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BACKGROUND OF THE INVENTION 
     a. Field of the Invention 
     The invention relates to bypass valves for use in wellbores, particularly but not exclusively to bypass valves used during the setting of hydraulic anchor packers. 
     b. Description of Related Art 
     The drilling industry often has need to monitor the depth and angular orientation of a tool (such as a whipstock) within a wellbore and to rigidly secure the tool within the wellbore once a required position has been achieved. The depth and orientation of a tool is typically determined through use of a measurement-while-drilling (MWD) tool. However, MWD tools require a flow of wellbore fluid through a string in order to communicate a measured depth and orientation to the surface and the flow rates involved are often sufficiently high to prematurely set the hydraulic anchor packer in use. 
     To overcome this problem, strings are often provided with a bypass valve located between the MWD tool and the anchor packer. When the depth and orientation of the string is being monitored, wellbore fluid is pumped through the MWD tool via the string bore and then bled to the wellbore annulus so as to prevent the pressure differential across the hydraulic anchor packer rising to the level required for setting. Once the string has been arranged in the desired position, the hydraulic anchor packer is set by increasing of the flow rate of wellbore fluid down the string. The increase in flow rate results in an associated increase in dynamic pressure at the bypass valve. Once this dynamic pressure increases to a predetermined magnitude, the bypass valve is activated and the fluid path between the wellbore annulus and the string bore is closed. The wellbore fluid is thereby directed downhole to the anchor packers where the appropriate setting pressure (typically a 1500-3000 psi differential between the inside and outside of the anchor packer) is then applied. 
     A conventional bypass valve incorporates a piston which slides within a cylinder in response to dynamic wellbore fluid pressure. The wall of the cylinder is provided with a plurality of holes through which fluid may pass from the string bore to the wellbore annulus. The piston is held by biasing means (such as a spring), a shear pin or a combination thereof so as to permit fluid flow through said holes in the cylinder. However, when the predetermined dynamic pressure is achieved, the biasing means and/or shear pin is overcome and the piston slides within the cylinder so that said holes become sealingly closed. 
     A problem associated with this type of bypass valve is that no warning is given at the surface of an imminent closing of the bypass valve and, consequently, of a potentially imminent setting of the anchor packer. A bypass valve is disclosed in UK patent application no. 9625547.6 (publication no. GB 2 307 932 A) which incorporates means for controlling the movement of the piston within the cylinder. The disclosed arrangement is such that movement of the piston is initially restricted so that the cylinder holes are only partially closed. The restricted passage to the wellbore annulus thereby created results in increased pressure losses which may be detected at the surface. Nevertheless, the dynamic pressure at the bypass valve has been allowed to rise to the predetermined activating magnitude and remedial action (i.e. a cycling of the bypass valve) must then be taken before full closure of the cylinder holes can be achieved. This remedial action is time consuming and, in certain applications, can be inconvenient and potentially problematic. 
     SUMMARY OF THE INVENTION 
     Further prior art bypass valves to which the present invention pertains are disclosed in U.S. Pat. Nos. 4,768,598 and 5,443,129. The latter document describes a bypass valve according to the preamble of the appended claims. However, this prior art valve requires a partial closing of the fluid path between the valve interior and exterior which is achieved by movement of the piston. 
     It is an object of the present invention to provide a bypass valve for use in a wellbore which communicates an imminent closure of the bypass valve to the surface. 
     The present invention provides a bypass valve for selectively isolating the interior of a downhole assembly from the exterior thereof, the bypass valve comprising: a body incorporating a wall provided with at least one opening extending therethrough; a piston slidably mounted in the body such that a first position of the piston relative to the body establishes a passage from the interior of the body to the exterior of the body via the opening and such that a second position of the piston relative to the body substantially isolates the interior of the body from the exterior of the body; and means for increasing the force exerted on the piston by a given flow of fluid through the bypass valve such that the resultant force on the piston is insufficient to move the piston to the second position; characterised in that the force increasing means increases the force exerted on the piston in response to a predetermined flow of fluid through the bypass valve. 
     Thus, a bypass valve according to the present invention may be employed in downhole operations in a similar manner to prior art bypass valves. However, if the rate of fluid flow through the bypass valve is increased (either intentionally or unintentionally) so that said predetermined fluid flow is achieved. then said means is activated. As a consequence, the force exerted on the piston by fluid flowing through the bypass valve is increased. Although the resultant force on the piston is not sufficient to move the piston so as to effect closure, the activation of said means generates a reactive force which resists the fluid flow. This resistance can be detected at the surface and thereby provides an indication that the fluid pressure differential across the length of the piston has increased to a predetermined level and that further unchecked increases will result in closure of the bypass valve. 
     The force increasing means preferably comprises means for restricting the passage of fluid past the piston. Furthermore, the passage of fluid past the piston is preferably provided by a fluid pathway comprising a longitudinal bore extending through the piston. The fluid pathway ideally also comprises at least one aperture in the piston providing fluid communication between the piston bore and a fluid route past the piston being at least partially located exteriorly of the piston. In such an arrangement, the passage restricting means preferably comprises a second piston mounted in said piston bore so as to be slidably moveable between positions in which said at least one aperture is either open, closed or partially closed. It is preferable for the second piston to be biased into a position wherein said at least one aperture is open. Said piston may be biased by mean of a spring. Alternatively, the second piston may be held by means of a shear pin in a position wherein said at least one aperture is open. Preferably, the second piston is moveable into a position wherein said at least one aperture is closed. The second piston is preferably provided with a longitudinal bore extending therethrough. 
     Preferably, the geometry of the piston is such that the piston, once in said second position, is biased into said second position by means of a static fluid pressure differential across said piston. 
     A bypass valve according to the present invention thereby has the advantage over the prior art of providing an indication at the surface of an imminent closure of the bypass valve. Once said indication is detected, the bypass valve may be closed, without the need for remedial action, by simply increasing the rate of fluid flow down the associated string. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional side view of a first embodiment of the invention arranged in an unset configuration; 
     FIG. 2 is a cross-sectional side view of said first embodiment arranged in a partially set configuration; 
     FIG. 3 is a cross-sectional side view of said first embodiment arranged in a set configuration; and 
     FIG. 4 is a cross-sectional side view of a second embodiment of the invention arranged in a set configuration. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first bypass valve  2  according to the present invention is shown in FIGS. 1,  2  and  3 . This bypass valve  2  comprises a cylindrical body  4  housing a number of internal components moveable in response to dynamic fluid pressure. 
     The cylindrical body  4  is defined by top and bottom subs  6 , 8  respectively threadedly engaged with the uphole and downhole ends of a central body element  10 . The top sub  6  is provided with a female connector  12  for threadedly engaging the uphole end of the bypass valve  2  with a string. Similarly, the bottom sub  8  is provided with a male connector  13  for threadedly engaging the downhole end of the bypass valve  2  with a string. The assembled elements of the cylindrical body  4  define a longitudinal bore  14  in which the aforementioned moveable components are located. Axial movement of said components within the bore  14  is restricted by means of a downhole facing internal shoulder  16  provided by the downhole end of the top sub  6  and an uphole facing internal shoulder  18  provided by the uphole end of the bottom sub  8 . Furthermore, fluid communication between the exterior of the cylindrical body  4  and the longitudinal bore  14  thereof is permitted by means of four apertures  20  extending laterally through the wall of the central body element  10 . The body apertures  20  are equispaced about the longitudinal axis of the bypass valve  2  and are arranged in a common plane which is prpendicular to said longitudinal axis. 
     The internal surface  22  of the central body element  10  is provided with a recess  24  located uphole of the body apertures  20  which, as will be described below, allows a secondary flow of fluid through the bypass valve  2  during use. Furthermore, the internal surface  22  is provided with an annular stop member  26 . This stop member  26  is located downhole of the body apertures  20  and radially projects into the bore  14 . In use, the stop member  26  provides means for constraining the aforementioned moveable components in addition to the downhole and uphole facing internal shoulders  16 , 18 . 
     Appropriate pressure relief means  28  (for example, a burst disc, a pressure relief valve. or a number of suitably sized nozzles) is provided in the bottom sub  8  so as to allow the escape of fluid from the bore  14  when the static pressure therein increases to a predetermined level. The fluid pressure within the bypass valve  2  may be thereby retained within acceptable limits. In this way, undesirable damage to the bypass valve  2  and the associated string, particularly during an anchor setting operation, may be avoided. 
     As mentioned above, a number of moveable components are retained within the bore  14  between the downhole and uphole facing internal shoulders  16 , 18 . These components include a primary piston  30 , a primary compression spring  32 , a primary piston extension member  34 , a secondary piston  36 , and a secondary compression spring  38 . 
     The primary piston  30  is generally cylindrical in shape and defines a primary piston bore  40 . The downhole portion of the primary piston  30  is provided with four laterally extending piston apertures  42 . The piston apertures  42  are similar to the body apertures  20  both in size and in arrangement. In addition to these apertures  42 , the uphole portion of the primary piston  30  is provided with a first set of secondary piston apertures  44 . These apertures  44  are equi-spaced about the longitudinal axis of the bypass valve  2  and are arranged in a common plane perpendicular to said axis. Furthermore, each of the secondary piston apertures  44  extends from the primary piston bore  40  in a downhole and radially outward direction. A generally central portion of the primary piston  30  is provided with a second set of secondary piston apertures  46 . The apertures  44 , 46  of the first and second sets are arranged about said longitudinal axis in an identical manner and are identical in size. However, the second set of secondary piston apertures  46  differs from the first set in that each aperture  46  of the second set extends from the primary piston bore  40  in an uphole and radially outward direction. The directions in which the secondary piston apertures  44 , 46  extend reduce the pressure losses associated with a fluid flow through the bypass valve  2 . Both said first and second sets are comprised of the six secondary piston apertures. An alternative number of apertures  44 , 46  may be used as appropriate. 
     The primary compression spring  32  is located downhole of the primary piston  30  and abuts the uphole facing internal shoulder  18 . The primary piston extension member  34  is located between the primary piston  30  and the primary compression spring  32 . The arrangement is such that the primary compression spring  32  presses the primary piston extension member  34  into abutment with the primary piston  30  which is in turn pressed uphole into abutment with the downhole facing internal shoulder  16 . 
     With the primary piston  30  pressed against the downhole facing internal shoulder  16  as shown in FIG. 1, the bypass valve  2  is arranged in an unset configuration. In this configuration, the primary compression spring  32  is sufficiently compressed to prevent premature downhole movement of the primary piston  30 . Furthermore, the geometry of the primary piston  30  is such that, when positioned as shown in FIG. 1 (i.e when the bypass valve  2  is in the unset configuration), the first set of secondary piston apertures  44  is located adjacent the uphole region of body element recess  24 , the second set of secondary piston apertures  46  is located adjacent the downhole region of the body element recess  24 , and the piston apertures  42  are located adjacent the body apertures  20 . 
     In the unset configuration, the first and second sets of secondary piston apertures  44 , 46  provide fluid communication between the primary piston bore  40  and the body element recess  24 . Thus, fluid passing through the bypass valve  2  will tend to flow both along the entire length of the primary piston bore  40  and also along a secondary path which bypasses a central section of the bore  40 . In following the secondary path, a downhole flow of fluid passes from the primary piston bore  40  through the first set of secondary piston apertures  44  and into an annular passage  48  defined by the body element recess  24  and the external surface of the primary piston  30 . Said fluid then flows downhole through the annular passage  48  and back into the primary piston bore  40  via the second set of secondary piston apertures  46 . 
     Furthermore, with the bypass valve  2  arranged in the unset configuration, fluid communication between the piston apertures  42  and the body apertures  20  is ensured by means of a circumferential recess  50  provided in the interior surface of the central body element  10  and a circumferential recess  52  provided in the exterior surface of the primary piston  30 . The circumferential recesses  50 , 52  are respectively provided in the region of the body apertures  20  and the piston apertures  42 . Accordingly, with the bypass valve  2  arranged in the unset configuration, the body apertures  20  and piston apertures  42  are in fluid communication with one another by means of an annular space  54  defined by the circumferential recesses  50 , 52 . A leakage of fluid from the annular space  54  (i.e. into any space between the central body element  10  and the primary piston  30 ) is prevented by means of two O-ring seals  56 , 58 . A third O-ring seal  60  is also provided so as to prevent the ingress of wellbore fluid through the body aperture  20  when the bypass valve  2  is in the set configuration shown in FIG.  3 . 
     The secondary piston  36  is located within the primary piston bore  40  between the first and second sets of secondary piston apertures  44 , 46  (when the bypass valve  2  is arranged in the unset configuration). The secondary piston  36  is generally cylindrical in shape and has a bore  37  extending therethrough. The downhole end portion of the secondary piston  36  is received within the primary piston bore  40  downhole of an uphole facing internal shoulder  62  provided on the interior surface of the primary piston  30 . An O-ring seal  64  located below said shoulder  62  prevents leakage of fluid between the primary and secondary pistons  30 , 36 . The uphole end of the secondary piston  36  is provided with a spring stop  66  which is annular in shape and retained adjacent the secondary piston  36  by means of a circlip (not shown). The secondary compression spring  38  is located between the spring stop  66  and the uphole facing internal shoulder  62  of the primary piston  30 . When the bypass valve  2  is in the unset configuration, the secondary compression spring  38  presses the secondary piston  36  uphole into abutment with a circlip  68  mounted in the primary piston bore  40 . The arrangement is such that the secondary piston  36  may be moved downhole relative to the primary piston  30  and close the second set of secondary piston apertures  46 . When the second set of secondary piston apertures  46  are closed in this manner, the bypass valve  2  is arranged in the partially set configuration (see FIG.  2 ). 
     During use, the bypass valve  2  is typically located in a string downhole of a MWD tool and uphole of a hydraulic anchor packer and is run down a wellbore in the unset configuration shown in FIG.  1 . In this way, fluid may be pumped down the string so that the depth and orientation of the packer may be monitored using the MWD tool. As in the prior art, premature setting of the packer is prevented by virtue of a bleeding of fluid from the interior of the bypass valve to the wellbore annulus. With reference to FIG. 1, it can be seen that the bleeding of fluid from the string is achieved by means of the fluid pathway provided by the body and piston apertures  20 , 42  and the annular space  54 . 
     If the rate of fluid flow through the bypass valve increases (either intentionally or unintentionally) to a predetermined level sufficient to overcome the bias of the secondary compression spring  38 , then the secondary piston  36  moves downhole within the primary piston bore  40 . The downhole movement of the secondary piston  36  is limited by means of a stop  70  provided on the primary piston  30 , but is sufficient to close the second set of secondary piston apertures  46 . The secondary flow of fluid via the annular passage  48  is thereby prevented. Consequently, with the bypass valve  2  arranged in the partially set configuration, all the fluid passing through the bypass valve  2  must flow through the primary piston bore  40  and the secondary piston bore  37 . This results in an increase in the force exerted by the fluid flow on the primary piston  30 . However, the stiffness of the primary compression spring  32  is such that this increased force is not sufficient to move the primary piston  30  downhole within the cylindrical body  4  and set the bypass valve  2 . Nevertheless, the increased force corresponds with an increased pressure loss which may be clearly detected at the surface. 
     Once in the partially set configuration, the bypass valve  2  may be set by further increasing the rate of fluid flow through the bypass valve. If the setting of the bypass valve  2  is not required, then the detected movement of the secondary piston  36  suggests that the fluid flow rate should be reduced so as to avoid accidental setting in the event of a unintentional further fluid flow rate increase. Appropriate remedial action may then be taken. 
     Once the fluid flow rate through the bypass valve  2  is sufficient to overcome the bias of the primary compression spring  32 , the primary piston  30  will move downhole within the cylindrical body  4  so as to sealingly close the body apertures  20 . All fluid entering the bypass valve  2  is then directed downhole through the string so that the required anchor setting pressure may be generated. Once the anchors have been set, the bypass valve  2  may be placed back into the unset configuration by simply reducing the rate of fluid flow. 
     A second bypass valve  90  according to the present invention is shown, in a set configuration, in FIG.  4 . The second bypass valve  90  is substantially identical to the first bypass valve  2  and corresponding components are labelled in the drawings with the same reference numerals. A minor difference between the two embodiments is the different number of secondary piston apertures  44 , 46  employed. However, the important difference between the two embodiments is in the design of the primary piston  30  which is provided with a downhole facing external shoulder  92  located between the O-ring seals  58 , 60  used to seal the body apertures  20  when in the set configuration. A corresponding uphole facing internal shoulder  94  is provided on the internal surface  22  of the central body element  10  at a location below the body apertures  20 . The arrangement is such that, when the second bypass valve  90  is in the set configuration, a static fluid pressure differential is generated across the length of the primary piston  30 , the magnitude of which is sufficient to resist the bias of the primary compression spring  32  and therefore maintain the bypass valve  90  in the set configuration without the need for a circulation of fluid through the string. Once set, the second bypass valve  90  may be opened by bleeding off fluid pressure at the surface. 
     The present invention is not limited to these specific embodiments described above. Alterative embodiments will be apparent to a reader skilled in the art.

Summary:
The present invention relates to bypass valves for use in wellbores, particularly but not exclusively to bypass valves used during the setting of hydraulic anchor packers. A bypass valve ( 2 ) is provided with a piston ( 30 ) slidably mounted adjacent a body member ( 4 ) having at least one opening ( 20 ) extending therethrough. The piston ( 30 ) is moveable between a first position establishing a passage from the interior of the body ( 4 ) to the exterior thereof via the at least one opening ( 20 ) and a second position isolating the interior of the body ( 4 ) from the exterior thereof. A second piston may be provided for increasing, in response to a predetermined fluid pressure differential across the length of the piston ( 30 ), the force exerted on the piston ( 30 ) by a given flow of fluid through the bypass valve ( 2 ) such that the resultant force on the piston ( 30 ) is insufficient to move the piston ( 30 ) to the second position. Thus, the bypass valve ( 2 ) is adapted to provide an indication at the surface of an imminent closure. Once the indication is received, the bypass valve ( 2 ) may be closed, without the need for remedial action, by simply increasing the rate of fluid flow.