Patent Publication Number: US-7721800-B2

Title: Flow control mechanism for a downhole tool

Description:
FIELD OF INVENTION 
   The present invention relates to a flow control mechanism for a downhole tool. The present invention also relates to a downhole tool assembly including a downhole tool and a flow control mechanism. 
   BACKGROUND TO INVENTION 
   To comply with safety regulations and to monitor the inclination of well boreholes, among other reasons, the hole may be surveyed periodically during drilling. It is important, for example, that the location of the drill bit relative to the mouth of the hole is known so that a relief well can be drilled in the event of a blow-out. 
   It is presently known to measure the inclination of a drilled hole using one of four types of devices. The first type of device is a drift indicator, the second is a magnetic single shot device, the third is a mechanical measuring-while-drilling device (MMWD), and the fourth is a directional measuring-while-drilling device (DMWD). 
   The first two types of device (the drift indicator and the magnetic single shot device) have been used for more than 50 years. They require a person drilling a well to lower the device into the hole, wait for the device to perform a reading, raise the device from the hole, and then check the measurement taken by the device. Frequently, a second measurement is required to confirm the accuracy of the first measurement. These devices are very expensive to use because the drilling procedure is halted while the device is being used to survey the hole. 
   The third type of device (the MMWD) has been used for more than 40 years. It is located above the drill bit in a purpose-built collar. This device uses a swinging mechanical pendulum to measure the inclination of the device with reference to the vertical plane. This inclination reading is linked to a mechanically activated plunger which, when activated, produces a pulse which is transferred to the surface. Each pulse represents 0.5 degrees of inclination. This provides a measurement of the verticality (the downhole inclination) of the hole. 
   The fourth type of device (the DMWD) is similar to the MMWD but conveys information about the inclination of the hole by means of binary code rather than by mechanically activated pressure pulses. At the drilling console, the code is received, decoded and the results are displayed to the drill operators. The DMWD has a number of disadvantages associated with it. For example, it usually needs at least one trained engineer to operate it correctly and it is more expensive than the other devices. 
   Presently, the most commonly used device is the MMWD device. It is relatively inexpensive to run and does not require an additional trained engineer to operate it. However, these devices are not very accurate or reliable. They are also very expensive to make because they are housed in collars which can cost more than the combined cost of the component parts inside them. A further disadvantage of these devices is that they are sometimes lost downhole, that is, they have to be abandoned, for example, in situations where the bottom hole assembly becomes stuck. 
   In the oil and gas exploration and production industry, a wide range of downhole tools are used for performing specific functions in the downhole environment. Many of these tools are fluid pressure activated and include relatively complex flow control mechanisms for controlling activation of the tool. Frequently these tools require a positive fluid flow for activation, for example, flow past the tool when located in a borehole. 
   Other tools, such as centralisers which are used for centralising a secondary tool in tubing in a well borehole, are mechanical and may include, for example, fins such as rubber fins or sprung arms. Where rubber fins are used, the fins are dimensioned to be a close fit within a tubular in which the centraliser is located whilst sprung arms are compressed inwardly on location of the centraliser within the tubular. In both cases, this acts to centralise a body of the centraliser and thus a tool coupled to the centraliser, such as a drill bit, within the tubular. However, fixed dimension centralisers such as these create potential problems when subsequently removed from the borehole, as a tool such as a packer, valve or jar may have been located in the borehole above the centraliser, these tools restricting the diameter of the borehole and making it difficult to withdraw the centraliser. 
   It is amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages. 
   SUMMARY OF INVENTION 
   According to a first aspect of the present invention there is provided a flow control mechanism for a downhole tool, the mechanism comprising: 
   a body defining a fluid chamber; 
   an inlet flow path for fluid flow into the chamber; 
   at least one tool flow path for fluid flow between the chamber and at least part of the downhole tool; 
   an exhaust flow path for fluid flow from the chamber to a fluid exhaust; and 
   control means including a control member mounted for movement within the chamber for controlling flow into and out of the chamber along the inlet flow path, the at least one tool flow path, and the exhaust flow path. 
   The invention therefore provides a control mechanism for controlling fluid flow within a downhole tool by movement of a control member of the mechanism. Thus the mechanism may be used to control exposure of the downhole tool to fluid pressure. The mechanism may be for controlling flow between the chamber and a fluid activated member of the downhole tool, such as a piston, valve or sliding sleeve. 
   The control mechanism may be activatable in response to applied fluid pressure, and may be activatable in response to a static fluid pressure, for example, the hydrostatic pressure of a fluid in which the control mechanism is located, such as the well pressure of fluid in a borehole of an oil or gas well. Accordingly, the mechanism may be adapted to be activated by hydrostatic well pressure and does not require fluid flow for activation, in contrast to prior assemblies. The mechanism may also be activatable in response to fluid flow, and may therefore be activatable in response to applied pressure of a flowing fluid. The mechanism may therefore function in a fluid flow environment, for example, where there is fluid flow past the downhole tool, or by supplying hydraulic fluid to the mechanism through control lines or the like. 
   The mechanism may be adapted to be provided as an integral part of a downhole tool, or as a separate mechanism adapted to be coupled to a downhole tool. The mechanism may comprise a control mechanism for a plurality of downhole tools and the control member may be movable for controlling flow between the chamber and parts of a plurality of downhole tools. The body may comprise a body of a downhole tool, or may comprise a separate body adapted to be coupled to a downhole tool. 
   The control member may be movable in a direction along a length of the body and may define an activating member. The control member may be movable for opening flow between the inlet flow path, the chamber and the part of the downhole tool, for supplying fluid to the downhole tool. The control member may also be movable for opening fluid flow between the downhole tool, the chamber and the exhaust flow path. 
   The mechanism may comprise an inlet flow port for flow into the chamber along the inlet flow path, a tool flow port for flow between the chamber and the downhole tool along the tool flow path and an exhaust flow port for flow between the chamber and the exhaust along the exhaust flow path. The mechanism may include a plurality of tool flow ports and associated tool flow paths for flow between the chamber and separate parts of the downhole tool, or between the chamber and parts of a plurality of downhole tools. 
   The control means may further comprise a plurality of seal elements which, together with the control member, are adapted to control flow into the chamber along the inlet flow path, flow between the chamber and the downhole tool, and flow out of the chamber along the exhaust flow path. The seal elements may be provided in the chamber and may be adapted to seal with a surface of the control member. The control member may be movable out of sealing abutment with the seals for opening fluid flow. 
   The control member may be movable between a position allowing fluid flow between the inlet flow path and the chamber and between the chamber and the downhole tool, and a further position allowing fluid flow between the downhole tool, the chamber and the exhaust. Thus simple, relatively small movements of the control member may control flow of fluid through the mechanism. The tool flow path may define a flow path for flow between the chamber and the downhole tool and vice-versa. This allows flow both to and from the downhole tool along a single tool flow path. The control member may be locatable in a position where flow between the inlet flow path and the chamber, the chamber and the downhole tool and the chamber and the exhaust, respectively, is prevented or closed, which may comprise a first, running-in position of the control member. This may allow the downhole tool to be run-in to a well without inadvertently activating the mechanism. The control member may be movable to second and third positions where flow is allowed, as described above. 
   In the first position of the control member, the member may be in sealing engagement with first and second seal elements for closing flow. In the second position, the control member may be out of sealing engagement with a first seal, for allowing flow into the chamber along the inlet flow path and flow between the chamber and the downhole tool; and in the third position, the control member may be out of sealing engagement with a second seal for allowing flow between the downhole tool and the chamber and between the chamber and the exhaust. Flow into the chamber along the inlet flow path may be adapted to be closed before or during movement of the control member to the third position. This allows the chamber to exhaust without further flow into the chamber along the inlet flow path. 
   The mechanism may further comprise an inlet flow path entrance port for supply of fluid into the inlet flow path. The entrance port may be adapted to be closed before or during movement of the control member to the third position. The mechanism may include a movable plug such as a sleeve or collar movable for closing the entrance port. 
   The mechanism may further comprise a filter for filtering fluid entering the inlet flow path. This allows the mechanism to be activated using well fluids or other fluids typically found in a well borehole. The movable plug may define the filter, and may define a passage between an inner surface of the plug and the body for flow of fluid into the inlet flow path, the passage dimensioned to prevent solids entering the passage. 
   In an alternative embodiment, the mechanism may comprise at least two tool flow paths, each tool flow path for fluid flow between the chamber and a respective separate part of the downhole tool, or separate downhole tools. Each flow path may be adapted for flow from the chamber to separate parts of the downhole tool, or from the chamber to respective parts of separate downhole tools, as well as for flow from separate parts of the downhole tool to the chamber, or respective parts of separate downhole tools and the chamber. Thus fluid may be supplied to and exhausted from the downhole tool. 
   The control member may be movable between a first position allowing flow into the chamber along a first tool flow path and from the chamber to the downhole tool; and a second position allowing flow into the chamber and from the chamber to the downhole tool along a second tool flow path. In the first position, the control member may also allow flow from the downhole tool to the chamber along the second tool flow path. This may facilitate movement of a fluid activated member such as a piston of the downhole tool coupled in a closed loop to the chamber, for example, by fluid flow to one end of the fluid activated member and fluid exhaust from the other end of the fluid activated member. In the second position, the control member may also allow flow from the downhole tool to the chamber along the first tool flow path. 
   In the first position of the control member, the member may be in sealing engagement with selected seal elements for allowing flow between the chamber and the downhole tool along tool flow paths. In the second position of the control member, the member may be in sealing engagement with selected other seal elements for allowing flow between the chamber and the downhole tool along tool flow paths. 
   The chamber may be subdivided into a number of secondary chambers which are adapted to be selectively fluidly isolated by the control member. The seal elements, together with the control member, may define the secondary chambers. The control member may include reduced dimension portions which are adapted to straddle a seal element to allow fluid flow. The control member may comprise a needle valve which may be generally rod shaped. 
   The mechanism may be adapted to be activated mechanically by application of a force to the control member for moving the control member within the chamber. The control mechanism may be adapted to be activated mechanically. For example, the assembly may include a release mechanism coupled to the control member, in a restraint position the release mechanism restraining the control member against movement and in a release position, the control member being movable within the chamber. The release mechanism may be moved between the restraint and release positions by a wireline coupled to the assembly. Alternatively, the release mechanism may be movable by applied fluid pressure, for example, by application of a pressure above a predetermined threshold, or by flow sequencing, for example, by application of fluid pressures in a determined sequence. In further alternatives, the release mechanism may be movable remotely and independently using electronic programming, for example, by electronic wireline coupled to the assembly, or by a combination of any of the foregoing. It will be understood that following activation in this fashion, the control mechanism may be subsequently activated by applied fluid pressure as described above. 
   The exhaust may comprise an exhaust chamber isolated from the hydrostatic pressure of fluid outside the mechanism. This allows flow to the exhaust chamber from the fluid chamber when required. The exhaust chamber may initially be at surface atmospheric pressure. 
   According to a second aspect of the present inventions there is provided a downhole tool assembly comprising: 
   a downhole tool including a fluid activated member; and 
   a flow control mechanism for controlling operation of the fluid activated member, the flow control mechanism comprising: a body defining a fluid chamber; an inlet flow path for fluid flow into the chamber; at least one tool flow path for fluid flow between the chamber and the fluid activated member; an exhaust flow path for fluid flow from the chamber to a fluid exhaust; and control means including a control member mounted for movement within the chamber for controlling flow into and out of the chamber along the inlet flow path, the at least one tool flow path, and the exhaust flow path. 
   Further features of the flow control mechanism are defined above. 
   The downhole tool may comprise a plurality of fluid activated members. The fluid activated members may be spaced along a length of the downhole tool, and may be rotationally spaced around the tool. The fluid activated member may be mounted in the body for movement substantially radially with respect to the body. 
   The downhole tool may comprise a centraliser and the fluid activated member may comprise a piston of the centraliser. Preferably, the centraliser comprises a plurality of pistons which are adapted to centralise the tool within a borehole of an oil or gas well, such as within a tubular such as casing, liner, production tubing or any other tubular. The centraliser may be adapted to centralise a downhole tool within a borehole. Thus the centraliser may be adapted to be coupled to a downhole tool for centralising the downhole tool and the tool may therefore be hydraulically self-centering within a borehole. 
   Most preferably, the centraliser comprises at least three pistons spaced around a circumference of the centraliser, the pistons being activatable to move outwardly and engage the borehole wall. The piston may be retractable from a radially extended position, allowing the tool assembly to pass through a bore restriction. The piston may be retractable when the control member is in the second position, allowing flow to the exhaust. The pistons may be equally rotationally spaced and where there are three pistons, may be spaced at 120° intervals for centralising the tool when the pistons are moved outwardly. The pistons may act as clamps for clamping a wall of a borehole. 
   The piston may be mounted in a cylinder coupled to the chamber, the cylinder initially containing a gas at a pressure less than the pressure of fluid supplied to the chamber. The piston may also define a first inner piston area greater than a second, outer piston area, the second piston area being open to well pressure. In this fashion, the piston experiences a force when fluid is supplied from the chamber to the piston cylinder, to move the piston radially outwardly. The piston may extend through a sealed opening in the cylinder and may include an abutment surface which may comprise a protective cover coupled to the piston, for exerting a force on a borehole to centralise the tool within the borehole. 
   Alternatively, the downhole tool may comprise a downhole tool for generating a fluid pressure pulse, such as a borehole inclination measuring (drift) tool, the tool including a fluid activated member in the form of a piston, the piston coupled to or defining flow restriction means mounted for movement between a first position and a second position where fluid flow is restricted compared to the first position. The piston may be movable in a direction along a length of the tool. The flow restriction means may include the fluid activated member such that movement of the flow restriction means depends upon movement of the fluid activated member, which movement is controlled by the control mechanism. 
   Further features of the downhole tool for generating a fluid pressure pulse will be defined below. 
   In alternative embodiments, other downhole tools may be provided incorporating the control mechanism or the control mechanism may be provided as part of a tool used to control other downhole tools; for example a downhole packer; a downhole valve such as an open/shut valve; a sliding sleeve; a downhole shutting tool (for shutting off a well); or a tool for providing temporary positioning of tools, such as cutting or patching tools, in tubing; or as a trigger for other devices such as sampling tools, perforating tools or any other downhole tool requiring positioning and/or activating. 
   The fluid activated member may comprise a piston coupled to a sliding sleeve, a valve element such as a ball valve or flapper valve or to any other fluid activated member of a downhole tool. 
   According to a third aspect of the present invention, there is provided a centraliser comprising a flow control mechanism and a fluid activated member movable outwardly for centralising the centraliser in a borehole, the flow control mechanism comprising: 
   a body defining a fluid chamber; 
   an inlet flow path for fluid flow into the chamber; 
   at least one tool flow path for fluid flow between the chamber and the fluid activated member; 
   an exhaust flow path for fluid flow from the chamber to a fluid exhaust; and 
   control means including a control member mounted for movement within the chamber for controlling flow into and out of the chamber along the inlet flow path, the at least one tool flow path, and the exhaust flow path. 
   The fluid activated member may be movable radially outwardly for centralising the centraliser in the borehole. 
   It will be understood that the term centralising within a borehole is intended to include centralising within a tubular within a borehole, for example, casing, liner or production tubing, as well as in an open (unlined) borehole. Further features of the centraliser are defined above. 
   According to a fourth aspect of the present invention, there is provided a downhole tool for generating a fluid pressure pulse, the downhole tool comprising a flow control mechanism and a flow restriction means, the flow restriction means including a fluid activated member movable between a first position and a second position where fluid flow is restricted compared to the first position, the flow control mechanism comprising: 
   a body defining a fluid chamber; 
   an inlet flow path for fluid flow into the chamber; 
   at least one tool flow path for fluid flow between the chamber and the fluid activated member; 
   an exhaust flow path for fluid flow from the chamber to a fluid exhaust; and 
   control means including a control member mounted for movement within the chamber for controlling flow into and out of the chamber along the inlet flow path, the at least one tool flow path, and the exhaust flow path. 
   The downhole tool may comprise a borehole inclination measuring (drift) tool. Further features of the tool for generating a fluid pressure pulse are defined above. 
   According to a fifth aspect of the present invention, there is provided a method of controlling the operation of a downhole tool, the method comprising the steps of: 
   coupling a control mechanism to the downhole tool to define: an inlet flow path for fluid flow into a chamber of the mechanism; at least one tool flow path for fluid flow between the chamber and at least part of the downhole tool; and an exhaust flow path for fluid flow from the chamber to a fluid exhaust; and 
   moving a control member of the mechanism within the chamber to control flow into and out of the chamber along the inlet flow path, the at least one tool flow path and the exhaust flow path. 
   According to a further aspect of the present invention, there is provided a downhole tool comprising: 
   a body defining a fluid flow path; 
   flow restriction means movably mounted in the body, for movement between a first position and a second position where fluid flow is restricted compared to the first position; and 
   activating means including a member movable in a direction along a length of the body to cause the flow restriction means to move between the first and second positions. 
   Preferably, the downhole tool is for generating a fluid pressure pulse. The flow restriction means may be movable between the first and second positions to generate a fluid pressure pulse. 
   Advantageously, movement of the activating member controls fluid communication between, for example, the exterior of the tool and the flow restriction means, for moving the flow restriction means between the first and second positions. The flow restriction means may comprise a first or upper part which is movable in response to movement of the activating member, and a second or lower part which restricts the flow of fluid through the body when the flow restriction means is in the second position. 
   The flow restriction means may be generally in the form of a piston. Preferably, the flow restriction means comprises a piston assembly which is movable in a direction along a length of the body in response to applied fluid pressure. At least part of the piston assembly may be hollow to selectively allow fluid to pass therethrough, and at least part of the piston assembly may be mounted in a cylinder defined by the body. The piston assembly may include a first or upper piston part which is movable in response to applied fluid pressure and a second or lower piston part. The first piston part may be hollow. A piston rod may couple the first and second piston parts. 
   The body may comprise a generally tubular outer housing of the tool and may include a first fluid inlet through which fluid may enter the body. It will be understood that, when the tool is located in, for example, a drill string, fluid may partly flow around the tool, but that the major part of the fluid flow is through the first fluid inlet into the body, passing through the body and exhausting into the string at a downstream location. The flow restriction means, preferably the second piston part may close the first fluid inlet when the flow restriction means is in the second position. The body may include a separate, second fluid inlet through which fluid may enter the body for moving the flow restriction means between the first and second positions. 
   The activating means may include a bore in the body and the activating member may be movably mounted in the bore. Preferably, the activating means further comprises a hollow control body mounted in the tool body, the control body defining the bore. The control body may include a sleeve in which part of the piston assembly, preferably the upper piston part, is mounted, and one or more housing rings coupled to the sleeve. The sleeve may define a cylinder, and the one or more housing rings may define the activating member bore. 
   The activating means, in particular the control body, may include a control flow port for allowing selective supply of fluid to the bore. In use, fluid may be supplied from the body second fluid inlet and to the control flow port. Preferably, the activating means, in particular the control body, includes four control flow ports opening on to the bore and associated with respective first, second, third and fourth control fluid flow channels, which channels may be defined by the control body. The first fluid flow channel may be for supplying fluid to the bore through the first control fluid port, and the second, third and fourth fluid flow channels may be for allowing fluid communication between the bore and the flow restriction means through the second, third and fourth flow ports, respectively. The second fluid flow channel may couple a first end of the upper piston part to the bore and the third fluid flow channel may couple a second end of the upper piston part to the bore. Advantageously therefore, when fluid is supplied from the bore to one end of the upper piston part, fluid is returned from the other end to the bore, and vice versa. Thus it will be understood that by controlling the flow of fluid to and from the flow restriction means, the movement of the flow restriction means and thus the generation of a fluid pressure pulse may be controlled. 
   The tool may further comprise a chamber for storing fluid evacuated from the bore. In particular, the chamber may be for storing fluid returned to the bore through the second and third channels. The fourth fluid flow channel may couple the bore and the chamber. Conveniently, the fourth fluid flow channel couples the bore with the hollow interior of the upper piston part, for exhausting fluid through the upper piston part into the chamber. The chamber may be dimensioned to contain fluid discharged from multiple, for example, at least one hundred and fifty cycles of movement of the flow restriction means between the first and second positions. 
   The bore may comprise a number of secondary chambers, which chambers may be selectively fluidly isolated by the activating member. A number of seals may be provided in the bore, said seals, together with the activating member, defining the secondary chambers. The activating member may be movable with respect to the seals, and may define fluid flow paths which are selectively isolated by the seals. In particular, the activating member may comprise a generally cylindrical rod, the rod including cut-away or reduced dimension portions, which may straddle a seal to define a flow path and allow fluid communication therethrough depending upon the position of the activating member. 
   Preferably also, the tool further comprises pressure isolation means for isolating the part of the piston assembly from the pressure of fluid outside the tool. The pressure isolation means may include an isolation chamber at least partly containing a gas, which may be at surface atmospheric pressure. The upper piston part and/or an end of the piston rod may be mounted partly in the isolation chamber. The lower piston part may experience equal fluid pressure on opposite piston faces thereof. This may prevent hydraulic lock of the piston assembly. 
   The tool may further comprise drive means for moving the activating member, which drive means may include a drive motor. The motor is conveniently battery powered, and may be operative in response to an applied fluid pressure. This is particularly advantageous in that a drive means is provided which does not require, for example, control lines or power lines extending to surface, with the associated disadvantages which will be appreciated by the skilled person. The activating member may be in the form of a partly screw threaded rod, which may be rotated by the drive means to move in the direction along a length of the housing. 
   According to a still further aspect of the present invention, there is provided a downhole tool comprising: 
   a housing defining a fluid flow path; 
   flow restriction means movably mounted in a first chamber in the housing, for movement in response to applied fluid pressure between a first position and a second position where fluid flow is restricted compared to the first position; and 
   activating means including a member movable to cause the flow restriction means to move between the first and second positions, whereby movement of the flow restriction means between the first and second positions displaces fluid from the first chamber into a second, storage chamber defined in the housing. 
   According to a yet further aspect of the present invention, there is provided a downhole tool comprising: 
   a body defining a fluid flow path; 
   flow restriction means movably mounted in the body, for movement between a first position and a second position where fluid flow is restricted compared to the first position; 
   activating means including a member movable to cause the flow restriction means to move between the first and second positions; and 
   pressure isolation means for isolating at least part of the flow restriction means from the exterior of the tool. 
   Preferably, the downhole tool is for generating a fluid pressure pulse. The flow restriction means may be movable between the first and second positions to generate a fluid pressure pulse. 
   By this arrangement, the pressure isolation means advantageously prevents hydraulic lock of the flow restriction means, in use. 
   According to a yet further aspect of the present invention, there is provided a method of generating a fluid pressure pulse in a borehole, the method comprising the steps of: 
   locating a body in the borehole to define a fluid flow path through the body; 
   providing a movable flow restriction means in the body, movable between a first position and a second position where fluid flow through the body is restricted compared to the first position; and 
   moving a flow restriction means activating member in a direction along a length of the body, to cause the flow restriction means to move to the second position, to restrict the flow of fluid through the body, generating a fluid pressure pulse. 
   The step of providing a movable flow restriction means may further comprise mounting a piston in the housing and selectively coupling the piston to a fluid pressure source. The step of moving the flow restriction means activating member may further comprise the step of coupling drive means to the activating member and activating the drive means to move the member. The fluid pressure pulse may provide an indication that the measurement of a desired parameter is to be transmitted, the magnitude of said measurement depending upon the length of time between pressure pulses. Thus the method may further comprise a method of transmitting data indicating the value of a desired parameter. 
   The step of moving the flow restriction member may further comprise the step of selectively supplying fluid to a first part of the flow restriction means whilst exhausting fluid from a second part of the flow restriction means. 
   It will be understood that one or more features of the above described aspects of the present invention may be provided singly or in combination. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings which are: 
       FIG. 1  a schematic illustration of a downhole tool incorporating a flow control mechanism in accordance with an embodiment of the present invention, the tool shown located in a drill string in a borehole; 
       FIG. 1A  an enlarged, longitudinal partial cross-sectional view of the downhole tool of  FIG. 1 , shown in an open position; 
       FIG. 1B  a schematic cross-sectional view of part of the downhole tool of  FIG. 1A , taken along line A-A of  FIG. 1A ; 
       FIG. 1C  an enlarged view of part of the downhole tool of  FIG. 1A , taken along line D-D indicated in  FIG. 1B ; 
       FIGS. 1D and 1E  further enlarged views of part of the tool of  FIG. 1 , taken along lines B-B and C-C of  FIG. 1B , respectively; 
       FIG. 2  a view of the downhole tool of  FIG. 1 , similar to the view of  FIG. 1A  but showing the tool in a closed position; 
       FIG. 2A  a graphical illustration of generation of fluid pressure pulses using the tool of  FIG. 1 ; 
       FIGS. 3A and 3B  enlarged views of part of the downhole tool shown in the open position of  FIG. 1A ; 
       FIGS. 4A and 4B  enlarged views of part of the downhole tool shown in the closed position of  FIG. 2 ; 
       FIG. 5  a schematic illustration of a downhole tool including a flow control mechanism in accordance with an alternative embodiment of the present invention, the tool shown located in a drill string in a borehole in a deactivated position; 
       FIG. 6  a view of the downhole tool of  FIG. 5  in an activated position; 
       FIG. 7  an enlarged view of the downhole tool of  FIG. 5 ; 
       FIG. 8  a top view of the downhole tool shown in  FIG. 7 ; 
       FIG. 9  a partially sectioned exploded view of the downhole tool shown in  FIG. 7 ; 
       FIG. 10  a schematic illustration of part of the flow control mechanism of the downhole tool shown in  FIG. 7 ; and 
       FIGS. 11 ,  12  and  13  schematic views of parts of the downhole tool of  FIG. 7  shown at various stages in a procedure of operating the tool. 
   

   DETAILED DESCRIPTION OF DRAWINGS 
   Referring initially to  FIG. 1 , there is shown somewhat schematically a view of a downhole tool in accordance with an embodiment of the present invention, the tool indicated generally by reference numeral  10 . The tool  10  takes the form of an Electronic Drift Tool (EDT) for use in measuring-while-drilling (MWD) techniques. The tool  10  is shown located within a string of tubing, typically a drill string  11 , by a baffle  13  of a type similar to that disclosed in United Kingdom Patent Publication Number 2 334 732, the content of which is incorporated herein by reference. Drilling fluid (not shown) is pumped through the string  11  to a drill bit  15  in the direction of arrow A′, before returning to surface through annulus  19 , in the direction of arrows B′, carrying entrained drill cuttings. Supply of the drilling fluid is controlled by a pump  21 , whose operation is governed manually by the drill operator  23 . A pressure sensor  25  measures the pressure of the fluid in the annulus  19 , to detect a pressure pulse, and the measurements are recorded by a processor  27 . The tool  10  includes an electronics package  29 , which includes a pressure sensor, inclinometer, accelerometer and a processor (not shown), together with a drive means in the form of a motor  22 , shown in  FIG. 1A  and which will be described in more detail below. The baffle  13  both locates the tool  10  in the drill string  11 , and constrains flow through the string  11  to be directed through ports  17  in the baffle  13  and, selectively, through the tool  10 . In fact, as will be described below, a major part of the fluid flow through the drill string  11  is directed through the tool  10 , when the tool  10  is in an open position. 
   Referring now to  FIG. 1A , there is shown a longitudinal cross-sectional view of the downhole tool  10  of  FIG. 1  in more detail. 
   A control mechanism  2  in accordance with an embodiment of the present invention is provided as part of the tool  10 . The control mechanism  2  includes a body in the form of an outer housing  12  of the tool  10 , the body defining a chamber or inner bore  34 ; an inlet flow path  92  for fluid flow into the chamber  34 ; at least one tool flow path  94 ,  96  for fluid flow between the chamber  34  and part of the tool  10 ; an exhaust flow path  97 ,  112  for fluid flow from the chamber to a fluid exhaust  40 ; and control or activating means, indicated generally by reference numeral  16 , which includes a control or activating member  18 . The control member  18  is mounted for movement within the chamber  34  for controlling flow into and out of the chamber  34  along the inlet flow path  92 , the at least one tool flow path  94 ,  96 , and the exhaust flow path  97 ,  112 , as will be described in more detail below. The control member  18  is movable along a length of the housing  12  to cause flow restriction means of the tool  10 , comprising a piston assembly  14 , to move between first and second positions. The tool  10  is shown in  FIG. 1A  in an open position, where the piston assembly  14  is in a first position and fluid flows through the housing  12 . In  FIG. 2 , the piston assembly  14  is shown in a second position where the piston assembly  14  has moved to close the tool  10 , to prevent fluid flow therethrough and thereby generate a fluid pressure pulse. 
   The structure of the tool  10  will now be described in more detail, viewing  FIGS. 1A and 2  top to bottom. At an upper end of the tool  10 , drive means  20  are provided for moving the control member  18 . The drive means  20  includes a fluid activated electric motor  22  powered by a battery (not shown) in the tool  10 , and a bearing and gearing assembly  24 . The control member  18  takes the form of a needle valve or activating rod, which is threaded at  19  and extends through an internally threaded upper guide housing  26 . When the motor  22  is activated, the motor  22  rotates the rod  18 , moving the rod  18  longitudinally along the housing  12  between the positions of  FIGS. 1A and 2A . The motor  22  is fluid activated according to the pressure of the fluid in the drill string  11 . 
   The control means  16  includes a control body which comprises five annular seal housing rings  28  and a lower sleeve assembly  30  defining a cylinder. Each of the seal housing rings  28  are secured together and to the sleeve assembly  30  by high tensile cap screws  32 , which ensure correct rotational orientation of the rings  28 . The rings  28  together define the chamber or inner bore  34  in which the rod  18  is located, and a number of seals  36  are provided between the rings  28  to seal the chamber  34 . 
   Referring now also to  FIGS. 1D and 1E , there are shown enlarged views of part of the tool  10 , taken along lines B-B and C-C of  FIG. 1B , respectively. The sleeve assembly  30  includes an outer sleeve  31  and an inner sleeve  38 , which defines a cylinder  42  in which part of the piston assembly  14  is located. The outer sleeve  30  and inner sleeve  38  are located by a sub  12   a  of the housing  12 , which is in turn, coupled to a lower housing part  12   b , and the housing part  12   b  defines the fluid exhaust which comprises a chamber  40 , as will be described below. The inner sleeve  38  carries a number of sets of seals  42   a ,  42   b  and  42   c , for sealing the inner sleeve  38  and for directing fluid into the cylinder  42 . 
   The piston assembly  14  includes an upper piston part  44  and a lower piston part  46  (shown to the right in  FIGS. 1A and 2 ), coupled together by a hollow piston rod  48 . When the tool  10  is open ( FIG. 1A ), fluid enters a muleshoe  12   d  of the housing  12  through a first flow port  50 , flowing along the housing  12 , exhausting through a lowermost outlet  51  and flowing to the drill bit  15 . Movement of the piston assembly  14  between the open position and the closed position ( FIG. 2 ), controlled by the control mechanism  2 , moves the lower piston part  46  to close the first flow port  50 , increasing annulus fluid pressure to generate a fluid pressure pulse. The piston part  46  includes a solid wall  47 , whilst the end faces  49 ,  53  include apertures to allow fluid flow axially through the piston to a pressure isolation unit  58 , which will be described below. Fluid is supplied to the control mechanism  2  to move the tool  10  between the first, open position and the second, closed position through an inlet path entrance port comprising a second tool fluid inlet  52 . The inlet  52  is defined by an upper end of the sub  12   a  that carries a filter  54  for removing relatively large particles from the drilling fluid. 
   The housing part  12   b  is coupled to a lower housing part  12   c  through a threaded, hollow seal end housing unit  56 . This unit  56  seals the fluid exhaust chamber  40  and the piston rod  48 , to prevent fluid escape from the chamber  40  during movement of the rod  48 . Below the seal end housing unit  56 , pressure isolation means in the form of a pressure isolation unit  58  isolates a lower end  60  of the piston rod  48 , and thus the upper piston part  44 , from the pressure of fluid outside the tool  10 . This allows the upper piston part  44  to move and prevents hydraulic lock. 
   The pressure isolation unit  58  includes a threaded housing  62  which couples the housing part  12   c  to the muleshoe  12   d , and which includes two passages  64  and a pressure isolation chamber  66 . The pressure isolation chamber  66  carries a seal  68  in which the lower end  60  of the piston rod  48  is moveably mounted, and is charged with a gas at surface pressure, before the tool  10  is run downhole. The passages  64  receive connecting rods  70 , which secure the lower piston part  46  to the piston rod  48  through upper and lower piston connectors  72  and  74 , respectively. Both the rods  70  are free to move within the passages  64 , and are unsealed for fluid communication through the annulus between the outer surface of the rods  70  and the inner surface of the passages  64 . This ensures that the pressure of the fluid on the upper and lower faces  76  and  78  of the lower piston part  46  are equal, to prevent hydraulic lock-up of the upper piston part  44 . During movement of the upper piston part  44  to the second, closed position of  FIG. 2 , the lower end  60  of the piston rod  48  compresses the gas in the pressure isolation chamber  66 . 
   In general terms, operation of the tool  10  to move between the open position of  FIG. 1A  and the closed position of  FIG. 2  is achieved in the following fashion. Fluid is supplied to the chamber  34  carrying the rod  18  from the drill string  11 , through the second fluid inlet  52 . Fluid supply from the chamber  34  into the cylinder  42  carrying the upper piston part  44  depends upon the longitudinal position of the rod  18 . Thus, movement of the rod  18  in a direction along the length of the housing  12  selectively supplies and exhausts fluid to the cylinder  42 . This moves the upper and lower piston parts  44  and  46  to close the first fluid inlet  50  ( FIG. 2 ), which prevents fluid flowing through the muleshoe  12   d  to the drillbit  15 . In this fashion, fluid flow is constrained to be directed through the baffle  13  only. This restricts the flow of fluid through the drill string  11 , increasing fluid pressure and generating a fluid pressure pulse. Typically, the tool  10  is held in a closed position only for a short duration to generate the pressure pulse. This is illustrated graphically in  FIG. 2A , which is a graph of annulus pressure (measured by pressure sensor  25 ) against time. The annulus pressure during a drilling operation (y 1  psi) is typically in the region of 500 psi to 10,000 psi, depending upon the depth of the borehole (and thus the hydrostatic pressure). When it is desired to transmit data such as borehole inclination, measured by sensors in the electronics package  29 , the tool  10  is closed ( FIG. 2 ), restricting fluid flow in the string  11 , increasing fluid pressure, and generating first fluid pressure pulse PP 1 . When detected at surface, this indicates that a measurement is about to be transmitted. The tool  10  is reopened ( FIG. 1A ) and closed again to generate pressure pulse PP 2 , which indicates the start of a measuring period. The tool  10  is then reopened once more, before a final pressure pulse PP 3  is generated, indicating the end of the data transmission. The magnitude of the parameter (for example, borehole inclination) transmitted is determined by measuring the peak to peak time x 1  between the pulses PP 2  and PP 3 , which is equal to x 3 −x 2 . Alternatively, the time may be measured between return of fluid pressure to level y 1 . 
   The structure and operation of the tool  10  will now be described in more detail with reference in particular to  FIGS. 1D ,  1 E and  FIGS. 3A to 4B . As shown in  FIG. 1A , fluid enters the tool  10  through the second fluid inlet  52 , passing through filter  54  and into an annulus  82  defined between the sleeve  30 , seal housing rings  28  and the outer housing  12 . A seal  84  is provided to seal an upper end of the annulus  82 . In a similar fashion, a seal (not shown) at a lower end of the annulus  82  directs fluid into the chamber  34 . In  FIGS. 1D and 1E , the five seal housing rings  28  have been numbered  28   a - 28   e , respectively, for ease of reference. Six annular seals  86   a - 86   f  are mounted in the chamber  34  and the control rod  18  is slidable within the seals between the positions of  FIGS. 1A and 2 . Each respective pair of seals  86   a/b ;  86   b/c ;  86   c/d ;  86   d/e ; and  86   e/f  separates the chamber  34  into a number of secondary chambers  88   a - 88   e , respectively. These chambers  88   a - 88   e  are isolated by the control rod  18 , depending upon its longitudinal position. The rod  18  includes cut-away portions  90   a ,  90   b  and  90   c , which are of reduced outer diameter compared to the remainder of the rod  18 . Depending upon the position of the rod  18 , these portions  90   a - 90   c  straddle respective ones of the seals  86   a - 86   e , to allow fluid communication between adjacent chambers  88   a - 88   e . 
   Also, the outer and inner sleeves  30  and  38 , together with the various seal housing rings  28   a - 28   e , define the inlet flow path  92 , tool flow paths  94 ,  96  and the exhaust flow paths  97 ,  112 . The inlet flow path  92  includes an inlet flow port  100  opening onto the chamber  34 . The tool flow path  94  defines a first tool flow path including a first tool flow port  102  opening onto the chamber  34  and a cylinder port  108  opening onto the cylinder  42 . In a similar fashion, the tool flow path  96  defines a second tool flow path including a second tool flow port  104  opening onto the chamber  34  and a cylinder port  110 , whilst the exhaust flow path  97  defines an exhaust flow port  106  opening onto the chamber  34 , and this channel  97  communicates with an upper end  112  of the cylinder  42 . The cylinder end  112  also forms an exhaust flow path in selective communication with a lower end of the chamber  34 , as will be described below. 
     FIGS. 3A and 3B  show part of the tool  10  in the open position of  FIG. 1A , and  FIGS. 4A and 4B  show the tool in the closed position of  FIG. 2 .  FIGS. 3A and 4A  are sectional views on line B-B of  FIG. 1B , whilst  FIGS. 3B and 4B  are sectional views on line C-C of  FIG. 1B . 
   To move the tool  10  to the closed position, where the first fluid flow port  50  is closed, generating a pressure pulse, the motor  22  is activated to move the control rod  18  longitudinally in a direction towards the motor  22 , to the position of  FIGS. 4A and 4B . In this position, the cut-away portion  90   b  straddles the seal  86   d , allowing flow between chambers  88   c  and  88   d . This allows fluid to flow through the inlet flow path  92 , through the inlet flow port  100  into the second tool flow port  104 , along the second tool flow path  96 , before discharging into the cylinder  42 . This fluid acts against an upper piston face  114  of the upper piston part  44  ( FIG. 4A ). 
   Simultaneously, the cut-away portion  90   a  straddles seal  86   b , allowing flow between the chambers  88   b  and  88   a . Therefore when fluid is supplied to the cylinder  42  through the second tool flow path  96 , fluid is simultaneously exhausted from the cylinder  42 , by downward movement of the upper piston part  44 . This fluid flows through the port  108 , through the first tool flow path  94  and into the chamber  34  through the first tool flow port  102 . This fluid is then exhausted across seal  86   b  and out of chamber  88   a  into the exhaust path defined by the upper end  112  of the cylinder  42 . The exhausted fluid flows through the inner bore  118  of the upper piston part  44 , and through exhaust ports  120  into the fluid exhaust chamber  40 , which is under a vacuum (reduced pressure) or contains gas at surface pressure. This movement of the upper piston part  44  brings the lower piston part  46  to the closed position of  FIG. 2 , generating the pressure pulse. 
   When it is desired to re-open the housing  12 , the upper piston part  44  is returned to the first position shown in  FIGS. 3A and 3B . This is achieved by activating the motor  22  to rotate the control rod  18  in the opposite direction, to move it longitudinally downwardly away from the motor  22 . In this position, the cut-away portion  90   b  now straddles the seal  86   c , allowing flow from chamber  88   c  to chamber  88   b . Fluid supplied to the chamber  34  through the inlet flow port  100  thus travels across seal  86   c  and enters the first tool flow path  94  through the first tool flow port  102 . This fluid is supplied through port  108  into the cylinder  42  to act against a lower piston face  116  of the upper piston part  44  ( FIG. 3A ). 
   Simultaneously, the cut-away portion  90   c  is moved to a position where it straddles the seal  86   e  ( FIG. 3B ), allowing flow across the seal  86   e  from chamber  88   d  to chamber  88   e . This allows fluid to be exhausted from the cylinder  42  through the port  110 , along the second tool flow path  96  and into the chamber  34 , through the second tool flow port  104 . The fluid travels across the seal  86   e  and into the exhaust flow path  97  via the exhaust flow port  106 . Therefore this fluid is exhausted from the cylinder  42 , through the chamber  34  and into the upper end  112  of the cylinder  42 , through inner bore  118  of the upper piston part  44  and into the exhaust chamber  40 . 
   The exhaust chamber  40  is of a volume sufficient to contain fluid discharged from a large number of such cycles of the tool  10 —typically of the order of 150 cycles. This is advantageous in that this allows multiple cycles of pressure pulses (and therefore transmission of data to surface) to be carried out before the tool  10  is pulled out of hole and the exhaust chamber  40  emptied ready for further use. As shown in  FIG. 1C , optional backup safety bleed ports  122  may be provided to provide a safety bleed from the cylinder  42  to annulus preventing surge. 
   Turning now to  FIG. 5 , there is shown a downhole tool in accordance with an alternative embodiment of the present invention, the tool indicated generally by reference numeral  200  and comprising a centraliser. Like components of the centraliser  200  with the drift tool  10  of  FIGS. 1-4B  share the same reference numerals incremented by 200. As will be described below, the centraliser  200  includes a control mechanism  202  in accordance with an alternative embodiment of the present invention, similar to the control mechanism  2  of the downhole tool  10 . 
   The centraliser  200  is shown in  FIG. 5  coupled to the drift tool  10  of  FIGS. 1-4B , for centralising the tool  10  within a drill string  211 , similar to the string  11  illustrated in  FIG. 1 . This ensures accurate inclination measurements of the borehole are obtained. The centraliser  200  is moveable between a deactivated position shown in  FIG. 5 , and an activated position shown in  FIG. 6  where fluid activated members comprising three centralising pistons  244  are urged radially outwardly to engage a wall  122  of the drill string  211 . The centraliser  200  is moved to the activated position under the control of the control mechanism  202 , which will now be described, in conjunction with a wireline  124 . 
   The centraliser  200  is shown in more detail in the enlarged view of  FIG. 7  and in  FIG. 8 , which is a top view of the centraliser shown in  FIG. 7 . The pistons  244  are shown in  FIGS. 7 and 8  in a retracted position and are rotationally spaced 120° apart around the centraliser  200  and axially staggered. The control mechanism  202  is shown in more detail in  FIG. 9 , which is a partially sectioned, exploded view of the centraliser  200 . For clarity, only one of the centraliser pistons  244  is shown in  FIG. 9 . 
   The flow control mechanism  202  controls the operation of the centraliser  200  and includes a body  212  defining a fluid chamber  234 , also shown in the enlarged schematic view of  FIG. 10 . The mechanism  202  also includes an inlet flow path  292  for fluid flow into the chamber  234 , at least one tool flow path in the form of tool flow path  294  for fluid flow between the chamber  234  and part of the centraliser  200 , and an exhaust flow path  297  for fluid flow from the chamber  234  to a fluid exhaust in the form of an exhaust chamber  240  ( FIG. 13 ), which is sealed from well pressure. Control means  216  of the mechanism  202  includes a moveable control rod or needle valve  218  mounted for movement within the chamber  234 , for controlling flow into and out of the chamber  234  along the inlet flow path  292 , the at least one tool flow path  294  and the exhaust flow path  297 . 
   Each centraliser piston  244  is mounted in a cylinder  242  for movement between the retracted and extended positions of  FIGS. 5 and 6 . The piston  244  is coupled to the chamber  234  through the tool flow path  294 , for selectively exposing the piston  244  to hydrostatic wellbore pressure, to urge the piston  244  radially outwardly for engaging the wall  122  of the drill string  211 . This movement is controlled by movement of the control rod  218  within the chamber  234 . 
   In more detail, the centraliser  200  includes an upper housing  126  and a shaft  128  coupled to the control rod  218  and threaded to the upper housing  126  for movement together. The upper housing  126  is coupled to the body  212  and moveable in an axial direction on a stub  130  of the body  212 . This movement of the housing  126  causes a corresponding movement of the control rod  218  within the chamber  234 . An annular collar or plug  132  is mounted around the stub  130  and is moveable independently of the upper housing  126 , and a hollow body  134  is movably mounted within the plug  132  and threaded to the stub  130 . The body  134  defines a passage  136  in which the control rod shaft  128  is movably mounted and a threaded coupling  138  of the body  134  defines the chamber  234 . The control rod  218  is mounted within the chamber  234  for movement with respect to first and second seal elements  286   a ,  286   b  which sealingly engage the control rod. 
   As shown in  FIG. 10  and the enlarged view of  FIG. 11 , the inlet flow path  292  extends through the body  134  and includes an inlet flow port  300  opening onto the chamber  234  and an entrance flow port  294  allowing fluid flow into the inlet flow path  292 . In the deactivated position of  FIG. 9 , the entrance flow port  298  is closed by the plug  132 , as will be described below. The entrance flow port  298  is closed during running-in of the centraliser  200 . This prevents the centraliser  200  from being inadvertently activated during run-in to the borehole. 
   The stub  130  includes an annular groove (not shown) in an upper surface forming part of the inlet flow path  292 . This facilitates connection of the body  134  to the stub  130  as the rotational orientation of the body  134  to the stub  130  as the rotational orientation of the body  134  does not need to be precisely determined; the groove ensures the inlet flow path in the stub  130  and body  134  are fluidly coupled. 
   Each piston  244  is mounted in an opening  140  in the tool body  212  and a threaded housing connector  142  is mounted and sealed in the opening  140 . The housing connector  142  is threaded to the body  212  and defines a passage  144  fluidly coupling the piston  244  to the tool flow path  294 , which opens into the opening  140 . The piston cylinder  242  is provided as a housing which is threaded and sealed to the housing connector  142  surrounding the piston  244  and the piston  244  includes a threaded stub  146  which extends through an opening  148  in an end of the cylinder housing  242 . The piston  244  includes a first O-ring seal  150  which is larger than a second O-ring seal  152  mounted in the opening  148  and thus defines a larger piston area than the seal  152 . A protective cover  153  is threaded to the piston stub  146  and defines an abutment surface for abutting and engaging the drill string wall  122 . The cylinder  242  is charged with a gas, typically air at surface atmospheric pressure, before the centraliser  200  is run into the borehole. Thus, when a first face  314  of the piston  244  is exposed to hydrostatic well pressure, the seal  150 , which is larger than the seal  152 , causes a pressure force to be exerted on the piston face  314 , which is greater than that exerted on the piston face  316 , such that the piston  244  is urged radially outwardly to the extended position of  FIG. 6 , engaging the drill string wall  122 . 
   The method of operation of the centraliser  200  will now be described in more detail, with reference to  FIGS. 11-13 , which are enlarged, schematic illustrations of parts of the centraliser  200 . In the running-in position of the centraliser  200  of  FIG. 5 , the upper housing  126  and thus the control rod shaft  128  and the control rod  218  are locked to the tool body  212 . In this position, the inlet path entrance port  298  is open, however, the control rod  218  seals against the seal elements  286   a ,  286   b , as shown in  FIG. 10 , to both close the tool inlet flow path  292  and the exhaust flow path  297 . Thus, although the inlet path entrance port  298  is open, there is no flow through the chamber  234 . 
   When the downhole tool  10  has been located in a baffle  213  in the drill string  211 , a locking mechanism (not shown) releases the upper housing  126 . A release tool  156  is mounted around the upper housing  126  and is located in an undercut  160  engaging a lower shoulder  158  of the housing. The release tool  156  is mounted on the wireline  124  and is moved upwardly by the wireline  124 , to carry the upper housing  126  a short distance upwardly with respect to the tool body  212 . Through the connection between the control rod shaft  128  and the control rod  218 , this moves the control rod  218  upwardly from the first, closed position of  FIGS. 9 ,  10  and  11  to a second position, illustrated in  FIG. 12  where there is a gap between an upper surface of the plug  132  and the upper housing  126 . In this position, the control rod  218  has moved past the lower seal  286   a , opening flow along the inlet flow path  292  into the chamber  234 . Fluid enters the inlet entrance port  298  along an annulus  154  ( FIG. 9 ) defined between the plug  132  and the upper housing  126 . This provides communication through the chamber  234  and along the tool flow path  294 . The tool inlet entrance port  298  is open to hydrostatic well pressure, therefore the piston  244  now experiences well pressure on the piston face  314 , and is urged radially outwardly. Similar movement of each of the other pistons  244  centralises the body  212  of the centraliser  200  and thus the downhole tool  10  within the drill string  211 . It will be understood that the hydrostatic pressure of fluids in the drill string  211  are sufficient to activate the pistons  244 , and thus that no fluid flow through the string is necessary. The centraliser  200  will operate at a pressure of as low as 250 psi and in a range of 250 to 10,000 psi. However, the centraliser  200  operates in fluid flow environments such as is typical downhole. When the centraliser  200  is in the activated position of  FIG. 6 , the centraliser  200  exerts a sufficiently large force on the string wall  122  to clamp the drift tool  10  centrally within the string  211 , and also resists the axial movement of the drift tool  10 . 
   When it is desired to deactivate the centraliser  200 , it is necessary to move the pistons  244  to the retracted positions of  FIG. 5 . This is achieved by engaging the release tool  156  in the undercut  160  in engagement with a lower shoulder  162  of the plug  132 , as shown in  FIG. 6 . A second upward movement of the release tool  156  initially carries the plug  132  upwardly to close the gap between the plug and the upper housing  126 , and to close the entrance port  298 . This closes the inlet flow path  292  and shuts off fluid communication between the chamber  234  and the exterior of the centraliser  200 . A further upward movement of the plug  132  now carries the upper housing  126  a further distance upwardly with respect to the tool body  212 . This moves the control rod  218  a further distance upwardly past the second seal  286   b , opening flow through the chamber  234  between the tool flow path  294  and the exhaust flow path  297 , whilst shutting off flow into the chamber  234  along the inlet flow path  292 , as shown in  FIG. 13 . 
   The exhaust chamber  240  is charged with a gas at surface atmospheric pressure or is under a vacuum. Accordingly, the pressure force exerted on the faces  314  of the pistons  244  is now greatly reduced. The force on the piston faces  316  is thus greater, urging the pistons  244  radially inwardly to the retracted position of  FIG. 5 . The centraliser  200  and tool  10  may then be recovered to surface through the drill string  211 . The centraliser  200  may then be re-set and run-in again when required to centralise a tool  10  within the drill string  211 . Alternatively, the centraliser  200  may be re-set downhole and thus may be used to perform a number of separate centralising operations before the centraliser  200  is pulled out of the borehole. 
   In alternative embodiments, other downhole tools may be provided incorporating the control mechanism of the present invention, or the control mechanism may be provided as part of a tool used to control other downhole tools; for example a downhole packer; a downhole valve such as an open/shut valve; a sliding sleeve; a downhole shutting tool (for shutting off a well); or a tool for providing temporary positioning of tools, such as cutting or patching tools, in tubing; or as a trigger for other devices such as sampling tools, perforating tools or any other downhole tool requiring positioning and/or activating. 
   The fluid activated member may comprise a piston coupled to a sliding sleeve, a valve element such as a ball valve or flapper valve or to any other fluid activated member of a downhole tool. 
   Various modifications may be made to the foregoing within the scope of the present invention. 
   It will be understood that the control mechanism essentially acts as a trigger mechanism for activating any fluid activated (hydraulic) downhole tool. The control mechanism has the ability to hold the tool in a desired position or activation state until the control member is moved, allowing fluid to be used as a motive fluid for activation of the downhole tool. 
   The centraliser may be used for centralising any downhole tool and may be used for centralising a tool within any tubular, such as casing, liner, production tubing or the like. The centraliser may equally be used in an open hole environment and thus may be used for centralising within an open borehole. 
   The control mechanism may be provided as part of a downhole tool or may be provided separately and coupled to a downhole tool for controlling operation of the tool. Thus the control mechanism may be provided in a separate body or housing coupled to the downhole tool to be controlled. 
   The control mechanism may be mechanically activated as described above, or may be activated in any other suitable fashion. Accordingly, the control member may be adapted to be moved in response to applied fluid pressure, fluid pressure sequencing or by electronic or electrical control. 
   The drift tool may include a control mechanism of the type described in relation to the centraliser and vice-verse. It will equally be understood that the flow control mechanism may be used for controlling any type of fluid activated downhole tool and thus of any fluid activated member of a downhole tool. 
   The centraliser or other downhole tool may include any suitable number of fluid activated members and may thus include any suitable number of pistons. The pistons may be provided at any desired rotational and axial spacing along a length of the centraliser.