GB2399361A

GB2399361A - Downhole bypass valve

Info

Publication number: GB2399361A
Application number: GB0305804A
Authority: GB
Inventors: Bruce Mcgarian; Rory Mccrae Tulloch
Original assignee: Smith International Inc
Current assignee: Smith International Inc
Priority date: 2003-03-13
Filing date: 2003-03-13
Publication date: 2004-09-15
Also published as: WO2004081337A1; GB0305804D0

Abstract

A bypass valve has a piston 22 slidably biassed by a spring 50 in a sleeve 20 which is in a casing 4. Holes 12, ports 32 and holes 40 can be aligned to allow flow from bore 42 to the annulus. This form is known. The invention puts a secondary closing means in each vent hole 12. The closing means comprises a ball 80, a ball seat 82 and a perforated cup 84. When the valve is mounted vertically the ball 80 moves off the seat so that flow both ways through the vent is possible. When there is sufficient pressure from the annulus to the interior of the valve the ball is forced into the seat so that flow is stopped. Two other forms of the secondary valve are disclosed in figs 4 and 7. The valve is described as being part of a downhole assembly that is used for cleaning a wellbore.

Description

239936 1 - 1

DOWNHOLE BYPASS VALVE AND METHOD

OF USING THE SAME

The present invention relates to a bypass valve for use in a wellbore and to a method of using a bypass valve in a wellbore cleaning operation.

It is common practice in the oil and gas drilling industry to incorporate a bypass valve in a drill string between a MWD (Measurement While Drilling) tool and a hydraulic anchor packer so that wellbore fluid may be pumped down the/drill string to operate the MWD tool without prematurely setting the anchor packer. One prior art bypass valve is disclosed in US 4,258,801 and comprises a deformable ball for closing a vent hole.

However, a more conventional bypass valve typically incorporates a piston which slides within a cylinder in response to dynamic fluid pressure. The wall of the cylinder is provided with a plurality of vent holes which allows fluid to pass from the drill string bore to the wellbore annulus. The piston is held in an open position by retaining means (such as a spring or a shear pin) and thereby allows wellbore fluid to operate a MWD tool located uphole of the bypass valve whilst preventing the generation of a pressure differential between the interior and exterior of the drill string sufficient to set an anchor packer. When the setting of the anchor packer is required, the flow of wellbore fluid down the drill string is increased so as to generate a dynamic pressure sufficient to overcome the retaining means. The piston then slides within the cylinder to a closed position in which the holes are sealed. A cross- sectional side view of this type of bypass valve is shown in Figure 1.

This type of conventional bypass valve can occasionally fail to move to the closed configuration when the appropriate fluid pressure is applied and this will often lead to costly and time consuming delays in a given downhole operation. In an attempt to overcome this problem, "sliding piston" type of bypass valves (such as the one described above) have been developed with a secondary closing means in addition to the primary closing means (the sliding piston). Such bypass valves are described in US 6,293,342.

With reference to this document it will be understood that, in the event that the primary piston within the cylinder fails to move to the closed position in response to an increase in dynamic pressure, the static pressure of the wellbore fluid in the annulus may be increased by a pump located at the surface, with the internal bore of the drill string having been sealed off, so as to generate a sufficient pressure above the downhole hydrostatic pressure to, for example, rupture a burst disc provided in the bypass valve casing and thereby apply a pressure differential across the length of a second piston. The pressure differential acts to press the second piston into a closed position in which the vent holes in the wall of the cylinder are sealed. Thus, although the primary closing means may fail to operate correctly, the bypass valve can nevertheless be moved into a closed configuration by the operation of the secondary closing means.

It will be appreciated however that, whereas the primary closing means may be reciprocated between open and closed configurations, the secondary closing means can only be moved from the open configuration to the closed configuration. Once in the closed configuration, the secondary closing means cannot be moved back to the open configuration unless the bypass valve is tripped out of hole. This is generally not problematic since the secondary closing means is used only occasionally (i.e. in the event of a failure of the primary closing means) and, once a decision has been taken to close the bypass valve with the secondary closing means, the need to immediately re-open the valve arises relatively infrequently. If this need does arise, then the bypass valve must be tripped out of hole. However, this should not necessarily be perceived as an added inconvenience since there was already a need to recover the bypass valve in order to repair the primary closing means.

However, the applicant has developed a need to use a bypass valve in an annulus cleaning operation and the use of the prior art bypass valves in such an operation is not satisfactory. A flushing of the wellbore annulus in order to remove undesirable debris involves pumping fluid down the annulus and up the interior of a downhole assembly at relatively high fluid flow rates. When using the aforementioned conventional bypass valves, it will be understood that a flow of fluid down the annulus and up the interior of the bypass valve will cause the primary closing means to move to an open configuration.

There is then a tendency for fluid to flow from the annulus into the bypass valve via the vent holes rather than flowing down the entire length of the assembly so as to maximise the length of wellbore which is cleaned. Even the prior art bypass valves with secondary closing means are not suitable. If such valves are used, then the flow of wellbore fluid down the annulus may be sufficient to activate the secondary closing means (i.e. rupture the burst disc) and move said means to a closed configuration. Fluid will then be prevented from flowing from the annulus into the bypass valve via the vent holes.

Although this will allow a desired annulus flushing, the bypass valve is effectively rendered useless for subsequent operations and must be tripped out of hole in order to be reset. This is particularly inconvenient considering that the primary closing means remains in working order. The prior art valves with secondary closing means are not therefore suitable for an annular flushing operation which takes place before a required conventional circulating of wellbore fluid down the assembly with the valve in both open and closed configurations.

It is an object of the present invention to provide an improved bypass valve and a method of using a bypass valve in a wellbore cleaning operation so as to improve the efficiency and effectiveness of downhole operations.

The present invention provides a downhole bypass valve according to the appended independent Claim 1. A bypass valve comprising further novel and advantageous features is defined in the appended dependent Claims 2 to 8.

Furthermore, the present invention provides a method of cleaning a wellbore according to the appended independent Claim 9. A method comprising further novel and advantageous features is defined in the appended dependent Claims 10 to 12. - 4

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional side view of a prior art bypass valve; Figure 2 is a cross-sectional partial side view of a first embodiment of the present invention; Figure 3 is a plan view of the exterior of a vent hole of the first embodiment; Figure 4 is a cross-sectional partial side view of a second embodiment of the present invention arranged with secondary closing means in an open configuration; Figure 5 is a plan view of the exterior of a vent hole of the second embodiment; Figure 6 is a cross-sectional partial side view of the second embodiment arranged with the secondary closing means in a closed configuration; Figure 7 is a cross-sectional partial side view of a third embodiment of the present invention; Figure 8 is a plan view of the exterior of a vent hole of the third embodiment; and Figure 9 is a cross-sectional side view of a wellbore cleaning assembly comprising the bypass valve of Figures 2 and 3 modified so as to include a multi-cycle pin and groove arrangement for controlling the reciprocating movement of the primary closing means.

The embodiments of the present invention will now be described as improvements to the prior art bypass valve of Figure 1. The bypass valve of Figure 1 is a conventional "sliding piston" bypass valve incorporating primary closing means only. This prior art bypass valve is described in detail below.

The apparatus of Figure 1 is a conventional bypass valve 2 comprising a plurality of internal parts mounted within the bore 6 of a casing 4. A shoulder 8 is provided in the bore 6 so as to prevent undesirable axial movement of the internal parts towards the lower end 10 of the bypass valve. Four vent holes 12 are located in the casing 4 uphole of the shoulder 8 and arranged so as to be coplanar and equispaced about the circumference of the casing bore 6. The vent holes 12 allow fluid to either enter the bypass valve from the wellbore annulus or enter the wellbore annulus from the bypass valve.

The plurality of internal parts includes a seal housing 18, a sleeve 20 and a piston 22. The seal housing 18 is substantially cylindrical in shape and has an outer diameter - s - similar to the diameter of the casing bore 6 defined by the portion of the casing 4 uphole of the shoulder 8. The seal housing 18 is located downhole of the vent holes 12 and is arranged so as to abut the shoulder 8.

The sleeve 20 is also substantially cylindrical in shape, the upper end thereof having an outer diameter similar to that of the casing bore 6. The lower end 28 of the sleeve 20 has an outer diameter which is less than that of the seal housing 18. The sleeve is arranged within the casing 4 with the lower end 28 of the sleeve 20 located in abutment with the seal housing 18. A vent chamber 30 in fluid communication with the vent holes 12 is thereby defined by the lower end 28 of the sleeve 20, the seal housing 18 and the casing 4. The vent chamber 30 defines an annular shape and is in fluid communication with a plurality of vent chamber ports 32.

The piston 22 is located in abutment with the inner surface 36 of the seal housing 18. The arrangement is such that the piston 22 may rotate and move axially within the sleeve 20 and the seal housing 18. The lower end 38 of the piston 22 extends beyond the vent chamber ports 32 and is provided with a plurality of piston holes 40 in the form of elongated slots. The piston holes 40 allow wellbore fluid to pass from the vent chamber in a piston bore 42 defined by the piston 22. The upper end 44 of the piston 22 is provided with connecting means 46 which allow the attachment of an appropriate nozzle (not shown) to the piston 22 so as to effectively reduce the diameter of the piston bore 42.

The attachment of a nozzle to the piston 22 reduces the flow rate of wellbore fluid required to move the piston 22 axially within the sleeve 20. The flow rate at which the bypass valve closes may therefore be varied with the inclusion of a suitable nozzle.

The piston 22 and the sleeve 20 define a piston spring chamber 48 in which a piston spring 50 is located. The piston spring 50 presses against the lower end 28 of the sleeve 20 and the upper end 44 of the piston 22, and thereby biases the piston 22 towards the upper end 52 of the bypass valve. Axial movement of the piston 22 is assisted by the venting of the piston spring chamber 48 to the vent chamber 30 by means of a piston spring chamber ports 56 located in the sleeve lower end 28. The axial movement of the piston 22 is restricted by a piston stop 58 and a piston circlip 60.

The sleeve 20 extends uphole of the piston 22 so as to abut a cross-over member (not shown) to which the casing 4 is threadedly connected. O-ring seals 62, 64, 66, 68 are - 6 provided in order to prevent undesirable ingress of wellbore fluid. Glyd ring seals 72, 74 are also provided to seal the interfaces of the piston 22 and to assist with the movement of the piston 22 within the sleeve 20 and the seal housing 18. Slyd rings 70, 76 are further provided as a bearing surface for the piston 22.

In use, the bypass valve 2 is run into a wellbore whilst arranged in an open configuration (i.e. with the piston 22 biased towards the upper end 52 of the bypass valve so that the piston holes 40 are substantially in line with the vent chamber ports 32) and thereby allows wellbore fluid to enter the drill string through the vent holes 12. At the option of the designer, debris may be prevented from entering the drill string by the inclusion of filter discs in the vent holes 12. The flow of wellbore fluid into the bypass valve equalises the very high hydrostatic pressures exerted on the outer surface of the drill string.

The wellbore fluid held within the drill string is circulated down the drill string bore at a predetermined flow rate sufficient for the operation of a MWD tool, but not high enough to generate the dynamic pressure required to activate the bypass valve. The wellbore fluid flows from the surface, through the MWD tool, into the wellbore annulus via the vent holes 12, and back to the surface through the annulus. Hydraulic anchor packers located downhole of the bypass valve 2 are not thereby exposed to a setting pressure differential.

Once the required position and orientation of the drill string within the wellbore has been obtained (measured with the MWD tool), the hydraulic anchor packers are set by moving the bypass valve into a closed configuration. In the closed configuration, the piston holes 40 are located downhole of the glyd ring seal 74 provided between the seal housing 18 and the lower end 38 of the piston 22, and the flow of wellbore fluid between the piston bore 42 and the wellbore annulus is thereby prevented. The movement of the bypass valve into the closed configuration is simply achieved by increasing the flow rate of wellbore fluid down the drill string and out through the vent chamber ports 32 so that sufficient dynamic pressure is generated across the length of the piston 22 to overcome the biasing force of the piston spring 50. Once the piston 22 sealingly closes the vent chamber ports 32, the required setting pressure differential at the anchor packers is generated. This results in a large pressure rise at the surface indicating that the anchor packers have been set.

A bypass valve according to the present invention may be advantageously used as an alternative to the bypass valve 2 described above and three embodiments are shown in Figures 2 to 9 of the accompanying drawings. The bypass valves shown in these figures have a substantially similar arrangement to the prior art bypass valve shown in Figure 1 and the components of these bypass valves which correspond to components of the prior art bypass valve have been labelled with the reference numerals used in Figure 1. The bypass valves shown in Figures 2 to 8 are identical to the bypass valve of Figure 1 other than in certain respects as shown in the partial views of Figures 2 to 8.

Common features between the three embodiments described herein and which differ from the prior art bypass valve of Figure 1 may be seen in the arrangement of the seal housing 18 of the prior art bypass valve which, in the three embodiments, is formed integrally with the lower end of the sleeve 20. Also, it will be seen that the embodiments each have larger vent chamber ports 32 and piston spring chamber ports 56 than in the prior art bypass valve. Furthermore, it will be seen that the wall thickness of the casing 4 is increased in the region of the vent holes 12. This allows a secondary valve assembly to be conveniently mounted within the vent holes 12. Also, as will be seen from Figures 2, 4 and 6, the three embodiments differ from the prior art bypass valve in the arrangement of seals and slyd rings in the region of the vent holes. In this regard, it will be noted that a Glyd-ring seal 63 is received in a circumferential groove on the exterior cylindrical surface of the sleeve lower end at a location downhole of the vent hole 12. This Glyd- ring seal 63 prevents an ingress of fluid between the casing 4 and the sleeve lower end and into the bore 6. Similarly, a Glyd-ring seal 71 is located in a circumferential groove in the interior surface of the sleeve lower end at a location downhole of the vent chamber ports 32. The Glyd- ring seal 71 serves to prevent an ingress of fluid between the lower end of the piston 22 and sleeve 20 when the piston 22 is in the closed position.

Furthermore, a Glyd-ring 75 is located in a circumferential groove in the interior surface of the sleeve 20 at a location above the vent chamber ports 32. The Glyd-ring seal 75 serves to assist the sliding movement of the piston 22 within the sleeve 20. - 8

With specific reference to the first embodiment 102 shown in Figures 2 and 3 of the accompanying drawings, each vent hole 12 is provided with secondary closing means for selectively closing the associated vent hole and thereby prevent a flow of fluid therethrough. More specifically, the secondary closing means serves to prevent fluid flowing through the vent hole 12 into the bypass valve 102 but not prevent fluid from flowing in the opposite direction through the vent hole 12. The secondary closing means also functions to allow fluid to flow at a relatively low rate through the vent holes 12 into the bypass valve. However, the secondary closing means is designed so that, above a predetermined rate of fluid flow into the bypass valve, the secondary closing means becomes activated so as to close the vent holes 12 and thereby prevent further flow into the bypass valve via the vent holes 12.

The secondary closing means associated with each vent hole 12 comprises a ball held captive within the associated vent hole 12 between a valve seat 82 and a perforated cup 84. The perforated cup 84 is retained within the vent hole 12 by means of a circlip 86 (although other suitable fixing means may be used). The ball 80 and/or valve seat 82 may be manufactured from or coated with a resiliently deformable material such as rubber so as to improve the fluid seal between these two components. The valve seat 82 has a frusto-conically shaped surface for sealing engagement with the ball 80. The sealing surface of the valve seat 82 is angled relative to the longitudinal axis of the bypass valve and, as a consequence, when the bypass valve is arranged with its longitudinal axis vertical or near vertical, the ball 80 will tend to roll down the sealing surface and away from a sealing position with the valve seat 82. The ball 80 is thereby biased away from a sealing position and fluid flowing at a relatively low rate will be able to flow past the ball and into the bypass valve via the vent hole 12 without displacing the ball 80 into a sealing position with the valve seat 82. Fluid flowing through the vent hole 12 does so through one of a plurality of apertures 88 extending through the perforated cup 84. The apertures form fluid passageways extending laterally through the vent hole 12.

The perforated cup 84 comprises a concave surface 90 (having a hemispherical shape) which functions to receive the ball 80 when the ball 80 moves from a sealing position with the valve seat 82. All but one of the fluid passageways extending through - 9 - the cup are located radially beyond the surface 90 and laterally extend sufficiently far into the vent hole 12 for fluid flowing into the bypass valve via the vent hole 12 to be directed past the ball 80 when the ball 80 is received on the surface 90. This allows fluid of low flow rates to pass the ball 80 without displacing the ball 80 into engagement with the valve seat 82. However, this is only the case for low flow rates and, as the flow rate increases, a further aperture 88 located centrally in the cup 84 and opening onto the hemispherical surface 90 ensures that the ball 80 is pressed into sealing engagement with the valve seat 82.

In use, the first embodiment 102 is run downhole as part of a wellbore cleaning assembly. As the bypass valve is run in hole, some fluid may enter the bypass valve via the vent holes 12 provided the flow rate through the vent holes 12 is not sufficiently high to move the balls 80 in the vent holes 12 into sealing engagement with their associated valve seats 82. Once the bypass valve has been run downhole and fluid pressure between the interior and exterior of the assembly has equalised, each ball 80 will tend to move to an open position. The bypass valve may then be used in a conventional way by pumping fluid through the bore 6.

Fluid vented through a vent hole 12 will press the associated ball 80 into engagement with the adjacent hemispherical surface 90. Although this will tend to seal the central aperture 88, the surrounding apertures 88 will remain open and allow fluid to flow from the ball 60 into the annulus. However, if the annulus is to be flushed with a downhole flow of fluid, the fluid flow rates involved will be sufficient to press the ball 80 into sealing engagement with the valve seat 82. All fluid will then be prevented from entering the bypass valve at the vent holes 12 and be directed past the bypass valve.

Debris may then be flushed down the annulus and up through the interior of the assembly.

Once the flushing operation has been completed and the fluid flow rate down the annulus is reduced, the ball 80 will tend to move to an open position. In any event, fluid attempting to flow from the interior of the bypass valve to the exterior thereof via a vent hole 12 will displace the ball 80 so that the vent hole 12 is opened. No more use of the bypass valve 102 may then be resumed. This sequence of events may be repeated as many times as is required. It will be understood that Figure 2 shows the secondary closing means in a closed configuration. - lo-

A second embodiment 202 of the present invention is shown in Figure 4. The second embodiment is identical to the first embodiment except that the frusto-conical surface of the valve seat 82 is provided with three elongate members 204 extending laterally therefrom. It will be apparent that only two of said members 204 are visible in the view of Figure 4. The elongate members 204 may be conveniently provided as screw threaded components which are screwed into the frusto-conical sealing surface. The members 204 are spaced equi-distant about the circumference of a circle concentric with the annular shape of the sealing surface, said circle having a diameter less than that of the ball so that the ball is unable to move past said members and sealingly engage the valve seat without deforming (see Figure 4). As a result, the rate at which fluid may flow into the bypass valve through the vent holes 12 is considerably greater than for the first embodiment.

The ball 206 is made from a resiliently deformable material which allows a predetermined rate of fluid flow through the vent hole, and into the bypass valve, to deform the ball 206 and push it passed said members into sealing engagement with the valve seat (see Figure 6). Once in engagement with the valve seat, the deformed ball is gripped by the three members 204. As a result, the ball will not readily move from the closed position unless forced to do so by fluid pressing from the interior of the bypass valve.

The third embodiment 302 comprises an entirely different secondary closing means to the first two embodiments 102,202. Rather than comprising a valve closure in the form of a ball, the third embodiment 302 comprises a closure in the form of a hinged flap 304 biased into sealing engagement (i.e. the closed position of Figure 7) by means of a torsion spring 306. The flap 304 is pivotally connected to a cylindrical base unit 308 by means of a hinge 310. The base unit 308 is retained in position within the associated vent hole 12 by means of a circlip 312 (although other suitable securing means could be used).

The base unit 308 has a longitudinally extending rib 314 projecting lateral from its exterior cylindrical surface (see Figure 8). The rib 314 locates in a corresponding groove in the vent hole 12 so as to prevent the base unit 308 from rotating within the vent hole 12. The secondary closing means of the third embodiment therefore serves to prevent any flow of fluid through the vent holes from the annulus but allow fluid flow in the opposite direction.

In use of the third embodiment 302, it will be understood that the flap 304 remains in a sealing position (so as to close the vent hole 12) unless forced open against the spring 306 bias by fluid pressing on the flap 304 from the interior of the bypass valve. Wellbore fluid does not therefore enter the bypass valve via the vent holes 12 as the bypass valve is run in hole. As such, the drill string may be filled with fluid from the surface periodically as the string is run in hole.

It will be apparent to a reader skilled in the art that the present invention may be embodied as a multi-cycle bypass valve. Such valves are described in US 6,095,249.

Indeed, any of the above described embodiments of the present invention may be modified to include a control groove and pin arrangement for controlling movement of the piston 22 relative to the sleeve 20. The first embodiment 102 is shown in Figure 9 modified in this way. The bypass valve 402 shown in Figure 9 differs from the first embodiment 102 in that two control pins 404 extend laterally from opposite sides of the casing 4 into engagement with a control groove 406 defined in the exterior surface of the uphole end of the piston 22. The control groove 406 circumscribes the piston 22 and, as will be understood by the skilled person, defines a path extending up and down the piston along which the control pins 404 may pass. In this way, the primary closing means of the bypass valve 402 may be cycled between two conditions wherein, in the first condition, the application of a fluid flow rate sufficient to move the piston 22 will open the vent holes, and, in the second condition, the application of a fluid flow rate sufficient to move the piston 22 will not open the vent holes.

The bypass valve 402 is shown in Figure 9 incorporated within a wellbore cleaning assembly 500 located in a 9 5/8 inch wellbore casing (which reduces to a 7 inch casing) 510. The assembly 500 comprises a 6 5/8 inch drill pipe 520, a bypass valve 402 according to the present invention, a first cross-over member 530, a 5 inch drill pipe 540, a casing scraper 550, a second cross-over member 560 and a mill 570.

The present invention is not limited to the particular embodiments described.

Further variations will be apparent to a reader skilled in the art. For example, the first embodiment may be modified so that the ball is biased by means of a spring into - 12 engagement with the valve seat. In this way, no fluid (even of a low flow rate) will be admitted from the annulus into the valve via the vent holes. This modified valve will then operate in a similar manner to the third embodiment.

The cleaning assembly shown in Figure 9 may also comprise a junk/debris catcher which will retain within the assembly junk/debris flushed from the annulus and into the assembly via ports in the mill (or bull nose). - 13

Claims

CLAIMS: 1. A downhole bypass valve for selectively isolating the interior

of a downhole assembly from the exterior thereof, the bypass valve comprising: a body having a bore for allowing the passage of wellbore fluid therethrough; a vent hole provided in the body for allowing fluid communication between the bore and the exterior of the bypass valve; primary means for closing the hole, the primary closing means comprising a piston slidably mounted in the bore and moveable between a first position, in which fluid communication between the bore and the exterior of the bypass valve by means of the hole is permitted, and a second position, in which fluid communication between the bore and the exterior of the bypass valve by means of the hole is prevented; means for retaining the piston in said first position; and secondary closing means for preventing fluid communication between the bore and exterior of the bypass valve by means of the hole; characterized in that the secondary closing means is moveable, in response to a predetermined fluid pressure, from a closed configuration, in which said fluid communication by means of the hole is prevented, to an open configuration, in which said fluid communication by means of the hole is permitted.
2. A downhole bypass valve as claimed in Claim 1, wherein the secondary closing means comprises means for releaseably retaining the secondary closing means in said closed configuration.
3. A downhole bypass valve as claimed in Claim 1 or 2, wherein the secondary closing means comprises means for releaseably retaining the secondary closing means in said open configuration.
4. A downhole bypass valve as claimed in Claim 3 when dependent upon Claim 1, or as claimed in Claim 2, wherein said retaining means comprises means for biasing the secondary closing means towards the configuration in which the secondary closing means is retained by said retaining means. - 14
5. A downhole bypass valve as claimed in Claim 1, wherein the secondary closing means comprises a valve seat defined on the body of the bypass valve, and a discrete ball held captive within the body of the bypass valve and moveable into engagement with the valve seat so as to locate the secondary closing means in said closed configuration.
6. A downhole bypass valve as claimed in Claim 5, wherein the valve seat is arranged so that the ball is moveable into engagement with the valve seat by a fluid flowing from the exterior of the bypass valve to the bore by means of the hole.
7. A downhole bypass valve as claimed in Claim 6, wherein means are provided for resisting movement of the ball towards a position in which the ball is in engagement with the valve seat.
8. A downhole bypass valve as claimed in Claim 7, wherein the resisting means comprises one or more projections extending between the ball and the valve seat, the or each projection being located so that, in use, the ball is unable, without deforming, to move passed the or each projection and locate the secondary closing means in said closed configuration, and wherein the ball is resiliently deformable.
9. A method of cleaning a wellbore, the method comprising the steps of making up a downhole assembly incorporating a downhole bypass valve as claimed in any of the preceding claims; running said assembly into a wellbore to be cleaned; and pumping fluid down the annulus defined between the wall of the wellbore and the bypass valve whilst the bypass valve is arranged with the primary closing means located with the piston in the first position and the secondary closing means located in the closed configuration.
10. A method of cleaning a wellbore as claimed in Claim 9, wherein, following the step of pumping fluid down the annulus, the secondary closing means is moved from the closed configuration to the open configuration. - 15
11. A method of cleaning a wellbore as claimed in Claim 9 or 10, wherein, following the step of pumping fluid down the annulus, the piston of the primary closing means is moved from the first position to the second position.
12. A method of cleaning a wellbore as claimed in any of Claims 9 to 11, wherein said assembly is run into the wellbore whilst the bypass valve is arranged with the primary closing means located with the piston in the first position and the secondary closing means located in the open configuration.
13. A method of cleaning a wellbore as claimed in any of Claims 9 to 1 1, wherein said assembly is run into the wellbore whilst the bypass valve is arranged with the primary closing means located with the piston in the first position and the secondary closing means located in the closed configuration.
14. Apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.
15. A method substantially as hereinbefore described with reference to and as shown in the accompanying drawings.