WELL ABANDONMENT AND SLOT RECOVERY

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 Ardyne AS, 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 US 8,365,826, the disclosure of which is incorporated herein in its entirety by reference.

W02018203064 describes a method and apparatus for casing recovery for well abandonment and slot recovery. A string is run-in, the string including a hydraulic jack, an anchor, a casing spear, a downhole flow pulsing device and a pressure drop sub. The casing spear grips an upper end of the length of casing to be pulled. The anchor is set in casing of a greater diameter above the length of cut casing. Fluid pumped through the string and through the pressure drop sub will linearly increase fluid pressure at the hydraulic jack to a first fluid pressure. Fluid pumped through the downhole flow pulsing device will vary the fluid flow superimposing a cyclic pressure on the first pressure. Fluid at the first pressure superimposed with the cyclic pressure enters the hydraulic jack and causes oscillation of an inner mandrel of the hydraulic jack. The jack moves the oscillating inner mandrel upwards relative to the anchor to pull the length of casing. W02018203064, incorporated herein in its entirety by reference, describes use of the DHPT of US 8,365,826 in combination with the Agitator™ downhole flow pulsing device available from National Oilwell Varco as described in US 6,279,670, and US 7,077,205, the disclosures of which are incorporated herein in their entirety by reference.

An object of the present invention is to provide apparatus for casing recovery with controlled oscillation of the hydraulic jack. It is a further object of the present invention is to provide a method for casing recovery in which oscillation of the hydraulic jack is controlled. According to a first aspect of the present invention there is provided apparatus for the recovery of a length of casing from a well, comprising a string for running into the well, the string being arranged to carry a fluid in a throughbore thereof and including :

a hydraulic jack, the hydraulic jack comprising an anchor for axially fixing the apparatus to a tubular in the well, and a mandrel extending from a lower end of the hydraulic jack and axially moveable relative to the anchor in response to a pressure difference in the fluid across a piston between a first pressure in the throughbore and a second pressure in an annulus surrounding the apparatus in the well;

a casing spear connected to the mandrel for engaging the length of casing;

a downhole flow pulsing device for varying fluid flow in the throughbore at an induced frequency and thereby superimpose a cyclic pressure on the first pressure and the second pressure at a wavelength;

at least one pressure drop sub to increase pressure of the fluid in the throughbore at the hydraulic jack to the first pressure;

a length of the apparatus between the piston and a lower end of the apparatus is configured to provide: an inner pathlength for fluid in the throughbore between the downhole pulsing device and the piston at the first pressure; an outer pathlength for fluid between the downhole pulsing device and the piston at the second pressure via the throughbore, the lower end of the apparatus and the annulus; and a path difference being the difference between the inner pathlength and the outer pathlength; wherein interference of the cyclic pressure superimposed on the first pressure and the cyclic pressure superimposed on the second pressure across the piston determines oscillation on the mandrel as it moves axially and pulls the length of casing. In this way, the path difference as a percentage of the wavelength will determine the degree of interference and hence the oscillation of the mandrel in the hydraulic jack can be controlled by selection of the length of the bottom hole assembly of the apparatus to give a matched or unmatched pressure differential across the piston in the hydraulic jack via the inside and outside pressures, thereby giving either a balanced or else reinforcing effect as desired by the particular operation.

Preferably, the inner pathlength and the outer pathlength are an integral number of half wavelengths. In this way the path difference can be selected to give a matched or unmatched pressure differential across the piston in the hydraulic jack via the inside and outside pressures, thereby giving either a balanced or else reinforcing effect as desired by the particular operation.

Preferably the path difference between the inner pathlength and the outer pathlength is an even number of half wavelengths. In this way, the fluid pressure differential across the piston is increased as the first and second pressure cycles arrive in-phase and this reinforces the oscillation on the mandrel. In this way, longer lengths of casing can be removed by creating a higher vibratory pull which will dislodge the 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.

Alternatively, the path difference between the inner pathlength and the outer pathlength is an odd number of half wavelengths. In this way, the fluid pressure differential across the piston is decreased as the first and second pressure cycles arrive out of phase and this reduces the amplitude of the oscillation on the mandrel. Such a reduction may be required if the oscillation on the piston and mandrel is considered to be detrimental to the long term performance of the hydraulic jack. Another way to consider the effect is at the downhole pulsing device. When the flow by area at plates or other restriction in the pulsing device is smallest, the pressure is highest above the plates and lowest below the plates to create the cyclic pressure. So a path difference of an even number of half wavelengths preserves this difference, with an even difference gives maximum pulsing at the jack and an odd difference cancels out at the jack. The path difference between the first pathlength and the second pathlength, is the distance from the plates/restriction to the lower end of the apparatus or string multiplied by two.

Preferably, the wavelength is of the order of 100m. So a string length below the plates of 25m gives a path difference of 50m being an odd number of half wavelengths and thus cancelling occurs. A string length of 50m gives a reinforcing, but 50m is rather impractical, so a better solution is (nearly) zero metres. While the effects are most noticeably seen at an integral number of half wavelengths, even if they are slightly out of phase (say O.lx of a half wavelength) the internal and external (first and second) pressures are still giving the desired benefit (they are still 95% in phase).

Preferably, the apparatus includes one or more lengths of pipe below the hydraulic jack to provide the length of apparatus. More preferably, the lengths of pipe are casing collars.

Preferably, the cyclic pressure amplitude is up to 4% of the first pressure, the first and second pressures being static pressures. More preferably, the cyclic pressure amplitude is up to 25% of the first pressure. An increased vibration on the mandrel may further assist in freeing the casing if it at first appears stuck.

Preferably, the hydraulic jack includes a housing supported in the well by the string and enclosing a plurality of axially stacked pistons generating a cumulative axial force, each of the plurality of pistons axially movable in response to the fluid at the first pressure; 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 string and enclosing a plurality of axially stacked pistons generating a cumulative axial force, each of the plurality of pistons axially movable in response to the fluid at the first pressure; 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. 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, the downhole flow pulsing device comprises a housing located in the string, a valve located in the throughbore defining a flow passage and including a valve member, the valve member being movable to vary the area of the flow passage to, in use, provide a varying fluid flow therethrough; and a fluid actuated positive displacement motor operatively associated with the valve for driving the valve member. In this way, the cyclic pressure variations on the fluid are as the fluid flows through the downhole flow pulsing device. The speed of fluid flow through the device determines the wavelength. Preferably the downhole flow pulsing device is the Agitator™ supplied by National Oilwell Varco.

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.

Preferably, the pressure drop sub comprises a housing located in the string and one or more apertures through a wall of the housing to provide at least one fluid flow path from the throughbore to an outer surface of the housing. Preferably the apertures are nozzles. In this way, the cross- sectional area of the nozzles is significantly less than the cross-sectional area of the throughbore so that a build-up of fluid pressure occurs when fluid is pumped down the string. This is used to create the first pressure for operating the hydraulic jack. Preferably the casing spear is located between the hydraulic jack and the downhole flow pulse device. Preferably the downhole flow pulse device is located between the casing spear and a pressure drop sub. There may be a pressure drop sub located between the casing spear and the downhole flow pulse device. Alternatively, the downhole flow pulse device may be located between two pressure drop subs. In this way, the downhole flow pulse device and the pressure drop subs are located in the length of casing and the hydraulic jack is anchored to tubular, preferably casing, having a greater diameter than the length of casing being pulled. 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 jack includes a right-hand threaded coupling interconnected to the inner mandrel for selectively releasing an upper portion of the tool from a lower portion of the tool.

Preferably, in the downhole flow pulsing device the speed of the motor is directly proportional to the rate of flow of fluid through the motor. Preferably, the positive displacement drive motor includes a rotor and the rotor is linked to the valve member. Preferably, the rotor is utilised to rotate the valve member. Preferably, the rotor is linked to the valve member via a universal joint which accommodates transverse movement of the rotor. Alternatively, the rotor may be linked to the valve member to communicate transverse movement of the rotor to the valve member. Preferably, the valve member cooperates with a second valve member, each valve member defining a flow port, the alignment of the flow ports varying with the transverse movement of the first valve member. Preferably, the positive displacement motor operates using the Moineau principle and includes a lobed rotor which rotates within a lobed stator, the stator having one more lobe than the rotor. Preferably, the motor is a 1 : 2 Moineau motor.

According to a second aspect of the present invention there is provided a method for the recovery of a length of casing from a well, comprising the steps: (a) providing apparatus according to the first aspect wherein a length of the apparatus is selected to provide a desired oscillation of the mandrel;

(b) locating the casing spear in an end of the length of casing and gripping the length of casing;

(c) setting an anchor of the hydraulic jack on tubing at a shallower depth in the well than the length of casing;

(d) flowing fluid through the string and through the pressure drop sub to thereby increase fluid pressure in the throughbore at the hydraulic jack to a first fluid pressure;

(e) varying fluid flow via the downhole flow pulsing device to superimpose a cyclic pressure on the first pressure and the second pressure;

(f) causing interference of the cyclic pressure superimposed on the first pressure having travelled via the inner pathlength and the cyclic pressure superimposed on the second pressure having travelled via the outer pathlength at the piston of the hydraulic jack to create a pressure differential across the piston; and

(g) oscillating the mandrel in response to variation in the pressure differential;

(h) axially moving the oscillating mandrel relative to the anchor to pull the length of casing.

In this way, controlling the path lengths of pressure signals generated at the downhole flow pulsing device provides controlled oscillation of the mandrel as the amplitude of vibration can be selected. Oscillations of the mandrel are transmitted to the length of casing via the casing spear which helps dislodge the 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. A longer length of casing is thus more easily removed from the well with a lower risk of being stuck. Preferably, the method includes the step of selecting the path difference to be an integral number of half wavelengths.

Preferably, the method includes the step of locating one or more lengths of pipe below the hydraulic jack to provide the length of apparatus. More preferably, the lengths of pipe are casing collars.

Preferably, the method includes selecting the length of the apparatus so that the path difference between the inner pathlength and the outer pathlength is an even number of half wavelengths. In this way, the fluid pressure differential across the piston is increased as the first and second pressure cycles arrive in phase and this reinforces the oscillation on the mandrel. In this way, longer lengths of casing can be removed by creating a higher vibratory pull which will dislodge the 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.

Alternatively, the method includes selecting the length of the apparatus so that the path difference between the inner pathlength and the outer pathlength is an odd number of half wavelengths. In this way, the fluid pressure differential across the piston is decreased as the first and second pressure cycles arrive out of phase and this decreases the amplitude of the oscillation on the mandrel. Such a reduction may be required if the oscillation on the piston and mandrel is considered to be detrimental to the long term performance of the hydraulic jack.

Preferably, the cyclic pressure amplitude is up to 4% of the first pressure. More preferably, the cyclic pressure amplitude is up to 25% of the first pressure. An increased vibration on the mandrel may further assist in freeing the casing if it at first appears stuck. Preferably, an axial force generated by a plurality of pistons in the hydraulic jack acts simultaneously on the anchor and on the inner mandrel, such that the apparatus anchoring force increases when the axial force on the inner mandrel increases.

Preferably, the anchor is set in response to axial movement of the plurality of pistons.

Preferably, step (e) includes driving a valve member in the downhole pulsing device and varying the cross-sectional area of the throughbore.

Preferably the method includes the final step of pulling the string via a top drive or elevator to surface. The method may include the further steps, before the final step, of:

(i) stroking the hydraulic jack to pull the length of casing;

(j) releasing the anchor;

(k) pulling the string so as to raise an outer housing of the hydraulic jack and the anchor;

(I) resetting the anchor and repeating steps (d) to (i).

Steps (i) to (I) can be repeated until the final step is achievable. In this way, the apparatus and method of the present invention have assisted casing recovery via a top drive/elevator.

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.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which:

Figure 1 illustrates apparatus for recovery of a length of casing in a well, according to an embodiment of the present invention; Figures 2(a) to 1(e) illustrate a method for recovery of a length of casing in a well, according to an embodiment of the present invention;

Figure 3(a) is a part sectional view of an actuator section of a hydraulic jack and Figure 3(b) is a part sectional view of an anchor of the hydraulic jack, according to an embodiment of the present invention; Figure 4(a) is a sectional view through a downhole flow pulsing device and Figure 4(b) is the lower portion in an expanded view, according to an embodiment of the present invention; and Figure 5 is a graph illustrating applied load against time for the linearly applied first pressure, the cyclic pressure and the first pressure superimposed with the cyclic pressure.

Figures 6(a) and 6(b) are graphs of the interference of the cyclic pressure superimposed on the first pressure and the cyclic pressure superimposed on the second pressure across the piston when they are (a) in-phase and (b) out of phase, according to embodiments of the present invention.

Reference is initially made to Figures 1 and 2(a) of the drawings which illustrate apparatus and a method of recovering casing from a well, according to an embodiment of the present invention. In Figures 1 and 2(a) there is shown a cased well bore, generally indicated by reference numeral 10, in which a length of casing 12 requires to be recovered. A tool string 16 including apparatus 11 is run in the well 10. Apparatus 11 includes a hydraulic jack 18, a casing spear 20, a downhole flow pulsing device 22, and a pressure drop sub 24.

The casing spear 20, downhole flow pulsing device 22, and pressure drop sub 24 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 or more parts may also be integrally formed and joined to any other part.

The tool string 16 is a drill string typically run from a rig (not shown) via a top drive/elevator system which can raise and lower the string 16 in the well 10. The well 10 has a second casing 14. Casing 14 has a greater diameter than casing 12. In an embodiment, length of casing 12 is 9 5/8" diameter while the outer casing is 13 3/8" diameter.

Casing 12 will have been cut to separate it from the remaining casing string. The cut casing may be over 100m in length. It may also be over 200m or up to 300m. Behind the casing 12 there may be drilling fluid sediments, partial cement, sand or other settled solids in the annulus between the outside of the casing 12 and the inside of a surrounding downhole body, in this case casing 14 but it may be the formation of the well 10. This material 26 can prevent the casing 12 from being free to be pulled from the well 10. It is assumed that this is the position for use of the present invention.

The hydraulic jack 18 has an anchor 28 and an actuator system which pulls an inner mandrel 30 up into a housing 32 of the jack 18. In a preferred embodiment the hydraulic jack is the DHPT available from Ardyne AS. It is described in US 8,365,826 the disclosure of which is incorporated herein in its entirety by reference. Referring to Figures 3(a) and 3(b) there is illustrated the main features of the hydraulic jack 18. Figure 3(a) shows a portion of the actuator system. The jack 18 has an outer housing 32 with a connection 34 to the tool string 16. There is an inner mandrel 30 which can move axially within the housing 32. A series of spaced apart outer stops 36 are connected into the housing 32. A series of spaced apart pistons 38 are connected to the inner mandrel 30. The pistons and stops 36,38 are stacked between each other so that an upper end face 40 of an piston 38 will abut a lower end face 42 of an outer stop 36. Only one set of piston/stop 36,38 are shown but this arrangement is repeated along the mandrel 30 to provide five sets of piston/stop 36,38 arrangements. The inner mandrel 30 includes a throughbore 43, with a number of ports 44 arranged circumferentially around the mandrel 30, at the upper end of each outer stop 36, when the piston 38 rests on the outer stop 36. A chamber 46 is provided at this location so that fluid can enter the ports 44 from the throughbore 43 and will act on the lower end face 48 of the piston 38. This will move the piston 38 upwards, crossing a vented space 50, until the upper end face 40 of the piston 38 abuts the lower end face 42 of the outer stop 36.

Space 50 is vented by virtue of a number of ports 51 which give access to the annulus 53 between the apparatus 11 and an inner surface 60 of the casing 14. This movement, caused by the pressure differential between a first pressure entering chamber 46 through ports 44 from the throughbore 43 and a second pressure in space 50 via the ports 51 from the annulus 53 outside the tool, to move the piston 38 across the vented space 50, constitutes a stroke of the jack 18.

Movement of the inner mandrel 30 is driven by movement of the inner pistons 38. As there are multiple stacked pistons 38, the combined cross- sectional areas of the end faces 40 when fluid pressure is applied generates a considerable lifting force via the inner mandrel 30.

Hydraulic jack 18 also includes an anchor 28, shown in Figure 3(b). Anchor 28 has a number of slips 52 arranged to ride up a cone 54 by the action of fluid entering a chamber 56 and moving the cone 54 under the slips 52. The outer surface 58 of the slips 52 is toothed to grip an inner surface 60 of the casing 14. The anchor 28 is connected to the outer housing 32 so that the inner mandrel 30 can move axially relative to the anchor 28 when the anchor is set to grip the casing 14.

Casing spear 20 operates by a similar principle to grip the inner surface 62 of the length of casing 12. The casing spear anchors as a slip designed to ride up a wedge and by virtue of wickers or teeth on its outer surface grip and anchor to the inner surface 62 of the casing 12. The casing spear 20 includes a switch which allows the casing spear to be inserted into the casing 12 and hold the slips in a disengaged position until such time as the grip is required. At this time, the casing spear 20 is withdrawn from the end 64 of the casing 12 and, as the switch exits the casing 12, it automatically operates the slips which are still within the casing 12 at the upper end 64 thereof. This provides the ideal setting position of the spear 20. In a preferred embodiment the casing spear 20 is the TYPHOON®

Spear as provided by the Ardyne AS. The TYPHOON® Spear is described in PCT/EP2017/059345, the disclosure of which is incorporated herein in its entirety by reference. The downhole flow pulsing device 22 is a circulation sub which creates fluid pulses in the flow passing through the device. This can be achieved by a rotating member or a rotating valve. In a preferred embodiment the downhole flow pulsing device 22 is the Agitator™ System available from National Oilwell Varco. It is described in US6279670, the disclosure of which is incorporated herein in its entirety by reference. For completeness we provide Figures 4(a) and 4(b) from the patent together with the accompanying description. Only reference numerals have been changed to distinguish from features in earlier figures. Reference is now made to Figures 4(a) and 4(b) of the drawings. The sub comprises a top section 110 connected by a threaded joint 111 to a tubular main body 112. A flow insert 113 is keyed into the main body 112 and flow nozzles 114 are screwed into the flow insert 113. The keyed flow insert 113 is attached to a motor stator 115 which contains a freely revolving rotor 116. The motor is of the positive displacement type, operating using the Moineau principle. The top section 110, keyed flow insert 113, flow nozzles 114, motor stator 115 and the main body 112 all allow drilling fluid to pass through the sub; in use, high velocity drilling fluid enters the top section 110. The flow is then channelled through the flow insert 113 and the flow nozzles 114. A balanced flow rate is achieved between the flow insert 113 and the flow nozzles 114 allowing the drilling fluid to rotate the rotor 116 at a defined speed in relation to the drilling fluid flow rate.

The lower end of the motor stator 115 is supported within a tubular insert 119 which has a threaded connection at its lower end 121 and has fluid passageways 120 to allow fluid to flow from the flow nozzles 114 over the motor stator 115 and into a chamber 122 defined by the insert 119.

The rotor 116 is connected at its lower end to a shaft 123 which in turn is connected to a tubular centre shaft 124. The shaft 124 extends into an intermediate outer body 117 connected to the main body 112 by way of a threaded connection. The connecting shaft 123 is located at either end by a universal joint 125 and 126. The rotor torque is thus directly translated through the connecting shaft 123 and universal joints 125 and 126 to the centre shaft 124.

A first valve plate 127 is attached to the lower end of the centre shaft 124 via a threaded connection 128. The valve plate 127 defines a slot opening 129 which provides a fluid passageway for drilling fluid to flow onto the fixed second valve plate 130 which also defines a slot 131; the slots 129, 131 thus define an open axial flow passage. The fixed valve plate 130 is attached to an end body 144 by way of threaded connection 146.

Drilling fluid is channelled through radial slots 132 in the upper end of the centre shaft 124 into the centre of the shaft 124 whilst the shaft rotates. Fluid then travels through the first slot 129 and as the two slots 129 and 131 rotate into and out of alignment with each other fluid flow is restricted periodically, causing a series of pressure pulses. The pressure drop sub 24 has a housing located in the string and apertures through a wall of the housing to provide multiple narrow fluid flow paths from the throughbore 43 to an outer surface of the housing. Nozzles are located in the apertures. The cross-sectional area of the nozzles is significantly less than the cross-sectional area of the throughbore 43 so that a build-up of fluid pressure occurs when fluid is pumped down the string. This is used to create the first pressure for operating the hydraulic jack. In Figure 1, the pressure drop sub 24 is located below the downhole flow pulsing device 22. Alternatively, the pressure drop sub can be located between the casing spear 20 and the downhole flow pulsing device 22. Such an arrangement reduces the pressure through the downhole flow pulsing device 22, which itself will also cause a pressure drop. There could be a pressure drop sub on either side of the downhole flow pulsing device 22 to provide both a suitable pressure to operate the hydraulic jack i.e. the first pressure and a suitable pressure for operating the downhole flow pulsing device 22. Referring again to Figure 2(a), the string 16 is run into the well 10 with the pressure drop sub 24, downhole flow pulsing device 22 and casing spear 20 being run-in the casing 12. The string 16 is raised to a position to operate the switch on the casing spear 20 and the slips 66 automatically engage the inner surface 62 of the casing 12 at the upper end 64 thereof. At this stage the string 16 can be pulled via the top drive/elevator to see if the casing 12 is stuck.

Referring now to Figure 2(b), slips 52 on the anchor 28 of the hydraulic jack 18 are operated to engage the inner surface 60 of the outer casing 14. As with the casing spear 20, an overpull on the string 16 will force the teeth on the slips into the surface 60 to provide anchoring.

With fluid flowing down the throughbore 43 of the string 16, the pressure of the fluid will build up by virtue of the restrictions at the nozzles of the pressure drop sub 24. This fluid pressure will linearly increase to a static first pressure/load 78. This linear increase is shown as a straight line in graph 70 but it may be a curve as long as it is smooth and increasing. This change in fluid pressure can be seen as line 72 in the graph 70 of applied load 74 against time 76 shown in Figure 4. At the same time, the fluid flow through the downhole flow pulsing device 22 will create pressure pulses seen as a cyclic variation of pressure and consequently applied load. For the downhole flow pulsing device 22 taken in isolation, the cyclic variation is illustrated by line 82. This provides an oscillation at a frequency of less than lOFIz. In preferred embodiments the frequency will be less than 5Flz, 2Flz or lhlz and even operate at 0.5Flz. This low frequency is selected so as to effectively influence the vibration on the inner mandrel 30. The cyclic variation induced by the downhole flow pulsing device 22 will be superimposed on the fluid pressure in the throughbore 43. The resulting fluid pressure and equivalent applied load is illustrated as line 80 on graph 70. The amplitude of the cyclic variations can be selected to determine the axial extent of the oscillatory movement on the inner mandrel 30. In contrast to the known arrangements of causing a percussive effect by using a shock sub in which the subs entire movement is oscillatory, the oscillatory motion of the inner mandrel 30 is only a small percentage so that the pulling force of the jack 18 is not affected. The amplitude of the cyclic pressure variation is selected to be up to 4% of the value of the first pressure. In an embodiment, the amplitude of the cyclic pressure variation can be up to 25% of the value of the first pressure.

Fluid at this first pressure superimposed with the cyclic pressure, at the frequency and wavelength determined from the downhole pulsing device 22, will enter the ports 44 on the jack 18 as a first pressure signal. In a preferred embodiment the wavelength is in the order of 100m. This fluid at a first pressure superimposed with the cyclic pressure will have travelled from the downhole pulsing device 22 to the piston 38 directly up the throughbore 43 and in through the ports 44. This distance can be considered as an inner or first pathlength 17, illustrated in Figure 1. Flowever it is also recognised that the cyclic pressure pulses will also travel down the throughbore 43 out of the lower end 25 of the apparatus 11 and up the annulus 53. This will enter the jack 18 via the ports 51 at a second pressure superimposed with the cyclic pressure to reach the piston 38, considered as a second or outer pressure signal. This is an outer second longer pathlength 19, also illustrated in Figure 1. The first and second pressures will interfere at the piston 38, and the static pressure to move the piston 38 is the difference between the first pressure and the second pressure, noting that the second pressure is significantly lower due to the presence of the pressure drop sub 24 and the longer pathlength travelled over a greater cross-sectional area. However, the oscillation on the inner mandrel 30 will be determined by the interference of the cyclic pressures. In order to control this, the length 27 of the apparatus 11 between the piston 38 and the lower end 25 is calculated and made-up to ensure that both the first pathlength 17 and the second pathlength 19 have an integral number of half wavelengths. This is determined from the frequency of the downhole pulsing device 22. The length 27 of the apparatus 11 is adjusted by locating further pipe sections 29 in the apparatus 11. These can be located above or below the downhole pulsing device 22 to achieve an integral number of half wavelengths for a preferred embodiment.

It will be recognised that the path difference is also given as the distance from the downhole pulsing device 22, and more particularly at the valve plates which provide a restriction when rotated, to the end 25 of the string multiplied by two.

It must also be determined as to the desired degree of interference of the first and second pressure with superimposed cyclic pressure desired. For the preferred embodiment, the path difference between the first pathlength 17 and the second pathlength 19 is an integral number of half wavelengths. Referring now to Figures 6(a) and 6(b) it is seen that the first and second pressures meeting at the piston 38, can be in-phase, as illustrated in Figure 6(a) which causes the amplitudes to increase and consequently a significant increase in the oscillation of the inner mandrel 30 occurs. Alternatively, they can be out-of-phase, as illustrated in Figure 6(b) which reduces the degree of oscillation on the inner mandrel 30. Length 27 is selected to provide an even number of half wavelengths between the first pathlength 17 and the second pathlength 19 to achieve the increased oscillations of Figure 6(a). Length 27 is selected to provide an odd number of half wavelengths between the first pathlength 17 and the second pathlength 19 to achieve the cancelling action and low to negligible oscillations of Figure 6(b).

It will be realised that the degree of oscillation can be varied by varying the path difference. While an integral number of half wavelengths is preferred, a substantially a even (or odd) number of half wavelengths can be used. Because even if they are slightly out of phase (say O.lx of a half wavelength) the internal and external pressures are still giving the desired benefit (they are still 95% in phase).

The first fluid pressure entering the ports 44 will be sufficient to move all the inner pistons 38 so forcing the inner mandrel 30 upwards into the housing 32. As the inner mandrel 30 is connected to the casing spear 20 which is in turn anchored to the length of casing 12, the force on the length of casing will match the applied load of the first pressure 78. This force should be sufficient to release the casing 12 and allow it to move. The cyclic pressure will act on the pistons 38 and through the inner mandrel 30. The inner mandrel will therefore vibrate or axially oscillate at the frequency created by the downhole flow pulsing device 22. The amplitude of the oscillations on the inner mandrel will be determined by the length 27 of the apparatus 11 selected to provide either constructive or destructive interference of the cyclic pressure on the first and the second pressure across the pistons 38. The inner mandrel 30 is directly connected to the spear 20 and the casing 12. Such vibration has been shown to assist in releasing stuck casing and thus this action can assist during the pulling of the casing 12 by the jack 18. The degree of interference of the superimposed cyclic pressure on each of the pressures is calculated depending on the type of operation required. While an increased amplitude of oscillation, Figure 6(a), will move the mandrel 30 over greater distances and should more greatly assist in dislodging the casing 12, the large degrees of movement can cause problems both in the mechanism of the jack 18, depending on the sizes of the space 51 and chamber 46, and on the operation of the spear 20, in the event that the casing 12 is stuck. Accordingly, for some operations the length 27 will need to be configured to ensure there is an odd number of half wavelengths between the first pathlength 17 and the second pathlength 19. This will provide a reduced oscillation as seen in Figure 6(b). The effect is can also be described by considering the cyclic pressure as it is generated at the valve plates in the Agitator. When the flow by area at the valve plates is smallest, the pressure is highest above the plates, but lowest below the plates. So a path difference of an even number of half wavelengths is required to preserve this pressure difference. Thus an even difference gives maximum pulsing at the jack while an odd difference from an odd in the number of half wavelengths cancels out at the jack.

In this example we can consider a wavelength of 100m in the superimposed cyclic pressure. The path difference is the distance from the valve plates to the end of the string, multiplied by two. So a string length below the plates of 25m gives a path difference of 50m being an odd number of half wavelength and so cancelling results. A string length of 50m gives reinforcing, but 50m is rather impractical, so a better solution is (nearly) zero metres, zero counting as an even number. With the mandrel 30 being moved upwards under the static pressure while being axially oscillated via the resultant cyclic pressure, it is hoped that the jack 18 can make a full stroke to give maximum lift to the casing 12. This is illustrated in Figure 2(c). If the casing 12 is still stuck only a partial stroke will be achieved. In either case, the anchor 28 is unset, by setting down weight, as shown in Figure 2(d).

Raising the string 16 will now lift the housing 32 with respect to the inner mandrel 30, repositioning the piston 38 to recreate the vented space 50. The jack is thus re-set in the operating position as illustrated in Figure 2(a). This is now shown in Figure 2(e) with the casing 12 now raised in the casing 14. As the string 16 is raised, the casing 12 may be free and then the entire apparatus 11 and the length of casing 12 can be recovered to surface and the job complete.

If the casing 12 remains stuck, the anchor 28 is re-engaged as illustrated in Figure 2(f) and the steps repeated as described and shown with reference to Figures 2(b) to 1(e). The steps can be repeated any number of times until the length of casing 12 is free and can be pulled to surface by raising the string 16 using the top drive/elevator on the rig.

The principle advantage of the present invention is that it provides apparatus for casing recovery with controlled oscillation of the hydraulic jack.

A further advantage of the present invention is that it provides a method for casing recovery in which oscillation of the hydraulic jack is controlled.

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. For example, the tool string may include other tools such as a cutting tool to cut the casing. Additionally, where reference has been made to shallower and deeper, together with upper and lower positions in the well bore, it will be recognised that these are relative terms and relate to a vertical well bore as illustrated but could apply to a deviated well.