Patent Publication Number: US-2022220820-A1

Title: Improvements In Or Relating To Well Abandonment and Slot Recovery

Description:
The present invention relates to apparatus and methods for well abandonment and slot recovery and in particular, though not exclusively, to an apparatus and method for casing recovery. 
     When a well has reached the end of its commercial life, the well is abandoned according to strict regulations in order to prevent fluids escaping from the well on a permanent basis. In meeting the regulations it has become good practise to create the cement plug over a predetermined length of the well and to remove the casing. This provides a need to provide tools which can pull long lengths of cut casing from the well to reduce the number of trips required to achieve casing recovery. However, the presence of drilling fluid sediments, partial cement, sand or other settled solids in the annulus between the outside of the casing and the inside of a surrounding downhole body e.g. outer casing or formation can act as a binding material limiting the ability to free the casing when pulled. Stuck casings are now a major issue in the industry. 
     Traditionally, cut casing is pulled by anchoring a casing spear to its upper end and using an elevator/top drive on a drilling rig. However, some drilling rigs have limited pulling capacity, and a substantial amount of power is lost to friction in the drill string between the top drive and the casing spear, leaving insufficient power at the spear to recover the casing. Consequently, further trips must be made into the well to cut the casing into shorter lengths for multi-trip recovery. 
     To increase the pulling capability, a downhole power tool (DHPT) available from the present Applicants, has been developed. After the casing has been located and engaged with a casing spear, hydraulically-set mechanically releasable slips anchor the DHPT to the wall of the larger ID casing above. A static pressure is applied to begin the upward movement of the cut casing, with the DHPT downhole multi-stage hydraulic actuator functioning as a hydraulic jack. After the stroke is completed, the anchors are released. The power section can be reset and the anchor re-engaged as many times as required. The DHPT is described in U.S. Pat. No. 8,365,826 assigned to the present Applicants, the disclosure of which is incorporated herein in its entirety by reference. 
     As in many downhole operations, it is practical to drive a hydraulic actuator by means of a liquid, typically a drilling fluid, which is pumped through a pipe string in which the tool is included. The actuator is then hydraulically connected in such a way that fluid may flow out of an access port in the pipe string and into the actuator. When pressure is to be created for driving an actuator in a downhole tool, it is known to close the flow of drilling fluid by means of a valve, which is placed below said access port. Most hydraulic actuators operate via movement of a piston across a chamber. The access port is arranged at a first end of the chamber and the fluid enters the chamber and acts on a first face or first side of the piston to move it through the chamber. An exhaust port is arranged in the opposing end of the chamber, so that fluid at the opposing face or second side of the piston is displaced out of the chamber to allow the required movement of the piston. This exhaust port is typically to the annulus surrounding the pipe string and tool in the well. When the actuator is to be re-set, the opposite displacement of fluid is required i.e. fluid on the first side is moved back into the pipe string while fluid enters the second side from the well annulus. In the DHPT, re-setting occurs by raising the pipe string which moves the piston relative to the chamber by virtue of each being connected to the pipe string or the cut casing. 
     However, when the hydraulic actuator is operated in a low level well, referred to as underbalanced, the hydrostatic imbalance between the column of fluid in the pipe string and the well annulus, prevents the piston being moved through the chamber so that the hydraulic actuator cannot be reset. 
     It is an object of the present invention is to provide a downhole assembly and method of operating an actuator on a pipe string in a low level well which obviates or mitigates at least some of the disadvantages of the prior art. 
     According to a first aspect of the present invention there is provided a downhole assembly for use in a low level well, comprising: 
     a hydrostatic equalisation device, the hydrostatic equalisation device having a first tubular member with a first end configured to connect to a pipe string, a second tubular member arranged to move relative to the first tubular member and at least one radial port being selectively openable and closeable to give fluid access between a throughbore of the pipe string and an annulus around the downhole assembly; 
     a hydraulic actuator to perform a task by the assembly downhole, the hydraulic actuator having a tool body including a central bore and a first end configured to connect to the hydrostatic equalisation device, a piston moveable in a chamber, the chamber having an access port from the central bore on a first side of the piston and an exhaust port to the annulus on a second side of the piston; and 
     a valve, the valve including an obturating member arranged to block fluid flow through the central bore to divert fluid flow from the throughbore into the chamber via the access port; wherein: 
     in a first configuration, the radial port is closed and the hydraulic actuator operates to perform the task by movement of the piston as fluid flows from the throughbore into the chamber via the access port to act on the first side of the piston; and 
     in a second configuration, the radial port is opened and fluid flows from the throughbore to the annulus to equalise pressure between the throughbore and the annulus, and on equalisation fluid flows into the chamber via the exhaust port allowing movement of the piston to re-set the hydraulic actuator in preparation to repeat the task. 
     In this way, the hydrostatic equalisation device allows the column of fluid in the throughbore to drain from the pipe string until equilibrium is reached with the volume of fluid in the annulus, to allow the hydraulic actuator to re-set. As the hydrostatic equalisation device operates independently of the actuator, by mechanical means rather than hydraulic, the actuator can be re-set when the piston is at any position in the chamber. 
     Preferably, the first tubular member and the second tubular member are biased to move telescopically with a sliding seal located therebetween and such telescopic movement opens and closes the at least one radial port. In this way, applying tension or compression to the downhole assembly can operate the hydrostatic equalisation device. Alternatively or additionally, the first tubular member and the second tubular member are arranged to rotate relative to each other with a sliding seal located therebetween and such movement opens and closes the at least one radial port. In this way, rotation of the pipe string can be used to operate the hydrostatic equalisation device. 
     Preferably, the hydrostatic equalisation device includes a first spring to bias the first tubular member and the second tubular member in the first configuration. More preferably, setting down weight on the first tubular member moves the second tubular member telescopically in relation to the first tubular member and aligns at least one radial port on the first tubular member with at least one radial port on the second tubular member. 
     Preferably, a diameter of the sliding seal is less than a predominant diameter of the pipe string. In this way, the hydrostatic equalisation device will remain closed in the first configuration even with the pressure difference between the fluid in the throughbore and the annulus due to the hydrostatic head. The hydrostatic equalisation device may include a second spring to bias the first and second tubular members, wherein a force of the second string is adjustable. This can be adjusted to assist in overcoming the imbalance force to open the at least one radial port when the diameter of the sliding seal does not equal the predominant diameter of the pipe string. 
     Preferably the assembly includes a hydraulic jack, the hydraulic jack comprising an anchor for axially fixing the assembly to a tubular in the well, and a mandrel connectable to a lower pipe string axially moveable relative to the anchor by activation of the hydraulic actuator. In this way, the downhole assembly is a downhole pulling tool. 
     Preferably the valve is connected below the hydraulic jack. In this way closure of the valve can be used to commence operation of the hydraulic jack. More preferably, the valve is closed by creating tension on the pipe string. In this way, the valve can be closed prior to actuating the hydraulic jack. Preferably the valve is the ALO valve available from Ardyne AS, Norway, which operates by opening and closing the pipe string by the application of tension on the pipe string as described in EP3063364 and incorporated herein by reference. Alternatively, the valve may be a ball seat sub which operates by dropping a ball down the throughbore of the pipe string to seat in a ball seat. 
     Preferably, the assembly includes a casing spear connected to the lower pipe string below the valve. In this way, the downhole assembly can be used to recover casing in a well bore. 
     The downhole assembly may include a casing cutter connected to the lower pipe string below the casing spear. In this way, casing may be cut and pulled on the same trip into the well bore. 
     Preferably, the hydraulic jack includes a housing supported in the well by the string and enclosing the hydraulic actuator, the hydraulic actuator comprising a plurality of axially stacked said pistons generating a cumulative axial force, each of the plurality of pistons axially movable in response to the fluid entering a plurality of the access ports; and wherein movement of the pistons also moves the mandrel, with the mandrel being an inner mandrel extending from the housing. In this way, a great pulling force can be created downhole at the jack. Preferably the hydraulic jack is the DHPT supplied by Ardyne AS. 
     Alternatively, the hydraulic jack includes an outer housing arranged around an upper mandrel connected to the pipe string and enclosing the hydraulic actuator, the hydraulic actuator comprising a plurality of axially stacked pistons generating a cumulative axial force, each of the plurality of pistons axially movable in response to the fluid entering a plurality of the ports; and wherein movement of the pistons also moves a mandrel, with the mandrel being a lower mandrel extending from a lower end of the outer housing. In this way, an alternative arrangement of a hydraulic jack is provided. The hydraulic jack may be as described in GB2533022, the contents of which are incorporated herein by reference. 
     Preferably, in the hydraulic jack the plurality of axially stacked pistons include a plurality of inner pistons each secured to the inner mandrel and a plurality of outer pistons each secured to a tool housing supported by the string. Preferably, the axial force generated by the plurality of pistons acts simultaneously on the anchor and on the tool mandrel, such that the tool anchoring force increases when the axial force on the tool mandrel increases. Preferably, the anchor includes a plurality of slips circumferentially spaced about the mandrel for secured engagement with an interior wall in the well. Preferably, an axial force applied to the plurality of slips is reactive to the force exerted on the casing spear by the plurality of pistons. 
     Preferably the casing spear comprises: a sliding assembly mounted on the inner mandrel; at least one gripper for gripping onto an inner wall of the length of casing, the gripper being coupled to the sliding assembly; the sliding assembly being operable for moving the gripper between a first position in which the gripper is arranged to grip onto the inner wall of the length of casing in at least one gripping region of the length of casing and a second position in which the gripper is held away from the inner wall; and a switcher which, when advanced into the length of casing, locks the sliding assembly to the inner mandrel with the gripper in the second position; and, when the casing spear is pulled upward out of the length of casing and the switcher exits the end of the length of casing, automatically allows engagement of the length of casing by the gripper in the first position. In this way, the length of casing is automatically gripped into engagement with the casing spear when the casing spear is at the top of the length of casing. Preferably the casing spear is the Typhoon® Spear supplied by Ardyne AS. 
     According to a second aspect of the present invention there is provided a method of operating an actuator on a pipe string in a low level well, comprising the steps:
         (a) locating a downhole assembly according to the first aspect on the pipe string with the hydrostatic equalisation device in the first configuration;   (b) running the pipe string into the well bore to a position at which the downhole assembly is to perform a task on operation of the hydraulic actuator;   (c) closing the valve;   (d) increasing fluid pressure in the throughbore at the access port to cause fluid to enter the chamber and act on a first side of the piston, thereby moving the piston to operate the hydraulic actuator and perform the task with the downhole assembly;   (e) switching the hydrostatic equalisation device to the second configuration by moving the first tubular member relative to the second tubular member and opening the at least one radial port;   (f) allowing fluid flow from the throughbore of the pipe string to the annulus outside the downhole assembly via the at least one radial port until equilibrium is reached between the throughbore and the annulus; and   (g) flowing fluid from the annulus to the chamber on the second side of the piston via the exhaust port and moving the piston relative to the chamber to re-set the hydraulic actuator.       

     In this way, the hydraulic actuator can be re-set in a low level well as the imbalance of hydrostatic pressures between the column of fluid in the pipe string and the low level fluid in the annulus would ordinarily prevent the piston from moving back to re-set the hydraulic actuator. 
     Preferably, the method comprises the additional step:
         (h) switching the hydrostatic equalisation device to the first configuration by moving the first tubular member relative to the second tubular member and closing the at least one radial port.       

     This re-cocks the downhole assembly ready to operate again. 
     The method may include repeating steps (d) to (h). In this way, the hydraulic actuator can be used again while the downhole assembly is in the well. 
     The method may include carrying out step (e) before the piston has fully stroked across the chamber. In this way, the hydraulic actuator can be re-set at any time and is not dependent on the piston travelling entirely through the chamber. 
     The method may include the step of pulling the pipe string and the downhole assembly from the low level well. 
     Preferably the hydraulic actuator operates a hydraulic jack. In this way the task is to provide a downhole pulling tool. 
     Preferably the method includes attaching a casing spear to a cut section of casing and pulling the cut section of casing as the task. 
     Preferably the method includes attaching a casing cutter to the downhole assembly and cutting casing in the well bore to provide the cut section of casing. 
     Preferably, the valve is closed by pulling the pipe string. Alternatively, the valve is closed by dropping a ball into the throughbore of the pipe string and seating the ball in a ball seat. 
     Preferably, the method includes the step of anchoring the downhole assembly to a wall of the well. The wall may be outer casing in the well. 
     Preferably, step (e) occurs by setting down weight on the pipe string. More preferably, the hydraulic actuator is fixed in relation to the wall of the well when step (e) occurs. The hydraulic actuator will be fixed if the downhole assembly is anchored to a wall of the well. 
     Preferably, the downhole assembly is selected to have sliding seal diameter less than or equal to a prominent diameter of the pipe string. 
     More preferably, a second spring on the hydrostatic equalisation device between the first and second tubular members is adjusted in length to vary the force holding the at least one radial port closed in the first configuration. 
     In the description that follows, the drawings are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce the desired results. 
     Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. Language such as “including,” “comprising,” “having,” “containing,” or “involving,” and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term “comprising” is considered synonymous with the terms “including” or “containing” for applicable legal purposes. 
     All numerical values in this disclosure are understood as being modified by “about”. All singular forms of elements, or any other components described herein including (without limitations) components of the apparatus are understood to include plural forms thereof. 
     Additionally, while relative terms such as ‘above’ and ‘below’ are used, this does not limit the invention to being used in a vertical well bore. The invention has equal application in inclined or deviated well bores. 
    
    
     
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which: 
         FIG. 1  is a schematic illustration of a downhole assembly according to an embodiment of the present invention; 
         FIG. 2( a )  is a part sectional view of an actuator section of a hydraulic jack and  FIG. 2( b )  is a part sectional view of an anchor of the hydraulic jack, according to an embodiment of the present invention; 
         FIGS. 3( a ) and 3( b )  are sectional views through a hydrostatic equalisation device, according to an embodiment of the present invention; 
         FIGS. 4( a ) and 4( b )  are sectional views through a valve, according to an embodiment of the present invention; and 
         FIGS. 5( a )-( d )  illustrate apparatus and method for casing recovery in a wellbore, using a downhole assembly, according to an embodiment of the present invention. 
     
    
    
     Reference is initially made to  FIG. 1  of the drawings which illustrates a downhole assembly, generally indicated by reference numeral  10 , located on a pipe string  12  in a well bore  13 . The assembly  10  includes a valve  14 , a hydraulic actuator  16  and a hydrostatic equalisation device  18 , in order according to an embodiment of the present invention. 
     The well bore  13  is a low level well, by which we mean the level of fluids in the annulus  30  between the assembly  10  and outer casing  54  is lower than the position of the downhole assembly  10  in the well bore  13 . More particularly the fluid level in the annulus  30  will be lower than the column of fluid found in the pipe string  12 . Such a well may be referred to as underbalanced. 
     From an upper end, being closer to the surface of the well bore, the pipe string  12  has the hydrostatic equalisation device  18  mounted therein. The hydrostatic equalisation device  18  has an inner tubular member  20  with an outer tubular member, sleeve  22  located around and supported thereon as is known in the art. The sleeve  22  is biased against the inner tubular member via spring  24 . The sleeve  22  includes a radial port  28  therethrough which when aligned with a radial port  26  on the inner tubular member  20  gives access for fluid flow between a throughbore  40  of the pipe string  12  and an annulus  30  around the assembly  10 . Alignment of the radial ports  26 , 28  can be achieved by compression of the sleeve  22  and tubular member  20  to move them towards each other and/or by rotation of the tubular member  20  within the sleeve  22 . Those skilled in the art will recognise that indexing and j-slot arrangements can be located between the member  20  and the sleeve  22  to control the movement and allow repeated opening and closing of the radial port  28  to give fluid communication between the throughbore  40  of the pipe string  12  and the annulus  30  in the well  13 . 
     The hydraulic actuator  16  may be any arrangement driven by an increase in fluid pressure against a piston  15 . In the illustration of  FIG. 1 , fluid flows through an access port  32  to move an inner mandrel  34  which forms a lower portion of the pipe string  12 . The piston  15  is contained within a chamber  17  and the access port  32  is arranged on a first side  19  of the piston  15 . On the second side  21  of the piston  15 , there is an exhaust port  23  which gives a fluid passageway between the inside of the chamber  17  and the annulus  30 . The inner mandrel  34  provides a central bore to the hydraulic actuator  16  which is a continuation of the throughbore  40 . 
     Below the hydraulic actuator  16 , the assembly  10  has a valve  14  which is shown as a ball seat sub mounted in the pipe string  12 . The ball seat sub  14  provides a ball valve seat  36  which is affixed to the inner wall of the pipe string  12 . The valve  14  operates by pumping a ball  38  down the throughbore  40  which will seat in the ball valve seat  36  and create a seal across the throughbore  40 , blocking fluid flow at this point. This is used to divert fluid flow from surface through the access port  32  to operate the hydraulic actuator  16 . In this embodiment, once the valve  14  is closed, the hydraulic actuator can only be reset by use of the hydrostatic equalisation device  18 . 
     The valve  14 , hydraulic actuator  16  and hydrostatic equalisation device  18  may be formed integrally on a single tool body or may be constructed separately and joined together by box and pin sections as is known in the art. Two parts may also be integrally formed and joined to the third part. 
     In use, the assembly  10  is mounted on a pipe string  12  with the sleeve  22  covering the radial port  28 . The pipe string  12  is run in the well and fluid can fill the throughbore  40 . With the assembly  10  at the desired position fluid is pumped down the throughbore  40 . The drop ball  38  is released into the pipe string  12  and is sized to pass through the hydrostatic equalisation device  18  and hydraulic actuator  16 . The ball  38  passes through the actuator  16  and is stopped at the ball valve seat  36 , seals the throughbore  40  and blocks the passage of fluid through the pipe string  12  at the valve  14 . By continuing to pump fluid from surface, the fluid pressure will increase above the ball  38  and consequently fluid entering the access port  32  on the hydraulic actuator  16  will have an increased pressure. The fluid will fill the chamber  17  on the first side  19  of the piston  15  and the fluid pressure will move the piston through the chamber  17  by acting on the first side  19  and operate the actuator  16 . In this embodiment the inner mandrel  34  will move relative to the upper pipe string  12 . As the ball valve seat  36  is fixed, the ball  38  will remain in the seat  36  and a maximum pressure can be applied to operate the actuator  16 . As we are in a low level well  13 , fluid pressure on the second side  21  of the piston  15  is much lower than the pumped fluid pressure and the piston  15  will move easily. 
     When we wish to reset the actuator  16 , in this case to move the pipe string  12  upwards relative to the inner mandrel  34 , we stop pumping fluid down the throughbore  40 . However, as we are in a low level well  13 , pulling on the pipe string  12  from surface will have no effect. This is because the weight of the column of fluid in the pipe string  12  supported on the ball  38 , provides a greater force on the first side  19  of the piston  15  than the pressure of fluid on the second side  21  of the piston  15  which will be the fluid pressure in the annulus  30 . This hydrostatic imbalance prevents the hydraulic actuator  16  being released. In the present invention, the hydrostatic equalisation device  18  is used to achieve this. 
     On run-in and activation of the actuator the hydrostatic equalisation device  18  can be considered to have been in a first configuration wherein the bias in spring  24 , held the sleeve  22  in a position in which the radial ports  26 ,  28  are misaligned. The radial port  28  is closed and fluid flow is entirely in the throughbore  40  past the device  18 . When required, the hydrostatic equalisation device  18  is switched to a second configuration by creating relative movement between the sleeve  22  and inner tubular member  20 . Dependent on the design of the device  18 , this can be done by setting down weight on the pipe string  12  i.e. slacking it off, pulling on the pipe string  12  i.e. applying tension and/or by rotation of the pipe string  12 , which will rotate the inner tubular member  20 . Such movement aligns the radial ports  26 , 28  and opens the fluid passageway between the pipe string  12  and the annulus  30 . It is noted that longitudinal movement is preferred over rotational as it can be more reliably performed in a well. 
     With the radial port  28  now open, the column of fluid which is present in the throughbore  40  will drain out of the pipe string  12 . This will continue until equilibrium is reached between the fluid pressure in the pipe string  12  and the fluid pressure in the annulus  30 . At this point, pressure on each side  19 , 21  of the piston  15  is balanced and if the pipe string  12  is pulled, the piston  15  can travel in the chamber  17  and be returned to its initial position. This will effectively reset the hydraulic actuator  16 . It&#39;s also noted that the action will also move the hydrostatic equalisation device  18  back to its first configuration and the assembly is re-cocked for use again. The hydraulic actuator  16  can thus be repeatedly activated without the requirement of removing the pipe string  12  from the well or opening the valve  14  to drain the entire column of fluid from the pipe string  12 . 
     In an embodiment the hydraulic actuator  16  operates a hydraulic jack  100 . A hydraulic jack  100  is illustrated in  FIGS. 2( a ) and 2( b ) . The hydraulic jack  100  has an anchor  128  and an actuator  116  system which pulls an inner mandrel  130  up into a housing  132  of the jack  100 . In the preferred embodiment the hydraulic jack is the DHPT available from Ardyne AS. It is described in U.S. Pat. No. 8,365,826, the disclosure of which is incorporated herein in its entirety by reference. 
     Referring to  FIGS. 2( a ) and 2( b )  there is illustrated the main features of the hydraulic jack  100 .  FIG. 2( a )  shows a portion of the actuator system  116 . The jack  100  has an outer housing  132  with a connection  134  to the pipe string  12 . There is an inner mandrel  130  which can move axially within the housing  132 . A series of spaced apart outer pistons  136  are connected into the housing  132 . A series of spaced apart inner pistons  138  are connected to the inner mandrel  130 . The pistons  136 , 138  are stacked between each other so that an upper end face  140  of an inner piston  138  will abut a lower end face  142  of an outer piston  136 . Only one set of pistons  136 , 138  are shown but this arrangement is repeated along the mandrel  130  to provide five sets of pistons  136 , 138 . The inner mandrel  130  includes a number of ports  144  arranged circumferentially around the mandrel  130 , at the upper end of each outer piston  136 , when the inner piston  138  rests on the outer piston  136 . A chamber  146  is provided at this location so that fluid can enter the ports  144  to operate the actuator  116  and will act on the lower end face  148  of the inner piston  138 . This will move the piston  138  upwards, crossing a vented space  150 , until the upper end face  140  of the inner piston  138  abuts the lower end face  142  of the outer piston  136 . This movement constitutes a stroke of the jack  100 . 
     Movement of the inner mandrel  130  is driven by movement of the inner pistons  138 . As there are multiple stacked pistons  138 , the combined cross-sectional areas of the end faces  140  when fluid pressure is applied generates a considerable lifting force via the inner mandrel  130 . 
     Hydraulic jack  100  also includes an anchor  128 , shown in  FIG. 2( b ) . Anchor  128  has a number of slips  152  arranged to ride up a cone  154  by the action of fluid entering a chamber  156  and moving the cone  154  under the slips  152 . The outer surface  158  of the slips  152  is toothed to grip an inner surface of the casing in which the anchor  128  is positioned. The anchor  128  is connected to the outer housing  132  so that the inner mandrel  130  can move axially relative to the anchor  128  when the anchor is set to grip the casing. 
     There is an alternative jack which may be used. This jack has the anchor located at the upper end and the hydraulic jack includes an outer housing arranged around an upper mandrel connected to the pipe string and enclosing the hydraulic actuator, the hydraulic actuator comprises a plurality of axially stacked pistons generating a cumulative axial force, each of the plurality of pistons axially movable in response to the fluid entering a plurality of the ports; and wherein movement of the pistons also moves the mandrel, with the mandrel being a lower mandrel extending from a lower end of the outer housing. This hydraulic jack is as described in GB2533022, the contents of which are incorporated herein by reference. 
     While  FIG. 1  shows a simplified hydrostatic equalisation device  18 , a difficulty with such pressure relief valves, circulation valves or unloader valves as they also may be referred to is in preventing relative movement between the inner tubular member and sliding sleeve until such time as they require to be operated. Any pressure differential created across the valve can cause relative movement. This is particularly the case when a lower end of the valve is fixed such as would occur when the valve is located above a DHPT. An embodiment of a hydrostatic equalisation device  118  designed to overcome this is illustrated in  FIGS. 3( a )-( b ) . Like parts to those of the earlier figures used for clarity. 
     Hydrostatic equalisation device  118  includes the features of a first tubular member  20  with a second tubular member or sleeve  22  located around it, a spring  24  between the tubular member  20  and sleeve  22 , and a radial port  28 , which can connect the throughbore  40  to an annulus  30  outside the device  118 . The first tubular member  20  has at a first end  42  a box section  44  for connecting the device  118  to a pipe string. At a lower end  46  there is a shoulder  48  on the outer surface  50  for the spring  24  to act against. Radial ports  26 , of which there are four in this embodiment, are arranged through the tubular member  20 . The sleeve  22  includes the radial port  28 , of which there are four in this embodiment, sized to match the radial ports  26  of the tubular member  20 . The sleeve  22  has at a lower end  52 , a pin section  56  to connect the device into a lower pipe string or to another tool such as the hydraulic actuator  16 . At an upper end  58  the sleeve  22  is supported on the tubular member  20  by a shoulder  60 . The shoulder  60  is at an end of a splined arrangement  62 , as is known in the art, which allows the sleeve  22  and tubular member  20  to move longitudinally with respect to each other without rotation. This telescopic movement without rotation is required to ensure that the radial ports  26 , 28  align and is shown in cross-section in  FIG. 3( b ) . At the upper end  58  is also arranged a sliding seal  64  between the outer surface  50  of the tubular member and the inner surface  66  of the sleeve  22  which prevents fluid passing through the device  118  when the radial port  28  is closed. Additional seals  68   a,b  are arranged at opposite sides of the radial port  28  also. 
     Hydrostatic equalisation device  118  includes two additional features: the first to prevent exposure and possible loss of a seal  68   a , when the tubular member  20  and sleeve  22  are moved relative to each other; and the second to assist in opening the radial port  28  when this is required. 
     A shoulder  69  on one side of the radial port  28  is initially aligned against a lower side of the radial port  26 , which together form an end of an annular chamber  70  between the tubular member  20  and the sleeve  22 , with the opposing end being a portion of the tubular member  20 . Within the chamber  70  is a piston sleeve  72  including an annular piston face  74  extending therefrom. A spring  76 , of lower strength than spring  24 , is located between the face  74  and the tubular member end of the chamber  70 . A lower end  78  of the piston sleeve  72  abuts the lower side of the radial port  26  and thereby covers the port  26 . The shoulder  69  includes the seal  68   a  held against the outer surface  50  of tubular member  20  when the hydrostatic equalisation device  118  is in the first configuration as described above. When the hydrostatic equalisation device  118  is switched to the second configuration, the tubular member  20  moves downwards relative to the sleeve  22  against the bias of spring  24 . The piston sleeve  72  will move down with the member  20  as it abuts it and is biased by the spring  76 . The piston sleeve  72  will then be stopped by the piston face  74  meeting the shoulder  69  with the tubular member continuing to move downwards. When the piston sleeve  72  is stopped, its lower end  78  will have travelled under the shoulder  69  and be covering the seal  68   a , so the seal has never been exposed. The lower end  78  is sized to the length of the shoulder  69  so that continued movement of the tubular member  20  aligns the radial ports  26 , 28  with each other and creates the fluid path from the throughbore  40  to the annulus  30 . Radial port  26  is prevented from passing radial port  28  by virtue of an upper end of the piston sleeve  72  reaching the end wall of the chamber  70  by virtue of compression of the spring  76 . Thus the radial ports  26 , 28  stay aligned as long as weight is set down on the device  118  and the seals  68   a,b  are never exposed in use. 
     The second feature is required as the device  118  is designed to be pressure balanced, which is the requirement that until a mechanical action is taken i.e. setting down weight, the device  118  will not activate so that the application of fluid pressure neither opens nor closes the device. This is particularly relevant were, as in a preferred embodiment, the sleeve  22  is axially fixed in the well. 
     If the sliding seal  64  diameter on the device  118  is the same as the ‘predominant running string diameter’ then pressure during run-in or when pumping fluid down the pipe string  12  will have no effect. Predominant running string diameter (PRSD) is the weighted average diameter of the pipe from surface down to the device  118 . Weighted average allows for restrictions at tool joints and variations in actual pipe inner diameter. It is not possible in practice to perfectly balance any tool. If the sliding seal  64  diameter is larger than the PRSD then the device  118  will want to stroke to open the radial port  28  when pressure is applied down the throughbore  40 , referred to as Design A in this analogy. Conversely if the sliding seal  64  diameter is smaller than the PRSD then no amount of pressure will open the valve, Design B. It is Design B that the device  118  must operate with because: when the assembly  10  is axially fixed in the well, pulling and applying tension strokes the device  118  out and closes the radial port  28 ; in the underbalanced well, the pipe string  12  must be filled in order to pressure up to activate the hydraulic actuator  16 ; and as the pipe string  12  is filled a pressure differential is created between the pipe string  12  and annulus  30  due to the hydrostatic head. If Design A is used, an ever larger pull on the pipe string  12  would be needed to keep the radial port  28  closed. However, with Design B the radial port  28  will not open. Design B is therefore required and the device  118  will only open the radial port  28  when the overpull is released and weight is set down. However, if the imbalance is too large then there may not be enough weight to overcome the force holding the device  118  closed. This force is: 
       Hydrostatic pressure×(PRSD Area−sliding seal diameter area)
 
     Thus the device  118  can only be opened if there is sufficient weight to overcome the imbalance force. In practice, different tool sizes are produced for each of the possible running string diameters. However, this is expensive. The present invention overcomes this by incorporating the second feature which reduces the number of different devices  118  for reasons of inventory and cost. 
     The second feature is based on the device  118  being connected to pipe string  12  where the PRSD is sized close to but larger than the sliding seal  64  diameter. An adjustable spring  80  is located between the upper end  58  of the sleeve  22  and an opposing shoulder  82  towards the first end  42  of the tubular member  20 . The force of this spring  80  is significantly greater than that of the other springs  24 ,  76  and will therefore increase the force supplied when weight is set down. The adjustable spring  80  is adjusted to meet the requirements of the mismatch in the PRSD area and the sliding seal diameter area. Adjustment is by varying the distance between the end  58  and shoulder  82 , which is achieved by having shoulder  82  on an adjustment sleeve  84  which is screw threaded to the outer surface  50  of the tubular member  20 . A lock sleeve  86 , as is known in the art, is also used. 
     In use, the device  118  is run in on the pipe string  12  in the first configuration with the radial port  28  closed by virtue of the misalignment of the radial ports  26 , 28 . The sleeve  22  is fixed and tension applied to the device  118  by pulling of the pipe string  12  will keep the radial port  28  closed. When the device  118  requires to be switched to the second configuration to open the radial port  28 , fluid pumping is stopped, tension is slackened off and the weight of the pipe string  12  allowed to act on the device  118 . This will cause the tubular member  20  to move longitudinally downwards telescoping into the sleeve  22 . The bias of the adjustable spring  80  is first taken up and then the bias of the spring  76  follows. This provides the final movement which will move the piston sleeve  72  over the shoulder  69  and expose the radial port  26  in the tubular member to the radial port  28  in the sleeve  22 . The column of fluid in the pipe string  12  will pass to the annulus  30  through the now aligned radial ports  26 , 28 . The application of tension by pulling on the string  12  can be used to switch the device  118  back to the first configuration as it repositions all the components and closes the radial port  28  again. 
     The valve  14 , is shown as a ball seat sub in  FIG. 1 . In a preferred embodiment the valve  14  is resettable so that fluid can be drained through the pipe string  12  and downhole assembly  10  when the downhole assembly  10  is to be pulled from the well  13 . A suitable valve  114  is illustrated in  FIGS. 4( a ) and 4( b ) . This is an ALO valve which is available from Ardyne AS, Norway, and operates by opening and closing the pipe string by the application of tension on the pipe string as described in EP3063364 and incorporated herein by reference. 
     The valve  114  operates in two positions. In the initial position, as shown in  FIG. 4( a ) , the pre-tensioned main spring  256  pushes the slider  248  and thereby the grooved shaft  258  with the actuating sleeve  274  in the direction of the opening and closing mechanism  278 . The telescope pipe  244  is pulled into the spring housing  236 , and the end face  294  of the actuating sleeve  274  pushes the valve body  280  towards the valve spring  286  and away from the valve seat  284 . The shoulder  268  of the grooved shaft  258  comes into abutment against the end piece  230  as an end stop. In this initial position there is a through-going fluid channel from the end piece  230  via the chamber  302 , the opening  300  in the valve sleeve  282 , the openings  304  in the actuating sleeve  274 , the bore  260  of the grooved shaft  258 , the bore  250  of the slider  248 , the passage  252  of the telescope pipe  244  and a bore  310  in the coupling piece  308 . 
     In the activated state, sufficient tensile force has been applied between the end piece  230  and the coupling piece  308  to overcome the force of the pre-tensioned main spring  256  and thereby pull the telescope pipe  2244  and the slider  48  in the direction against the spring  256 . The grooved shaft  258  and the actuating sleeve  274  follows the movement of the slider  248  and the valve spring  286  moves the valve body  280  towards the valve seat  284 . The opening and closing mechanism  278  closes as the valve body  280  lands on the valve seat  284 , and fluid cannot flow in at the end piece  230  and out at the coupling piece  308 . 
     Reference is now made to  FIGS. 5( a )-( d )  which illustrate a method of casing recovery using a downhole assembly  110 . Those parts referred to in  FIGS. 1 to 4  have been given the same reference numeral. The assembly  110  now includes a casing spear  88  and a casing cutter  90 . The assembly  110  is mounted on the pipe string  12  and the pipe string  12  is a drill string typically run from a rig (not shown) via a top drive/elevator system which can raise and lower the string  12  in the well  13 . The casing cutter  90  and casing spear  88  are run into a first casing  92  in the well  13 . The well  13  has a second casing  54  in which the first casing  92  is located. In an embodiment, casing  92  is 9⅝″ (244 mm) in diameter while the outer casing  54  is 13⅜″ (340 mm) diameter. 
     In a preferred embodiment the casing spear  88  comprises: a sliding assembly mounted on an inner mandrel; grippers  94  for gripping onto an inner wall  96  of the length of casing  92 , the grippers  94  being coupled to the sliding assembly; the sliding assembly is operable for moving the grippers  94  between a first position in which the grippers  94  are arranged to grip onto the inner wall  96  of a length of casing  92  in at least one gripping region of the length of casing  92  and a second position in which the grippers  94  is held away from the inner wall  96 ; and a switcher which, when advanced into the length of casing  92 , locks the sliding assembly to the inner mandrel with the grippers  94  in the second position; and, when the casing spear  88  is pulled upward out of the length of casing  92  and the switcher exits the end of the length of casing  92 , automatically allows engagement of the length of casing  92  by the grippers  94  in the first position. In this way, the length of casing  92  is automatically gripped into engagement with the casing spear  88  when the casing spear  88  is at the top  98  of the length of casing  92 . In a preferred embodiment the casing spear  88  is the Typhoon® Spear supplied by Ardyne AS. 
     Casing cutter  90  may be any tool which is capable of cutting casing downhole in a well bore. A pipe cutter, section mill, jet cutter, laser cutter and chemical cutter are a non-exhaustive list of possible casing cutters. 
     As shown in  FIG. 5( a )  the downhole assembly  110  is run in the well and the casing cutter  90  has been used to cut the casing  92  to separate it from the remaining casing string. The cut casing may be over 100 m in length. It may also be over 200 m or up to 300 m. Behind the casing  92  there may be drilling fluid sediments, partial cement, sand or other settled solids in the annulus between the outside of the casing  92  and the casing  54 . This material  102  can prevent the casing  92  from being free to be pulled from the well  13 . On run-in the downhole assembly  110  is in the first configuration with the radial port  28  closed on the hydrostatic equalisation device  118  and the valve  114  is open so that all flow is along the throughbore  40 . With the casing  92  cut, the pipe string  12  is raised so that the casing spear  88  grips the upper end  98  of the casing  92 . 
     To operate the hydraulic jack  100 , the string  12  is pulled and tension applied to the valve  114 . As the valve  114  is axially fixed by attachment to the casing  92  via the casing spear  88 , the valve  114  closes, sealing the throughbore  40  and blocking the passage of fluid through the pipe string  12  at the valve  114 . By continuing to pump fluid from surface, the fluid pressure will increase above the valve  114  and consequently fluid entering the ports  144  on the hydraulic actuator  16  will have an increased pressure. Initially fluid will enter the chamber  156  of the anchor  128  and set the slips  152  against the inner wall  104  of the outer casing  54 . With the hydraulic jack  100  held in place, fluid at the increased pressure will enter the actuator  116 , through ports  144  and move the pistons  138  thereby raising the inner mandrel  130  relative to the upper pipe string  12 . As the inner mandrel  130  forms a lower pipe string  12  and is connected to the casing spear  88 , the cut section of casing  92  is raised. As tension is maintained on the valve  114  it remains closed. Tension has also been maintained on the hydrostatic equalisation device  118  and the radial ports  28  remain closed so that a maximum pressure can be applied to operate the actuator  116 . This is as illustrated in  FIG. 5( b ) . 
     It is hoped that the jack  100  can make a full stroke to give maximum lift to the casing  92 . This is illustrated in  FIG. 5( b ) . If the casing  92  is still stuck only a partial stroke will be achieved. In either case, the anchor  128  now needs to be unset. Fluid pumping is stopped and the pipe string  12  is slackened off so that weight is set down on the tubular member  20 . The radial ports  26 , 28  align and the column of fluid in the throughbore  40  at the device  118  drains from the pipe string  12  until equilibrium is reached with the annular volume  30 . The anchor  128  is unset by the setting down of weight and the hydraulic jack  100  can be repositioned by pulling on the pipe string  12  to extend the mandrel  130  from the outer housing  132  of the jack  100 . This occurs as fluid can enter the second side  21  of the piston  15  in the actuator  116  as the pressure across the pistons is balanced.  FIG. 5( c )  shows the hydraulic jack  100  in a raised position with the mandrel  130  extended. 
     If the section of casing  92  is free, the pipe string  12 , downhole assembly  110  and recovered casing  92  can be raised out of the well  13  as illustrated in  FIG. 5( d ) . On raising the string  12 , if a drop ball seat is used as the valve  14 , then the throughbore  40  has remained blocked and a column of fluid in the pipe string  12  remains. This results in a need for handling a wet string at surface. If the ALO valve  114  is used, when pulling the string  12 , the valve  114  will open and the remaining column of fluid can drain down the throughbore  40  and out of the end of the pipe string  12 . This advantageously removes the requirement to handle a wet string at surface. 
     If the section of casing  92  is not free, the pipe string  12  will stop when the inner mandrel  130  is fully extended, at  FIG. 5( c ) . To use the hydraulic jack  100  again, the procedure is repeated to set down weight, close valve  114 , set anchor  128 , operate actuator  116  to activate jack  100  and pull on casing  92 , open port  28  on hydrostatic equalisation device  118 , drain string  12  to reach pressure equilibrium and release and re-cock the hydraulic jack  100 . The steps can be repeated until the cut section of casing  92  is free and the downhole assembly  110  and casing  92  can be pulled from the well  13 . 
     The downhole assembly  110  may further include a hydraulic disconnect, which releases under load, between the jack  100  and valve  114 . This allows for a contingency release of the string  12  from the casing  92  due to well constraints on torque. 
     The principle advantage of the present invention is that it provides a downhole assembly and method of operating a hydraulic actuator on a pipe string which allows repeated operation of the hydraulic actuator in a low level well. 
     A still further advantage of the present invention is that it provides a downhole assembly and method for casing recovery in a low level well which allows repeated operation of a hydraulic jack in the well. 
     The foregoing description of the invention has been presented for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention herein intended with the invention being defined within the scope of the claims.