to the drill string again, and lowering the drill string until the tool assembly lockingly engages and opens the stage collar; e) sealing off the drill pipe below the stage collar; f) pumping second stage cement down the drill pipe and through the stage collar into the annulus around the casing string; g) and thereafter exerting an upward pull on the drill string which first, via the tool as sembly, closes and locks closed the stage collar, and then breaks the locking engagement between the tool assembly and the stage collar.

In order that the invention may be fully understood, one presently preferred embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal view showing part of the drill string located within a borehole casing string, in order to illustrate the running-in sequence; Figs. 2 and 3 illustrate the steps of the first stage cementing operation; Figs. 4 to 8 illustrate the steps of the running procedure in the second stage cementing operation; Fig. 9 is a longitudinal view, partly in section, showing the latching pack-off tool assembly on a larger scale; and, Fig. 10 is a longitudinal view, partly in section, showing the stage collar on an enlarged scale.

As indicated above, the purpose of the method and apparatus of the present invention is to displace cement by a two-stage process, down through a drill pipe which extends axially down through a borehole casing string. In the illustrated embodiment, the stage collar is shown as a 26 inch collar but it should be understood that the invention is applicable to drill pipes and casing strings of any dimensions.

Referring first to Fig. 1, this shows how the inner string cementing system is set up for use, i.e. it illustrates the running-in sequence.

The borehole casing is indicated generally at 10 and is made up of a plurality of casing sections which are secured together. Towards the bottom of the string of casing sections is located a stab-in float collar, indicated generally at 12. Included within the casing string 10 is a stage collar indicated generally at 14.

This stage collar 14 is shown in greater detail in Fig. 10. It can be seen from Fig. 1 that the stage collar 14 comprises an outer case 15 provided with radial cementing ports 16, a sliding sleeve 17 within the outer case 15 and provided with radial cementing ports 18, and a seat 19 held on the internal wall surface of the sliding sleeve 17.

The central drill string is made up from an assembly of parts. At the lower end of the drill string is a latch-in stinger 20 which is engageable in the float collar 12. The stinger 20 is made up on to the first section of drill pipe 22. Above drill pipe 22 is positioned a latch-in inner string seal assembly, indicated generally at 24. A short drill string collar 26 is positioned above the seal assembly 24. This short drill string collar 26 may be 3 metres long for example. Fitted above the drill string collar 26 is an "L" latch pack-off tool assembly, indicated generally at 28. The packoff tool assembly is shown in greater detail in Fig.9. Further sections 30 of drill pipe extend upwardly above the pack-off tool assembly 28, up towards the drilling rig rotary table. At the top of the drill string is a cement head, indicated generally at 32.The cement head 32 is pre-loaded with first and second drill pipe wiper darts 33 and 34 respectively, and also with two drill pipe opening balls 35 and 36 respectively.

The pack-off tool assembly 28 is shown in more detail in Fig. 9, to which particular reference is now made. The assembly is secured to the drill pipe 30 which passes through it by means of an "L"-type latch. This comprises a pair of L-shaped slots 37 at the top of the assembly 28 and diametrically opposed locking pins 38 extending radially from the drill pipe. A spring-loaded ball 39 is preferably also included as part of the latch. The packoff tool assembly 28 is also provided with external unlatch drag springs 40 which engage the internal wall surface of the casing string.

The assembly 28 includes a pack-off cup 42 which contacts the internal wall of the casing string and which has a circumferential flange portion which is capable of flexing movement to adapt itself to the contour of the casing string. Four by-pass passages 44 run from above the cup 42 to below the cup, through the body of the assembly. Each of these bypass passages 44 is fitted with a bleed-off pressure ball valve 46, for running out of the borehole. The lower part of the tool assembly 28 comprises a reduced diameter portion provided with circumferentially spaced radial cementing ports 48, below which is a latching portion indicated generally at 50. This latching portion has a circumferential seal 52, four external equispaced pivotable lugs 54, and an internal L-type latch 56 with an upwardly tapering lead-in surface 58.The external lugs 54 latch into the seat 19 in the stage collar 14.

The internal latch 56 engages with catches which are provided at the upper part of the seal assembly 24, and the lead-in surface 58 is tapered to match the external taper on the seal assembly 24. Engagement of the tool assembly latch 56 with the seal assembly catches ensures that the cementing ports 48 are correctly aligned with radial ports 60 (Fig.1) through the upper part of the seal assembly when the seal assembly and pack-off tool assembly are locked together.

The stage collar 14 is shown most clearly in Fig. 10 and comprises the outer case 15 which is made up from a plurality of annular sections which are secured together. It includes anti-rotation lugs 62 which ensure that the correct alignment of the internal cementing ports 18 is maintained. Circumferential seals 64 and 65 are provided within the outer case 15 respectively above and below the external cementing ports 16. The internal sliding sleeve indicated generally at 17 is formed in two parts. The first part 66 which is provided with the internal cementing holes 18 equispaced around its circumference is provided internally with a plurality of ribs which engage with corresponding ribs on the annular seat 19 which has a stepped internal configuration as shown in Fig. 10.The seat 19 is itself formed in two parts and is preferably made of a synthetic plastics material of appropriate strength and durability. Connecting pins 67 fasten the two parts of the seat 19 together. At the lower end of the one sleeve part 66 there is provided a circumferential groove in the outer wall of the sleeve part, which contains a snap ring 68. This snap ring 68 is normally held within the groove by the presence of the other sleeve part 70 which encircles the lower end of the one sleeve part 66 and which issecured to it by a plurality of circumferentially spaced pins 72. The second sleeve part 70 is provided at its upper internal margin with a groove 74 adapted to receive the snap ring 68 in locking engagement therewith as will be explained hereinafter.The lower end of the outer sleeve part 70 is provided with external locking ribs 76 which are arranged to engage with correspondingly shaped ribs 78 which are provided internally within the outer case 15 below the sleeve 17.

The method of operation in accordance with the invention will now be described.

RUNNING IN SEQUENCE Initially, the inner string cementing system is set up for operation as illustrated in Fig. 1.

The latch-in stinger 20 is first made up on the bottom of the drill pipe 22. The inner string seal assembly 24 is then fitted so that the centre of the seal assembly 24 will be located approximately 3 metres below the centre of the stage collar 14. The short drill collar 26 is then made up on to the seal assembly 24 and the pack-off tool assembly 28 is made up on to the drill collar 26. in making up the packoff assembly 28 it should be arranged that the assembly is positioned approximately 3 metres above the centre of the stage collar 14, so that the seal assembly 24 and tool assembly 28 are approximately equidistant below and above the stage collar 14 respectively. The drill pipe is made up until the inner drill string is complete.The drill pipe inner string is then set down in place within the borehole casing string by applying a thrust to it of 10,000 pounds which will latch the stinger 20 fully into the float collar 12. A pull-up force of 5,000 pounds is then applied to the drill string to check that the stinger 20 is fully latched into the float collar 12. When the inner string has been latched into place, only approximately one metre of the drill string should extend above the drilling rig rotary table. The cement head 32 is then connected to the upper end of the drill pipe, the cement head being pre-loaded with the wiper darts 33,34 and with the opening balls 35,36.

FIRST STAGE CEMENTING OPERATION The first stage cementing operation is illustrated in Figs. 2 and 3. Firstly, the borehole is circulated with drilling fluid for conditioning purposes. Then the lower drill pipe opening ball 35 is released from the surface and pumped down the hole until it lands in a ball catcher sleeve 80 (Fig.2) in the float collar 12.

Fig. 2 shows the ball 35 passing down the drill pipe 22. The ball catcher sleeve 80 is held in place by shear pins 82. A reading of 1000 psi at the surface will indicate that the pins 82 holding the sleeve have sheared after receiving the ball 35, allowing the ball and sleeve to fall into a catcher housing further down the float collar, as can be seen from Fig. 3. When this happens the float valve is converted to a back pressure fiap valve based upon a flap 84 which pivots into place through 90". Further drilling fluid is circulated to condition the hole, and then the required amount of cement is mixed and pumped down the drill pipe.When the required amount of cement has been pumped down the drill pipe, the lower drill pipe wiper dart 33 is released from the surface and is pumped down the drill pipe behind the cement slurry until it lands in a receptacle located in the float collar 12. This is shown in Fig. 3. A pressure reading of 1000 psi on a rig floor pressure gauge will confirm that the wiper dart 33 has sealed in the float collar 12. The pressure on the cementing unit is then checked for returns, and if there are no returns then an applied overpull of 12,000 pounds on the drill string will release the stinger 20 from the float collar 12, leaving the wiper dart 33 latched into the float collar. This completes the first stage cementing operation.

SECOND STAGE CEMENTING OPERATION The running procedure for the second stage cementing operation is illustrated sequentially in Figs. 4 to 8. In this second stage operation one carries out further filling with cement from a higher level on to the cement of the first stage operation which is still in a fluid state.

The drill string is raised until the latch-in inner string seal assembly 24 is 3 metres above the stage collar 14 (Fig. 4). The drill string is then rotated through 1800 to the right in order to unlatch the pack-off tool assembly 28 from the drill string. The tool assembly 28 will then remain located at that position within the casing string due to the pressure of the pack-off cup 42 against the internal wall of the casing.

The drill string is then raised a further 12 metres. This causes the seal assembly 24 to latch into the tool assembly 28. The drill string is then rotated through another 1800 to the right, thus causing the seal assembly 24 and the pack-off tool assembly 28 to lock together by the engagement of the [atch 56.

The drill string is then ready to be used to open the stage collar 14. At this time the sliding sleeve 17 in the stage collar 14 is held by shear pins 90 (Fig.10) in a raised position in which the internal cementing ports 18 and the external cementing ports 16 are not aligned. In other words, at this stage the stage collar is positively held closed.

In order to open the stage collar 14 the drill string is lowered until a weight indicator at the surface shows 30,000 pounds less than the hook load of the made-up string weight. This lowering operation causes the pins 90 to shear and the sliding sleeve 17 to move downwards (Fig.5) until the cooperating ribs 76 and 78 engage to lock the sliding sleeve in the open position where the internal and external cementing holes 18 and 16 are in alignment. This lowering of the drill string also causes the pack-off tool assembly 28 to be locked into engagement with the stage collar 14 by virtue of the pivotable lugs 54 engaging behind the internal seat 19 in the stage collar.

As will be seen from Fig. 5, in this position the cup 42 engages positively with the wall of the casing to prevent any upward movement of cement out of the chamber defined by the stage collar and the pack-off cup. The primary reason for locking open the stage collar sleeve 17, and also for locking the pack-off tool assembly 28 to the seat 19, is that a very heavy cement mix is normally needed for the second stage operation. This gives a back pressure from the annular inner string through the cementing ports in the stage collar of some 35 to 40 bar pressure when the cement slurry is pumped. This locking mechanism involving the stage collar 14 is therefore important for a satisfactory cementing operation.

Then, the the remaining drill pipe ball 36 is released. This ball 36 lands in a sleeve inside the seal assembly 24. As the ball 36 also closes off the drill pipe below the sleeve in the seal assembly, circulation can then be started through the stage collar ports 18, 16 and is continued until all the ports are cleaned and the mud is in good condition.

After the cement slurry has been pumped down the drill string, the second drill pipe wiper dart 34 is then released from the surface and is pumped down the hole behind the cement slurry (Fig. 6). The dart 34 will continue downhole until it lands in the seal assembly (Fig. 7). At this point all the cement will have been dissipated out into the annulus around the casing string through the cementing ports 18, 16 in the stage collar. An overpull of 30,000 pounds is then applied to the drill string in order to close and lock the stage collar 14 shut. When this overpull is applied to the drill string the seat 19 in the stage collar tears away from the sleeve part 66 and is carried upwards to the surface with the drill string, as shown in Fig. 8.Additionally, the pins 72 holding the two parts 66 and 70 of the sliding sleeve 17 together shear, enabling the sleeve part 66 to rise while the other sleeve part 70 is held locked in place by the interengaging ribs 76 and 78. As the sleeve part 66 rises so the snap ring 68 will engage in the recess 74 and will lock the two parts of the sliding sleeve together but in a displaced relationship so that the stage collar is not only closed but is also locked closed. This action of locking the stage collar closed after the cementing operation is very important, because, when all the cementing tools have been returned to the drilling rig floor, the next operation is then to drill deeper using a smaller size of pipe.It would be disastrous if, during this drilling operation, the stage collar 14 was to open, as the drilling fluid used in the drilling operation could then force its way out of the stage collar ports into the recently cemented section. This could result in the rig "losing circulation" which would be a very expensive event, requiring drilling to stop and drilling time to be lost.

When the stage collar 14 has been closed and locked closed the second stage cementing process is complete and the pack-off tool assembly 28 and seal assembly 24 are raised to the surface with the drill string. When these assemblies have passed through the rotary table, the cementing ports and latching mechanisms are flushed out with water so that these components can be re-used. Thus, the whole unit is retrievable. The borehole is then clear for the next drill bit without any need for drilling out a collar, as is often necessary with conventional systems.

Claims

1. A stage collar insertable as a section within a borehole casing string, the collar comprising an outer case provided with first cementing ports, and a sleeve slidable within the outer case and provided with second cementing ports, the collar being open when the first and second ports are in alignment and closed when the said ports are out of alignment, wherein the sleeve is provided with internal seating means engageable by a tool movable within the casing string to control both the opening and the closing of the collar, and wherein the sleeve is arranged to be locked irreversibly in a position where the collar is closed when the sleeve slides from a position where the collar is open to a position where the collar is closed.

2. A stage collar as claimed in claim 1, in which the sleeve comprises two annular parts which are movable jointly when the sleeve is displaced initially to open the collar, one of said parts thereby becoming locked in position relative to the outer case, and the other of said parts being displaceable relative to said one part, when the collar is subsequently closed, into a position in which it also is locked relative to the outer case.

3. A stage collar as claimed in claim 2, in which said two sleeve parts are axially slidable and a detent means locks said other part to said one part after a predetermined axial displacement of the other part relative to the locked one part.

4. A stage collar as claimed in claim 3, in which the detent means comprises a snap ring housed in said other sleeve part and which is receivable in a groove in said one sleeve part for said locking engagement.

5. A stage collar as claimed in any preceding claim, in which the seating means is arranged to latch the tool into a locked relationship with the collar and to be detachable from the sleeve for removal with the tool when the tool is withdrawn to close the collar.

6. A stage collar as claimed in claim 2, 3 or 4, in which the locking of said one sleeve part to the outer case is by interengaging ribs on said respective elements.

7. In combination with a stage collar as claimed in any of claims 1 to 6, a drill string comprising a drill pipe and a pack-off tool assembly which can be latched to and unlatched from the drill pipe and which includes a pressure pack-off cup arranged to contact the internal wall of the casing string, wherein the tool assembly also includes latching means by which it can be locked into engagement with the stage collar whereby the tool assembly can be used both to open and close the stage collar with the tool assembly locked in place.

8. The combination as claimed in claim 7, in which the pack-off tool assembly includes a plurality of detent lugs below the cup and arranged to make latching engagement with a seat of stepped configuration positioned internally of the sleeve.

9. The combination as claimed in claim 7 or 8, in which the drill string includes a seal assembly fixedly connected in the drill string below the pack-off tool assembly, the seal assembly being arranged latchingly to engage with the tool assembly by unlatching the tool assembly from the drill pipe and raising the drill string until the seal assembly engages the tool assembly.

10. The combination as claimed in any of claims 7 to 9, in which the pack-off assembly is coupled to the drill pipe by an "L"-type latch.

11. The combination as claimed in any of claims 7 to 10, in which the pack-off tool assembly comprises friction drag means adjacent to its upper end above the cup and engageable with the casing wall surface.

12. A method of carrying out a two-stage cementing operation in a borehole fitted with a casing string incorporating a stage collar as claimed in any of claims 1 to 6, which comprises the steps of: (a) making up a drill string comprising a drill pipe and a pack-off tool assembly which can be latched to and unlatched from the drill pipe and which includes a pressure pack-off cup arranged to contact the internal wall of the casing string; (b) locating the lower end of the drill string in a collar or shoe within the casing string and pumping first stage cement down the drill pipe through the collar or shoe; (c) releasing the drill string from the collar or shoe;; (d) unlatching the tool assembly from the drill string above the stage collar, raising the rest of the drill string, latching the tool assembly to the drill string again, and lowering the drill string until the tool assembly lockingly engages and opens the stage collar; (e) sealing off the drill pipe below the stage collar; (f) pumping second stage cement down the drill pipe and through the stage collar into the annulus around the casing string; (g) and thereafter exerting an upward pull on the drill string which first, via the tool assembly, closes and locks closed the stage collar, and then breaks the locking engagement between the tool assembly and the stage collar.

13. A method as claimed in claim 12, in which the drill string includes a seal assembly which is used to seal off the drill pipe below the stage collar, and in which when the drill string is initially located in a collar or shoe for the first stage cementing operation the seal assembly is positioned a distance below the stage collar approximately equal to the distance which the tool assembly is positioned above the stage collar.

14. A method as claimed in claim 13, in which step (d) includes raising the drill string until the seal assembly is located a predetermined distance above the stage collar, rotating the drill string through 180 in a first sense to unlatch the tool assembly, raising the drill string until the seal assembly latches into the tool assembly, rotating the drill string through a further 1800 in the same sense to lock the seal assembly and the tool assembly together, and lowering the drill string into contact with the stage collar.

15. A stage collar substantially as hereinbefore described with reference to Figs. 1 to 8 and 10 of the accompanying drawings.

16. Borehole cementing apparatus substantially as hereinbefore described with reference to the accompanying drawings.

17. A method of cementing a borehole substantially as hereinbefore described with reference to the accompanying drawings.