Valve assembly and method of controlling fluid flow in an oil, gas or water well

Abstract

A valve assembly (1) for use particularly in a deviated wellbore of an oil, gas or water well comprises a body (50) with an axis and first and second resiliently deformable seats (20, 25) to seat a valve closure member (10a) such as a ball, the seats being deformable to allow passage of the ball at different first and second fluid pressures acting on the seated valve closure member. The first and second seats are axially spaced from one another on a control sleeve (60) on opposite sides of an inner end of a selectively operable fluid outlet conduit connecting the bore with an external surface of the valve assembly, and operation of the valve assembly at pressures between the first and second pressures opens and closes the outlet while maintaining the ball between the first and second seats.

Claims

1. A valve assembly for use in a wellbore of a well, the valve assembly comprising: a body with a bore for flow of fluid through the valve assembly, the bore having an axis; a valve closure member; a control sleeve which is axially movable within the bore relative to an outlet port provided in the body between open and closed configurations of the control sleeve to open and close fluid communication between the outlet port and the bore, and first and second resiliently deformable seats axially spaced from one another in the bore and each adapted to seat the valve closure member in the bore, to resist the passage of fluid through the bore past the seated valve closure member, wherein the first and second seats are disposed in the control sleeve, wherein the first seat is adapted to resiliently deform from a first resting configuration to a second deformed configuration of the first seat to allow passage of the valve closure member past the first seat at a first threshold pressure of fluid acting on the valve closure member seated on the first seat, wherein the second seat is adapted to resiliently deform from a first resting configuration to a second deformed configuration of the second seat to allow passage of the valve closure member past the second seat at a second threshold pressure of fluid acting on the valve closure member seated on the second seat, wherein the second threshold pressure is higher tha n the first threshold pressure, and wherein the first and second seats are axially spaced from one another along the axis of the bore on opposite sides of an inner end of a selectively operable fluid outlet conduit connecting the bore with an external surface of the valve assembly.

2. The valve assembly of claim 1, wherein each of the first and second seats has an inner diameter providing a restriction in the bore at an apex of the respective first and second seats, and wherein the apexes of the first and second seats are axially spaced apart by a distance greater than the inner diameter at the apex of at least one of the first and second seats.

3. The valve assembly of claim 2, wherein the apex of at least one of the first and second seats comprises the narrowest part of the bore.

4. The valve assembly of claim 1, wherein the first and second seats are each radially compressible, with an inner diameter:outer diameter ratio which increases as the valve closure member passes through each of the first and second seats.

5. The valve assembly of claim 1, wherein an inner diameter and a radial thickness of each of the first and second seats recover resiliently to the respective first resting configurations after axial passage of the valve closure member through the first and second seat.

6. The valve assembly of claim 1, wherein the first seat is closer to an entry of the wellbore into a formation in which the wellbore is drilled than the second seat.

7. The valve assembly of claim 1, wherein the second seat has a higher elastic modulus than the first seat.

8. The valve assembly of claim 1, further comprising a resilient device, wherein the control sleeve is biased resiliently against the direction of fluid flow through the bore of the valve assembly by the resilient device.

9. The valve assembly of claim 1, wherein the first and second seats comprise mutually parallel rings extending circumferentially around an inner surface of the control sleeve.

10. The valve assembly of claim 1, wherein the first and second seats each extend radially inwards from an inner surface of the control sleeve, creating a throat in each of the first and second seats that is narrower than a bore of the control sleeve and a sealing diameter of the valve closure member.

11. The valve assembly of claim 1, wherein rotation of the control sleeve relative to the outlet port is restricted.

12. The valve assembly of claim 1, wherein the control sleeve is biased in the closed configuration and wherein fluid pressure above the seated valve closure member at the first threshold pressure is insufficient to move the control sleeve from the closed configuration.

13. The valve assembly of claim 1, wherein the control sleeve is biased in the closed configuration and wherein fluid pressure above the seated valve closure member between the first and second threshold pressures causes the control sleeve to move from the closed configuration to the open configuration.

14. The valve assembly of claim 13, further comprising a resilient device, wherein seating of the valve closure member in the second seat leads to a build-up of fluid pressure uphole of the second seat which overcomes the force of the resilient device biasing the control sleeve into the closed configuration, such that the control sleeve is urged axially under the fluid pressure relative to the outlet port from the closed configuration into the open configuration in which the outlet port is at least partially in fluid communication with the bore.

15. The valve assembly of claim 1, further comprising an outlet sleeve that is fixed in the bore of the body over the outlet port in the body, wherein the outlet sleeve comprises a leading edge formation at an uphole end of the outlet sleeve, formed as a radially inwardly extending shoulder having a throat that narrows to a diameter at a downhole end of the shoulder that is at least as narrow as an inner diameter of a bore of the control sleeve.

16. The valve assembly of claim 1, further comprising a shoulder extending radially into the bore above the first and second seats.

17. The valve assembly of claim 16, wherein the shoulder has a maximum diameter at an uphole end of the shoulder, and tapers to a narrower diameter towards a downhole end of the shoulder.

18. The valve assembly of claim 1, wherein the bore is adapted to receive the valve closure member and a second valve closure member, wherein the second valve closure member is inserted into the bore after the valve closure member is retained in the second seat and wherein the fluid outlet conduit is adapted to be obstructed by the second valve closure member, and wherein build-up of fluid pressure within the bore above the second valve closure member to the second fluid pressure threshold is adapted to force the valve closure member and the second valve closure member through the first and second seats.

19. A method of diverting fluid flow in a wellbore of a well, the method comprising: flowing fluid through a valve assembly having a body comprising a bore with an axis, a control sleeve which is axially movable within the bore relative to an outlet port provided in the body between open and closed configurations of the control sleeve to open and close fluid communication between the outlet port and the bore, and first and second seats disposed in the control sleeve, the bore being in fluid communication with the wellbore, wherein the first seat is adapted to resiliently deform from a first resting configuration to a second deformed configuration of the first seat to allow passage of a valve closure member past the first seat at a first threshold pressure of fluid acting on the valve closure member seated on the first seat, and wherein the second seat is adapted to resiliently deform from a first resting configuration to a second deformed configuration of the second seat to allow passage of the valve closure member past the second seat at a second threshold pressure of fluid acting on the valve closure member seated on the second seat, wherein the second threshold pressure is higher than the first threshold pressure; admitting the valve closure member into the bore of the body and seating the valve closure member on the first seat; raising fluid pressure acting on the seated valve closure member on the first seat to the first threshold pressure to move the valve closure member past the first seat and seating the valve closure member on the second seat; raising the pressure acting on the seated valve closure member on the second seat to an activation pressure between the first and second threshold pressures and diverting the fluid flowing in the bore through a fluid outlet in communication with the bore and disposed between the first and second seats; retaining the valve closure member between the first and second seats during activation; and raising the pressure above the seated valve closure member on the second seat to the second threshold pressure to move the seated valve closure member through the second seat to open the bore of the valve assembly.

20. The method of claim 19, further comprising: obturating the bore of the valve assembly by seating the valve closure member on the second seat; and actuating the valve assembly from a first configuration in which fluid flow is directed axially through the bore, to a second configuration in which fluid flow is directed radially through at least one outlet port disposed in a side wall of the valve assembly.

21. The method of claim 20 wherein the control sleeve is biased resiliently against the direction of fluid flow through the bore of the valve assembly by a resilient device, and wherein the method further comprises building fluid pressure uphole of the valve assembly when the bore is obturated by the valve closure member to urge the control sleeve in a downhole direction against the biasing force of the resilient device.

22. The method of claim 19, further comprising inserting a second valve closure member into the bore and seating the second valve closure member on the first seat to close off communication between the bore and the fluid outlet.

23. The method of claim 19, further comprising increasing the fluid pressure in the bore until it reaches the second threshold pressure, and forcing the valve closure member through the second seat.

24. The method of claim 19, further comprising returning the valve assembly to a closed configuration in which fluid travels in an axial direction through the bore by expansion of a resilient device.

25. The method of claim 19, further comprising reducing downward thrust acting on one of the first and second seats by restricting fluid flow through the bore axially uphole of one of the first and second seats.

26. A valve assembly for use in a wellbore of a well, the valve assembly comprising: a body with a bore for flow of fluid through the valve assembly, the bore having an axis; a valve closure member; first and second resiliently deformable seats axially spaced from one another in the bore and each adapted to seat the valve closure member in the bore, to resist the passage of fluid through the bore past the seated valve closure member; a resilient device; and a control sleeve having an outlet aperture, the first and second seats being axially spaced from one another on the control sleeve and disposed on opposite sides of the outlet aperture, the control sleeve biased resiliently against the direction of fluid flow through the bore of the valve assembly by the resilient device, and the control sleeve further being axially movable within the bore relative to an outlet port provided in the body between open and closed configurations of the control sleeve to open and close fluid communication between the outlet port and the bore, wherein the first seat is adapted to resiliently deform from a first resting configuration to a second deformed configuration of the first seat to allow passage of the valve closure member past the first seat at a first threshold pressure of fluid acting on the valve closure member seated on the first seat, wherein the second seat is adapted to resiliently deform from a first resting configuration to a second deformed configuration of the second seat to allow passage of the valve closure member past the second seat at a second threshold pressure of fluid acting on the valve closure member seated on the second seat, wherein the second threshold pressure is higher than the first threshold pressure, and wherein seating of the valve closure member in the second seat leads to a build-up of fluid pressure uphole of the second seat which overcomes the force of the resilient device biasing the control sleeve, such that the control sleeve is urged axially under the fluid pressure relative to the outlet port from the closed configuration into the open configuration in which the outlet port is at least partially in fluid communication with the bore.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings, FIGS. 1-8 show a sequence of views showing sequential steps in the operation of one example. In the drawings:

(2) FIG. 1 shows a close up section view of a valve assembly in a running in (outlet port closed and central bore open) configuration with no valve closure member seated;

(3) FIG. 2 shows the valve assembly of FIG. 1 with a first valve closure member seated on a first valve seat;

(4) FIG. 3 shows the valve assembly of FIG. 1 with the valve closure member passing through the first valve seat under fluid pressure from above it, e.g. from surface pumps;

(5) FIG. 4 shows the valve assembly of FIG. 1 with the valve closure member seating on the second seat;

(6) FIG. 5 shows the valve assembly in a circulating “open” configuration when under fluid pressure from above with the bore occluded by the seated closure member on the second seat, and a radial outlet port through the wall of the body of the valve assembly diverting fluid flow through the wall for circulation tasks;

(7) FIG. 6 shows the valve assembly in the open configuration of FIG. 5 with a further valve closure member approaching the first seat;

(8) FIG. 7 shows the valve assembly in the FIG. 6 configuration, but with the further valve closure member now driven seated on the first seat and engaging the first valve closure member on the second seat;

(9) FIG. 8 shows the final configuration similar to FIG. 1, after both valve closure members are driven through the seats;

(10) FIGS. 9-16 show views of the valve assembly according to FIGS. 1-8 in the context of a tubular adapted for connection into a string.

DETAILED DESCRIPTION

(11) Referring to the drawings, which show an example of a valve assembly 1 for use in a wellbore of an oil, gas or water well, comprises a body 50 which can be in the form of a tubular having box and pin connections or similar, and adapted to be connected into a string of tubulars, for example a drill string, having a drill bit at the lower end (which is the right hand end as shown in the drawings). The body 50 has a bore 50b in fluid communication with the bore of the string, and the bore 50b houses a number of valve components optionally in the form of sleeves. In this example, the bore 50b has an outlet sleeve 70 fixed in the body at an axial location of an outlet port 52, with which it communicates via an aperture 72 aligned with the outlet port 52, and a control sleeve 60. The outlet sleeve 70 surrounds a portion of the control sleeve 60, which has a bore 60b with an axis that is generally co-axial with the bore 50b of the body and the bore of the outlet sleeve 70. The bores of the sleeves 60, 70 are in fluid communication with the bore 50b of the body 50. Seals are provided between the body 50 and outlet sleeve 70 and between the outlet sleeve 70 and the control sleeve 60, above and below the outlet port 52.

(12) The outlet sleeve 70 is fixed in the body 50 in both rotational and axial position by fixing members in the form of pins 54, which are inserted through the wall of the body 50, into receiving bores in the outlet sleeve 70. The pins 54 can be removed in order to facilitate removal and replacement of the outlet sleeve 70 when necessary, for example in the event of erosion of the aperture 72. The pins 54 further extend radially inwards to engage the outer surface of the control sleeve 60, and are adapted to be received in axial slots 60s in the outer surface of the control sleeve 60 in the bore of the outlet sleeve 70, to restrict rotational movement of the control sleeve 60 while permitting relative axial movement of the control sleeve 60 within the body 50.

(13) The outlet sleeve 70 provides a replaceable “hanger” in the bore for the connection of the other components, and protects the outlet port 52 from erosion damage by fluid flow through the outlet port 52. The outlet sleeve 70 can be readily removed and replaced when damaged by erosion, or if a different size of inner bore is needed.

(14) A resilient device, in this example in the form of a compression spring 80, circumferentially surrounds a downhole end of the control sleeve 60, and is held in compression to bias the control sleeve 60 upwards in the bore into a first running in configuration as shown in FIG. 1, in which the bore is open and the outlet port 52 is closed. In the first configuration shown in FIG. 1, the spring 80 is energised in compression between an optional spring retainer 85 surrounding the control sleeve 60 at the spring's downhole end and abutting against an upwardly facing shoulder in the bore (optionally formed by a sleeve in the bore), and a downwardly facing shoulder of the control sleeve 60 at its uphole end. The spring 80 is optionally preloaded in compression in the FIG. 1 state, and its expansion urges the control sleeve 60 in an uphole direction within the bore 50b against the fluid flow until the pin 54 abuts a lower end of the slot 60s on the control sleeve 60, which limits the further axial travel of the control sleeve 60 (and the expansion of the spring 80) within the bore 50b in the uphole direction. The spring 80 can be compressed further as will be described below.

(15) The control sleeve 60 is adapted to slide axially in the bore 50b (but in this example is resistant to rotation in the bore due to the pin 54 and slot 60s), to open and close at least one fluid pathway in the assembly connecting the bore 50b with the outlet port 52, in this example to divert the fluid flowing through the bore 50b of the body and the bore 60b of the control sleeve 60 and out into the annulus of the wellbore, through the outlet port 52 in the body 50.

(16) In the first configuration shown in FIG. 1, the control sleeve 60 is positioned within the body 50 such that an outlet aperture 62 through the control sleeve 60 is out of alignment with the outlet aperture 72 on outlet sleeve 70, closing off fluid communication between the bore 50b and the outlet port 52, and maintaining axial fluid flow F.sub.1, with the direction of flow as illustrated by the arrow in FIG. 1, in a downhole direction within the bore 60b of the control sleeve. This is the running in configuration of the valve assembly before any circulation tasks are started.

(17) The valve assembly 1 is actuated between different configurations to permit and restrict fluid communication between the bore of the valve assembly 50b and an external surface of the valve assembly. When the valve assembly 1 is in the running in configuration shown in FIG. 1, the outlet port 52 is obturated by the control sleeve 60, which is urged axially upwards relative to the outlet port 52 to cover it in the first configuration. Annular seals are optionally compressed between the outlet sleeve 70 and the control sleeve 60 in axial positions above and below the outlet port, so that in the closed configuration in FIG. 1, the control sleeve 60 seals off all fluid communication between the bore 60b and the outlet port 52, so fluid flows in the axial direction through the bore. The control sleeve 60 has at least one and in this case, two outlet apertures 62 which pass radially through a wall of the control sleeve 60 at the same axial location on the control sleeve 60, and which are spaced diametrically from one another around the circumference of the control sleeve 60. When the control sleeve 60 is in the first configuration shown in FIG. 1, the apertures 62 are above the apertures 72, out of axial alignment with the outlet port 52, and in this configuration, the outlet port 52 is closed and the fluid flowing through the bore 50b above the valve assembly 1 flows through the bore 60b of the control sleeve and on through the tubular string to the drill bit (for example) in a generally unobstructed manner.

(18) When the outlet port 52 is to be opened and fluid flow is to be diverted to the outlet ports 52 for example in a circulation operation, the control sleeve 60 moves axially down the bore from the first configuration shown in FIG. 1 with axial fluid flow F.sub.1, to the second configuration with radial fluid flow F.sub.2, as shown in FIG. 5, to open the outlet port 52 as will be described below. The axial travel of the control sleeve 60 can result in the outlet port 52 being fully open (as shown in FIG. 2), fully closed (as shown in FIG. 1), or partially open (an intermediate position between the two).

(19) In this example, the control sleeve 60 further comprises a first seat 20 and a second seat 25 situated respectively on opposite sides (above and below) of the outlet apertures 62. When the control sleeve 60 is in the first configuration of FIG. 1 and the outlet port 52 is closed the seats 20, 25 do not offer any substantial obstruction to axial flow of the fluid through the bore 60b. The seats 20, 25 are adapted to seal the bore by seating at least one valve closure member, for example, a ball, a dart, a plug etc. The valve closure member is normally dropped from surface or otherwise released into the tubular above the seats 20, 25, and travels with the fluid flow in a downhole direction to the seats 20, 25, where its further axial travel in the bore 50b is prevented, and it closes or substantially obturates the bore of the control sleeve 60 by seating on the seat 20, 25. Each seat 20, 25 is optionally formed as an annular ring of inherently resilient material such as rubber, plastics etc.

(20) Each of the seats 20, 25 is adapted to deform resiliently from the first resting configuration seating the ball 10a into a second radially compressed configuration to allow passage of the ball 10a past the seats when the force urging the ball 10 downwards in the bore overcomes the inherent resilience of the material of the seat 20, 25 reacting against it. The valve seats 20, 25 extend radially inwards into the bore 60b of the control sleeve 60. The inner radial dimension of each seat 20, 25 in a resting configuration where no force is acting on it is smaller than the maximum radial dimension of the ball 10a so that the ball 10a seats on the seats 20, 25. The inner radial dimension of each seat 20, 25 is adapted to expand radially during deformation and axial passage of the ball through the seat 20, 25, such that the radial thickness of each seat 20, 25 reduces transiently during deformation. Thus as the ball 10a passes through the valve assembly under the force of the fluid pressure above it, the inner faces of the seats 20, 25 are resiliently compressed in a radially outward direction by the non-deformable ball 10a acting under the force of fluid pressure directed downhole from the surface. Each seat 20, 25 optionally maintains a consistent outer radial dimension and volume in the resting and deformed configurations, and merely changes shape when deforming.

(21) In this example, the control sleeve 60 optionally comprises an assembly of separate sleeves which are interconnected, mainly for reasons of easy assembly and disassembly for the replacement or servicing of the various parts. In the present case, the control sleeve 60 comprises a central sleeve 61 having a radially inwardly extending shoulder extending into the bore 60b and accommodating the outlet apertures 62 within that shoulder. Below the shoulder, the central sleeve 61 is internally threaded for connection to a lower sleeve 63 having an external threaded section which is received within the bore of the central sleeve 61. The lower sleeve 63 also has an internally threaded bore at its lower end, which accommodates spring sleeve 65, which is assembled together with the spring which fits over the spring sleeve 65 before the spring sleeve 65 is offered to the threaded internal bore of the lower sleeve 63. In this example, the lower sleeve 63 and spring sleeve 65 can be secured after assembly of the spring by a grub screw 64 extending between each after the thread has been made up. Other methods of connecting the sleeves can of course be used. The assembly of the lower sleeve 63 and spring sleeve 65 is then offered into the bore of the central sleeve 61. At the upper end of the central sleeve 61, a cap 67 is threaded onto the upper end of the control sleeve. This multipart structure allows the easy assembly, disassembly and replacement of the seats 20, 25, which in this example are in the form of annular rings of resilient material which are offered to the opposing end bores of central portion 61 before assembly of the other components, and which abut the radially inwardly extending shoulder on opposite sides of the outlet aperture 62. For example, after insertion of the second seat 25 into the bore of the lower end of the central portion 61, the assembled lower sleeve 63 and spring sleeve 65 is then connected to the central portion 61 to secure it in place. The cap 67 is fitted to the upper end in the same way, once the first seat 20 has been received in place against the upper face of the inwardly radially extending shoulder. Seals are provided between the various sleeve components so that when assembled, the connected sleeves effectively perform as a single unit 60 while allowing disassembly for replacement and servicing of components such as the seats 20, 25.

(22) FIGS. 2 and 3 show the valve assembly 1 of FIG. 1, with a first valve closure member in the form of a first ball 10a seated on (FIG. 2) and passing through (FIG. 3) the first seat 20. The first seat 20 deforms radially to allow passage of the ball 10a under a relatively small pressure differential applied by the fluid above the seated ball 10a, which is insufficient to compress the spring 80. As the fluid pressure builds above the seated ball 10a to the first pressure threshold, the ball 10a moves from the FIG. 2 seated position to the FIG. 3 “passing through” position as the first seat 20 deforms radially outwards to allow the passage of the ball 10a through the first seat 20. As the spring 80 remains uncompressed at these relatively low pressures, the control sleeve 60 remains still and the outlet port 52 remains closed, so fluid pressure above the seated ball 10a does not escape from the bore 50b. Eventually the ball 10a squeezes through the first seat 20 and seats on the second seat as shown in FIG. 4. The second seat 25 has a higher elastic modulus than the first seat 20, so does not initially deform in response to the pressure differential which is still relatively low at this point, just above the first pressure threshold. However, since the bore 50b is now blocked, as the ball 10a is seated on the second seat 25, the pressure differential across the seated ball 10a on the second seat 25 continues to build as the fluid is pumped from the surface.

(23) As the increasing fluid pressure differential across the seated ball 10a starts to overcome the force of the spring 80 supporting the control sleeve 60, the pressure is generally higher than the first pressure threshold needed to push the ball through the relatively soft first seat, but lower than the second pressure threshold needed to push the ball 10a through the harder second seat, so while the control sleeve 60 is urged axially under the fluid pressure the ball 10a stays seated on the second seat 25 blocking the bore. The fluid pressure acting on the seated ball 10a increases eventually to a required activation pressure needed to overcome the spring force, and at this point the force of the fluid above the seated ball 10a pushes the control sleeve 60 axially in a downhole direction. The pins 54 allow the control sleeve 60 to translate in an axial direction without a rotational component, thus maintaining the axial alignment of the aperture 62 with the outlet sleeve aperture 72 and the outlet port 52. The axial movement of the control sleeve 60 compresses the spring 80 between the control sleeve 60 and the spring retainer 85 as can be seen in FIG. 5. As the control sleeve 60 moves in a downhole direction relative to the outlet sleeve 70 and the body 50, the aperture 62 moves into alignment with the aperture 72 and the outlet port 52, which allows the pressurised fluid to escape in a radial direction into the annulus of the wellbore for circulation of the fluid above the drill bit for example. These high pressure jets of fluid can be used for, for example, cleaning the annulus, or washing drill cuttings back to the surface. The fluid is prevented from flowing into the space between the body 50 and the outlet sleeve 70 by a pair of seals situated just uphole and just downhole of the outlet sleeve aperture 72. The space between the control sleeve 60 and the outlet sleeve 70 is similarly sealed off. Thus, the fluid is directed to flow solely out of the outlet port 52 and is prevented from escaping through other paths.

(24) Once the ball 10a is in the FIG. 5 position, the pumps can be driven continuously to maintain the pressure differential and perform circulation tasks. The pumps can optionally be switched off to remove the pressure differential, and release the ball 10a from the second seat 25, causing the spring 80 to expand and return the control sleeve 60 to the FIG. 1 position, and this can be done without fear of losing the ball 10a for example in a deviated wellbore, because the first seat 20 resiliently recovers to its original form shortly after passage of the ball 10a, regaining the apex ID that is narrower than the ball 10a which deformed it to squeeze through, and hence after passage through the first seat 20, the ball 10a is trapped between the first and second seats 20, 25, and can readily be re-applied to the second seat by just switching on the pumps again. This is especially useful if the wellbore is deviated or horizontal, as the resiliently recovered first seat stops the ball from rolling back up the bore 50b away from the seat 25. The seats 20, 25 are spaced apart by a relatively short distance, optionally between 1× and 2× the diameter of the ball 10a, or the ID of the seats 20, 25. The distance between the apexes of the seats is optionally chosen so that when the first ball 10a is seated on the second seat 25, the second ball 10b is supported by the first ball 10a (and therefore its movement is arrested by the seated first ball 10a) at the same time as the second ball 10b is seated on the first seat 20. Hence, the ball 10a is kept available for re-application to the second seat 25 when required, and the pumps do not need to be continuously operated, and can be switched on and off as required to open and close the port during the circulation operations. This can happen as many times as is needed during circulation operations.

(25) The force required to deform the second seat 25 is higher than that required to deform the first seat 20, and is also higher than what is required for most circulation operations, so in the FIG. 5 configuration, especially since the fluid pressure is escaping the bore 60b via the open outlet port 52, the second seat 22 has not yet resiliently deformed and continues to seat the ball 10a between the first and second seats 20, 25, which are axially spaced from one another along the axis of the bore 50b on opposite sides of the outlet apertures 62 and outlet ports 52. Circulation operations can thus be performed as needed with the ball 10a in the FIG. 5 position. When circulation operations are completed and the valve assembly is to be reset to the starting configuration, for example for the resumption of drilling, or for the performance of operations below the valve assembly, the ball 10a is unseated from the second seat 25 as follows.

(26) Unseating of the ball 10a from the second seat 25 can be initiated when the control sleeve is still in the FIG. 5 configuration, with the outlet port 52 radially aligned with the control sleeve aperture 62 and the ball 10a seated on the seat 25. In order to reset the valve assembly 1 to the initial drilling configuration and to unseat the ball 10a, a second valve closure member in the form of a ball 10b is inserted into the bore 50b of the body 50 above the seat 25 while the first ball 10a is seated on the second seat 25. The second or further ball 10b lands on first seat 20 as shown in FIG. 6, blocking the bore once more and forcing the second ball 10b through the first seat 20 as the pressure differential exceeds the first pressure threshold.

(27) The axial distance between the apexes of the first and second seats 20, 25 is chosen to be between 1× and 2× the maximum outer diameter of the balls 10a, 10b, so that when the second ball 10b has is seated on the first seat 20 with its maximum OD engaged in the apex of the first seat 20, the second ball 10b is abutting the first ball 10a seated on the second seat 25. In this configuration, shown in FIG. 7, the second ball 10b seals off the bore 60b of the control sleeve 60 above the aperture 62, thereby substantially obturating the bore 60b of the control sleeve 60 and effectively preventing escape of the fluid through the outlet port 52. Fluid pressure within the closed bore 50b above the seated second ball 10b then rapidly builds up to a second fluid pressure threshold that is higher than the first fluid pressure threshold, higher than the activation pressure for opening the valve outlet, and also higher than normal operating pressures for the circulation tasks. At the second pressure threshold the fluid pressure above the obturated bore has increased to a level at which the force urging the balls 10b, 10a downwards in the bore 60b is greater than the resilient force maintaining the ball 10a on the second seat 25, and the higher force exerted by the fluid forces the first and second balls 10a, 10b rapidly through the second seat 25, which resiliently deforms as the balls 10a, 10b pass through it, before returning to its original configuration. The balls 10a, 10b are optionally caught in a ball catcher device (not shown) after they have passed through the seat 20.

(28) In this example, the first seat 20 and the second seat 25 optionally take different forms. According to one option, as best seen in FIG. 8, the first seat has a generally asymmetric cross-section, with an upper funnel section having a wide upper mouth converging to a reduced ID above the apex, which has the narrowest ID of the seat. The apex is disposed below funnel section, nearer to the lower end of the first seat 20 than the upper end. When a ball or other valve closure member seats on the first seat 20, it is received in the upper funnel section, and in this example, the diameter of the ball and the inner diameter of the apex are selected so that the ball 10 seats on the first seat 20 most firmly when the maximum OD of the ball 10 has passed the funnel section and is engaged on the apex. As can be best seen in FIG. 8, the apex of the first seat is generally cylindrical with a generally consistent ID along its axial length, and a good seal is obtained between the ball maximum OD and the apex of the first seat 20 substantially along the whole axial distance of the cylindrical apex. This is useful because it ensures that a consistent seal is formed between the first seat 20 and the second ball 10b while the first ball 10a is being pushed through the apex of the second seat 25. At that time, both balls are moving down the bore 60b under the relatively constant pressure. The second ball 10b optionally only moves out of the cylindrical apex of the first seat 20 after the first ball 10a has passed through the apex of the second seat 25, so the pressure differential across the second ball 10b is maintained relatively constant until the second ball 10b has dropped through the cylindrical apex in the first seat 20. In some examples, the apex does not require to be of consistent ID, and can have a radius. The lower end of the seal 20 below the cylindrical apex optionally has a chamfered section that diverges radially outwardly from the apex, to reduce the resilient force applied to the ball as it moves axially out of the apex, and to reduce the erosion experienced by the seat 20 in use. The cylindrical apex of the first seat optionally has a narrower ID than the apex of the second seat below it.

(29) The second seat 25 typically has around the same axial length as the first seat 20, but the ID is radiused and non-linear as best shown in FIG. 8. The radiused ID of the second seat 25 reaches its narrowest point at an apex, which is generally at the centre of the seat 25, so the second seat 25 is generally symmetrical. The apex of the second seat 25 has a very slightly wider ID than the apex of the first seat 20 The ball 10a is most firmly seated on the second seat 25 when its maximum OD is just above the apex, as shown in FIG. 5, and the ball is less firmly retained by the seat 25 as soon as the maximum OD has moved below the radiused apex of the second seat 25. This is a useful feature as it allows the second seat 25 to retain the first ball 10A firmly on the upper side of the apex, as shown in FIG. 5, and does not delay its retention in the seat as soon as it has squeezed past the apex. In fact, since the ID of the second seat 25 is continuously expanding on an arc below the apex, the resilient recovery of the second seat after deformation can optionally assist in the ejection of the ball 10A from the second seat 25 at the second pressure threshold.

(30) The inherent resilience of the material of the seats 20, 25 is optionally such that the original configuration as shown in FIG. 8 is not immediately recovered after the balls are forced through in response to the second pressure threshold, so the second seat 25 is optionally still partially resiliently deformed as the second ball 10b passes it under the force of the second pressure threshold. Optionally the second ball 10b is very slightly smaller than the first ball 10a, so that it can pass more easily through the seats 20, 25, and is not retained in the second seat 25 when the second pressure threshold is applied.

(31) In this example, the outlet apertures 62, 72 and the outlet port 52 are optionally directed at least in part in a non-perpendicular direction with respect to the axis of the bore 50b. Thus, each of the outlet apertures 62, 72 are at least partially directed radially outwardly at an angle toward the lower end of the tool (to the right as shown in the drawings). The outlet port 52 in the body 50 has a radial upper section, and a diverging lower section, which diverts the jet of fluid passing through the outlet apertures 62, 72 in a generally downward direction, as well as radially outwardly. This can be useful in directing jets of fluid to particular areas of the bore hole, beneath the outlet port 52 which require particular cleaning or maintenance, and the canted angle of the jets can in some cases perform better cleaning operations than perpendicular jets.

(32) The first and second pressure thresholds can optionally vary in different examples, but an optional first pressure threshold could be similar to what a wellbore would withstand in a normal circulation operation. In the present example, a suitable pressure to open the ports and allow flow is around 100-300 psi, for example, 150 psi, which is optionally sufficient to overcome the force of the spring, and the resilience of the first seat 20, but not the resilience of the stiffer second seat 25. The second pressure threshold is optionally higher than the first pressure threshold, and could be from 1000-2000 psi, for example 1500 psi and is optionally sufficient to overcome the resilience of the second seat 25 and to force the balls 10 through the seat 25. The spring strength is optionally chosen in light of the likely operating pressure which will influence the desired first pressure threshold.

(33) Once the balls 10a, 10b have passed through the seat 20, the obstruction of fluid flow through the bores 50b, 1b is removed, and the fluid pressure drops suddenly, reducing below the level needed to compress the spring 80. The spring 80 then returns the control sleeve 60 under its upward biasing force to the initial first configuration shown in FIG. 8, where the aperture 62 is situated uphole of the outlet sleeve aperture 72, out of alignment with the aperture 72 and the outlet port 52, and the outlet port 52 is closed off from the bore 50b by the control sleeve 60 and its seals. Fluid flow through the radial pathway F.sub.2 is thus prevented and flow resumes along the axial pathway F.sub.1. Drilling can then resume with the fluid being directed to the drill bit to wash cuttings back to the surface.

(34) In the present example, the cap 67 disposed at the uphole end of the control sleeve optionally includes a bladed component, which is urged resiliently against the inner surface of the wall of the outlet sleeve 70, and in this example is in the form of a resilient wiper 68, but a rigid scraper or similar could also or alternatively be provided. The wiper 68 can be formed from a resilient material, for example a plastic or rubber material. The wiper 68 covers the upper end of the annulus between the control sleeve 60 and the outlet sleeve 70, and reduces the amount of debris accumulating therein. As the control sleeve moves in the bore of the outlet sleeve 70, the wiper 68 scrapes against the inner surface of the outlet sleeve and cleans off debris. The inner diameter of the cap 67 is larger than the inner diameter of the seat 20, in order to avoid any erroneous seating of the ball 10a in the cap 67 before it reaches the seat 20.

(35) The threaded connection of the cap 67 with the control sleeve 60 allows removal of the component for repair or replacement without requiring complete disassembly of the other valve sleeves. This also permits, for example, the insertion of components to narrow the bore of the control sleeve 60 further for use with different sizes of balls or other shapes of plugs.

(36) At the uphole edge of the outlet sleeve 70, there is a cap 75 connected by threaded attachment to the outlet sleeve 70. The cap 75 has an upper end which offers a leading edge 40 facing in an uphole direction, against the fluid flow F. The outer wall of the cap 75 is cylindrical with parallel sides to match the inner bore 50b, but the inner wall 75w of the cap has a shaped profile which tapers radially inwards into the bore of the cap 75 to a throat 75t, which is narrower than the upper end of the bore of the cap 75. The inner wall of the cap 75w therefore forms a funnel in the bore, which acts to reduce turbulence and drag within the flow of the fluid, and to smooth out any eddies that would otherwise have been created by the upper end of the outlet sleeve 70. The funnel provided by the inner wall 75 directs fluid into the bore 60b, with a diameter that is at least equal to the diameter of the bore 60b, but can optionally be less than the diameter of the bore 60b.

(37) In another optional feature, the control sleeve 60 is optionally castellated at its downhole end with arches 65a cut out of the sleeve material, but other shapes may be used. The arches 65a permit fluid flow to the annular space in between the control sleeve 60 and the valve body 50, into the cavity where the spring 80 is retained. In this case, when the control sleeve 60 moves in a downhole direction, the spring is free to compress as fluid is forced out of the cavity through the arches 65a and into the bore 50b. Similarly, when the control sleeve 60 is travelling back in an uphole direction to its initial configuration, the spring 80 must extend, and fluid can flow through the arches 65a into the spring cavity to fill the vacuum that the extension creates. This feature reduces the risk of hydraulic lock of the control sleeve 60. The spring retainer 85 likewise optionally has similar formations 85a allowing fluid communication and preventing or alleviating risks of hydraulic locking of the moving parts of the assembly 1.

(38) An operation using the above example will now be described. During wellbore operations, for example downhole drilling, fluid is normally pumped axially down the drill string to the drill bit for cooling the bit, and for washing cuttings back to the surface. The option of diverting the fluid being pumped down the bore of the string into a radial fluid flowpath can be desirable in order to e.g. clean drill cuttings from the annulus of the wellbore. In this example, the ball 10a is dropped from the surface and travels through the bore of the string under the combined force of gravity and fluid being pumped down the well by positive displacement pumps at the surface. The ball 10a enters the bore 50b of the valve assembly 1 and passes through the cap 75 of the outlet sleeve 70. The ball 10a then passes through the cap 67 of the control sleeve 60, landing on the first seat 20. When engaged with the first seat 20, the non-deformable ball 10a forces deformation of the resilient first seat 20 under the initial force of fluid pressure in the bore behind the ball 10a once the pressure differential reaches the first (relatively low) pressure threshold. As the ball 10a passes through the apex of the first seat 20, the seat 20 is radially compressed by the ball 10a, such that its radial thickness is reduced and the diameter of the bore increases in a transient and reversible manner, but while the outer diameter of the seat 20 and its volume remains unchanged. After passing the first seat 20, the ball 10a seats on the second seat 25 on the other (lower) side of the outlet aperture 62. The second seat 25 requires more force to deform and allow passage of the ball 10a, and so the ball 10a is thus held seated on the second seat 22 at the relatively low first threshold pressure.

(39) The seating of the ball 10a in the second seat 25 obturates the axial fluid flowpath F.sub.1, as the seat 25 sealingly engages with the ball 10a. The resulting increase in fluid pressure uphole of the valve assembly 1 and into the bore 50b applies a correspondingly increasing force to the uphole-facing surface of the seated ball 10a. Once the fluid pressure has reached a threshold where the force applied to the ball 10a is greater than the opposing biasing force of the spring 80 (the activation pressure) the control sleeve 60 begins to travel axially in a downhole direction, and is guided in an axially-travelling path by the inner ends of the pins 54 occupying axial slots on the outer surface of the control sleeve 60. Any rotational movement of the control sleeve 60 at this point could lead to the aperture 62, through the wall of the control sleeve 60, being misaligned relative to the aperture 72, through the wall of the outlet sleeve 70, and the outlet port 52, through the side wall of the body 50. Hence, preventing rotation via the pins 54 increases consistency of fluid flow through the open outlet port 52.

(40) The spring 80 is compressed between the spring retainer 85 and the lower end of the control sleeve 60, with the compression increasing as the control sleeve 60 travels axially downwards. The control sleeve aperture 62 begins to cross the outlet aperture 72, allowing a small volume of fluid to be diverted out of the outlet port 52, which is fully aligned with the aperture 72. This diversion of fluid can sometimes slightly reduce the fluid pressure acting on the control sleeve 60, and pumping from the surface can optionally increase accordingly in order to maintain sufficient force to continue compressing the spring 80. Once the control sleeve 60 has reached the full extent of its travel, the apertures 62, 72 and the outlet port 52 are fully aligned, and the flow of fluid is diverted along the radial flowpath shown as arrows F.sub.2 in FIG. 5, through the apertures 62, 72, and outlet port 52, into the annulus of the well bore. Full alignment is not strictly necessary for satisfactory performance, but it is convenient to shift the control sleeve 60 by the same amount each time. The downward axial travel of the control sleeve 60 in the bore can optionally be limited by a travel stop formed by an internal shoulder on the body 50 engaging the lower end of a component of the control sleeve 60, and/or by the pin 54 reaching the upper end of the slot 60s.

(41) Once the function of the radial flow of fluid into the annulus has been performed (and repeated as needed) and the operator wishes to return the fluid flow to an axial direction through the valve assembly 1, a second ball 10b is dropped from the surface, and travels through the string to the valve assembly 1 under the combined force of gravity and fluid flow. The ball 10b is slightly narrower than the ball 10a. It passes through the narrowed bore of the cap 67 and seats on the first seat 20, as shown in FIG. 7, at the same time, abutting on the uphole-facing surface of the first ball 10a, which remains retained in the second seat 25. The second ball 10b seating on the first seat 20 obturates the bore 60b at a position uphole of the aperture 62, thus blocking the bore 60b.

(42) Fluid pressure increases above the seated balls 10a, 10b, (and can optionally be increased from the surface as required) to a second pressure threshold which is optionally considerably higher than the first threshold and higher than the activation pressure. This increases the force bearing down on the uphole-facing surface of the seated second ball 10b, which in turn bears down on the first ball 10a. Since the second ball 10b is supported from below by the first ball 10a seated on the second seat 25, it cannot move through the first seat 20, remaining within the cylindrical apex thereof, and the pressure therefore cannot escape through the outlet 52. The downhole-directed force applied by the higher second pressure threshold finally drives the non-deformable ball 10a down the bore 60b to begin deformation of the second valve seat 25 and press into the narrow apex of the second seat 25. The ball 10a causes the second seat 25 to compress in a radially outward direction, transiently increasing the diameter of the bore formed by the second seat 25 (while optionally maintaining outer diameter and volume), and allowing both balls 10a, 10b to pass through the second seat 25. Since the ball 10b is slightly narrower than the ball 10a, it is not seated as firmly in the second seat 25. In some examples, it is sufficient that the OD of the ball 10b is slightly larger than the ID of the second seat 25, so it can seat on the second seat 25, but since the balls 10a, 10b pass through in quick succession, while second seat 25 is still resiliently recovering to its initial resting configuration after passage of the larger first ball 10a, passage through the second seat 25 by the second ball 10b is facilitated by the transient deformation of the seat 25 by the passage of the first ball 10a.

(43) In some examples, and in the case of this example, the OD of the second ball 10b is very slightly smaller than the ID of the second seat 25, so the second ball 10b does not actually seat on the second seat 25, and passes through it without restriction. The balls 10a, 10b, are then optionally caught in a ball catcher downhole of the valve assembly (not shown). The first and second seats meanwhile resiliently return to their initial uncompressed configuration.

(44) Once the balls 10a, 10b have passed through the seat 20, the fluid pressure is relieved through the axial bore 50b, and there is nothing to maintain the compression of the spring 80 which returns the control sleeve 60 to its original upper position. As the control sleeve 60 moves in an uphole direction, the wiper 68 wipes against the inner surface of the outlet sleeve 70 and cleans away debris, reducing the risk of the control sleeve 60 jamming and maintaining the smooth running of the control sleeve within the outlet sleeve 70, and keeping any debris from entering the annulus between the control sleeve 60 and the outlet sleeve 70, and degrading the seals therein. Once the control sleeve 60 has returned to its initial position, the aperture 62 is wholly out of alignment with the aperture 72 and the outlet port 52 and the fluid flow returns to an axial path, shown as arrow F.sub.1 in FIG. 1.