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A modulator is disclosed for creating a pressure pulse in a fluid-filled well. The modulator comprises a tool body on which a plurality of extendable arms are mounted. The arms may be retracted into a stowed position substantially adjacent the tool body, or may be extended to meet the wall of the bo

A modulator is disclosed for creating a pressure pulse in a fluid-filled well. The modulator comprises a tool body on which a plurality of extendable arms are mounted. The arms may be retracted into a stowed position substantially adjacent the tool body, or may be extended to meet the wall of the borehole. The arms are preferably resilient bowsprings that may be flexed outwards from the tool by means of an actuator pushing on at least one end of the springs. A flexible valve sleeve or bag is suspended between the arms and cooperates with a valve mounted adjacent the sleeve on the tool body. The valve sleeve creates a fluid-flow path through the valve, and in operation, the valve closes one end of the valve sleeve to create the pressure pulse. Sealing of the valve sleeve against the wall of the well is a result of the fluid pressure inflating the sleeve against the well wall. As a result, sealing takes place over an extended area of the bag and is dynamically responsive to changes in fluid flow or pressure.

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1. An apparatus for downhole use in a fluid-filled well, the apparatus comprising: a longitudinal tool body;a plurality of extendable arms mounted on the tool body for opening and closing within the fluid-filled well;a flexible valve sleeve attached to the plurality of extendable arms and moveable b

1. An apparatus for downhole use in a fluid-filled well, the apparatus comprising: a longitudinal tool body;a plurality of extendable arms mounted on the tool body for opening and closing within the fluid-filled well;a flexible valve sleeve attached to the plurality of extendable arms and moveable between a stowed position and an unstowed position by movement of the extendable arms, wherein in the unstowed position the valve sleeve is arranged to receive a flow of fluid from the well and the pressure of the fluid in the valve sleeve causes at least a portion of the valve sleeve to seal against a wall of the well, and wherein in the unstowed position, the valve sleeve causes the flow of fluid to exit a downstream end of the valve sleeve such that the diameter of the flow of fluid upon exiting the valve sleeve is greater than a diameter of the longitudinal tool body;wherein the flexible valve sleeve has at least a first tapered section, a first end of the tapered section of the sleeve having a first diameter and a second end of the tapered section having a second diameter, the first diameter being greater than the second diameter and greater than an anticipated internal diameter of the well, and wherein the first tapered section is attached to the extendable arms, such that in use the portion of the valve sleeve that seals against the wall of the well is intermediate the first and second diameters. 2. The apparatus of claim 1, wherein the valve sleeve is non-porous substance. 3. The apparatus of claim 1, wherein the valve sleeve is made of a reinforced material. 4. The apparatus of claim 1, comprising a plurality of constant force springs, each spring slideably mounted at one end on the tool body and attached at the other to the periphery of the first tapered section of the valve sleeve having the first diameter, wherein the slideably mounted constant force springs are concentrically mounted such that by sliding in one direction they push the periphery outwards from the tool body, and by sliding in the other they draw the periphery in towards the tool body. 5. The apparatus of claim 4, comprising releasable connectors for connecting the valve membrane to the extendable arms. 6. The apparatus of claim 5, wherein the releasable connectors are connected to the constant force springs at a location intermediate the ends of the constant force springs. 7. A modulator apparatus for downhole use in a fluid-filled well, the modulator comprising: the apparatus of claim 1;a valve for modulating the pressure of the fluid in the fluid-filled well by at least partly closing one end of the valve sleeve to restrict the flow of fluid. 8. The modulator apparatus of claim 7, wherein the valve comprises: a valve seat mounted on the tool body;one or more valve members for closing against the valve seat to at least partly stop the flow of fluid in the valve sleeve. 9. The modulator apparatus of claim 8, comprising an actuator for operating the one or more valve members to close onto the one end of the valve sleeve and press it against the valve seat. 10. The apparatus of claim 9, wherein the valve sleeve has a second substantially tubular section joined at one end to the first tapered section at the end with second diameter, and joined at the other end to the one or more valve members. 11. The modulator apparatus of claim 8, wherein the valve seat has a number of ridges arranged radially around the tool body, and the valve members are arranged to cooperate with the ridges on closing. 12. The modulator apparatus of claim 8, wherein the valve seat is made of resiliently compressible material. 13. The apparatus of claim 8, wherein the tool body comprises an over-pressure release mechanism, the over pressure release mechanism comprising: a telescopic section of the tool body,a tool body housing for receiving the telescopic section;wherein one end of each arm in the plurality of extendable arms is mounted on the telescopic section of the tool body, and the other end is mounted on the tool body housing;a retention mechanism for retaining the telescopic section in a retracted position in the housing, wherein the retention mechanism can be overcome by a force acting on the valve sleeve and through the plurality of extendable arms. 14. The apparatus of claim 13, comprising a spring biasing the telescopic section to remain in the tool body housing. 15. The apparatus of claim 13, wherein the retention mechanism comprises a groove on one of the telescopic section or tool body housing and a restriction on the other of the telescopic section or tool body housing; and a deformable ring received in the groove. 16. The apparatus of claim 15, wherein the deformable ring is a canted coil spring. 17. A modulator apparatus for downhole use in a fluid-filled well, the modulator comprising: a tool body;a valve for modulating the pressure of the fluid in the fluid-filled well, the valve having a valve seat disposed on the tool body and one or more valve members for closing against the valve seat;a retractable fluid flow channel for at least partly sealing against the wall of the fluid-filled well and for channeling the fluid in the fluid-filled well to the valve; anda flexible valve sleeve attached to the plurality of extendable arms and moveable between a stowed position and an unstowed position by movement of the extendable arms, wherein in the unstowed position, the valve sleeve causes a flow of fluid to exit a downstream end of the valve sleeve such that the diameter of the flow of fluid upon exiting the valve sleeve is greater than a diameter of the tool body;wherein the flexible valve sleeve has at least a first tapered section, a first end oldie tapered section of the sleeve having a first diameter and a second end of the tapered section having a second diameter, the first diameter being greater than the second diameter and greater than an anticipated internal diameter of the well, and wherein the first tapered section is attached to the extendable arms, such that in use the portion of the valve sleeve that seals against the wall of the well is intermediate the first and second diameters. 18. The modulator apparatus of claim 17, wherein the fluid flow channel has a diameter that tapers from an end distal to the valve to an end proximate the valve. 19. The modulator apparatus of claim 18, wherein the retractable fluid flow channel comprises: a plurality of extendable arms mounted on the tool body for opening and closing within the fluid-filled well;a flexible valve sleeve attached to the plurality of extendable arms and moveable between stowed position and an unstowed position by movement of the extendable arms, wherein in the unstowed position the valve sleeve is arranged to receive a flow of fluid from the well and the pressure of the fluid in the valve sleeve causes at least a portion of the valve sleeve to seal against a wall of the well. 20. The modulator apparatus of claim 19, comprising art actuator for operating the one or more valve members to close onto the one end of the valve sleeve and press it against the valve seat. 21. The modulator apparatus of claim 20, wherein the valve seat has a number of ridges arranged radially around the tool body, and the valve members are arranged to cooperate with the ridges on closing. 22. The modulator apparatus of claim 21, wherein the valve seat is made of resiliently compressible material. 23. A tool string comprising the apparatus of claim 17.