WO2019239100A1

WO2019239100A1 - Improvements in or relating to well abandonment and slot recovery

Info

Publication number: WO2019239100A1
Application number: PCT/GB2019/051431
Authority: WO
Inventors: Steffen Hansen
Original assignee: Ardyne Holdings Limited
Priority date: 2018-06-14
Filing date: 2019-05-24
Publication date: 2019-12-19
Also published as: GB2574647A; GB201809740D0; GB2574647B

Abstract

A method and apparatus for determining a free point (114) of stuck casing (12) in a well bore. Apparatus includes a jack (18) being anchored to upper casing (14) and a casing spear (20) connected to a first portion, typically the mandrel (30), of the jack and to the upper end of the stuck casing. The jack (18) includes a measurement system (90) for determining the distance of travel of the mandrel relative to the fixed body of the jack when the jack applies tension to stretch the stuck casing. In an embodiment, shear rings (92, 94, 35) at fixed distances apart in the jack (18) are sheared as the mandrel moves over the fixed distances with each shear being detectable at surface. The tension applied to move the fixed distance can be used to calculate the free point of the stuck casing. Embodiments include cutting the casing above the free point and recovering the cut casing section on a single trip in the well bore.

Description

IMPROVEMENTS IN OR RELATING TO

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 a method for determining the free point of casing.

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.

It is useful to know the free point of the casing i.e. the position at which there is nothing preventing movement of the casing so that the casing can be cut at this location and successfully recovered. Currently, the free point is determined by running a casing spear on a drill string and anchoring the spear to the upper end of the casing. The drill string is then pulled at surface to apply a tensile force to the casing. This application of tension will increase the length of the drill string and casing between the stuck point and surface. With a knowledge of the diameters, weights and materials of the drill string and casing, the distance over which the drill string can be pulled at surface can be used to calculate the position of the stuck point which is also the free point of the casing. A disadvantage of this method is that the drill string will typically be much longer than the length of casing to the stuck point. When the tensile force is applied, the drill string will experience a greater stretch compared to the much heavier casing. Consequently, a large portion of the distance measured at surface will be attributable to the drill string with minimal to the casing, which limits the accuracy to which the stuck point on the casing can be determined.

US6851476, US7389183, US8284074, US9879523 all describe methods of determining the stuck point using freepoint tools. These tools measure the local elongation of a drill string when a tensile force is applied at surface on the drill string. The freepoint tool is moved to successively higher points in the string until a response is received at the tool which indicates elongation of the drill string. The stuck point is therefore determined to be between the current position and the last measurement position. These tools are only used on stuck drill pipe not on heavier casings, they have problems in attachment to the inside of the drill pipe if they require to be anchored to take a measurement, they must be moveable in the string for measurements to be made at multiple locations, the sensors within them are typical fragile and expensive and a sophisticated telemetry system is required to transfer the data measurements to surface.

It is an object of the present invention to provide a method of determining the free point of stuck casing which obviates or mitigates at least some of the disadvantages of the prior art.

It is an object of the present invention to provide apparatus for determining the free point of stuck casing 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 method of determining a free point of stuck casing in a well bore, the well bore including a first casing, the first casing located closer to surface than the stuck casing, comprising the steps:

(a) anchoring a jack to the first casing in a well bore;

(b) attaching a first portion of the jack to a casing spear;

(c) anchoring the casing spear in the stuck casing;

(d) activating the jack to pull the casing spear and the stuck casing to thereby apply a tensile force on the stuck casing;

(e) using the tensile force applied and a distance of travel of the first portion to determine the free point of the stuck casing.

By applying the tensile force effectively at the end of the drill string to which the jack will be attached, the tensile force is now applied across the stuck casing and the distance of travel can be attributed to the stuck casing and not the drill pipe. This gives a more accurate determination of the free point of the stuck casing.

Preferably, the first portion is arranged telescopically relative to a second portion of the jack. In a preferred embodiment the first portion is a mandrel and the second portion is a cylindrical housing. Alternatively, the first portion is the housing moveable over a mandrel being the second portion. Preferably, step (d) is achieved by pumping fluid down a drill string to the jack. In this way, the jack can be a hydraulically operated jack.

Preferably, step (d) includes sealing a central bore below the jack to allow a fluid pressure increase in the jack and thereby activate the jack. By closing the central bore, pressure from surface can be used to create the tensile force. Preferably, the method includes creating a signal detectable at surface when the first portion has moved over a fixed distance of travel. In this way, no sophisticated telemetry is required. Preferably, step (e) includes measuring the change in pressure over time for the first portion to move over the fixed distance. In this way, small lengths of stretching do not have to be measured, it is the change in pressure and hence the tensile force required to move the stuck casing which is effectively being measured. Pressure is a standard measurement made at the surface of the well so no sophisticated measurement apparatus is required.

Preferably, the change in pressure is recorded for more than one fixed distance of travel of the first portion. This further improves the accuracy by providing multiple measurements.

Preferably, the change in pressure is recorded for successive fixed distances of travel of the first portion, with the signal being detectable at surface when each fixed distance of travel is achieved. In this way, measurements are made over the full stretch of the stuck casing.

In a preferred embodiment, the method includes shearing a shear ring between the first portion and the second portion of the jack to create the signal, wherein at least two shear rings are spaced apart by the fixed distance along the first portion. In this way, a simple signal creation system is provided in the jack.

The method may include the additional steps of cutting the stuck casing at a position above the free point and pulling the cut section of casing. In this way, a casing cut and pull operation can be achieved on a single trip, with the maximum length of free casing being recovered. According to a second aspect of the present invention there is provided apparatus for determining a free point of stuck casing in a well bore, the well bore including a first casing, the first casing located closer to surface than the stuck casing, the apparatus comprising :

a jack having a first portion moveable longitudinally relative to a second portion with a central bore therethrough, gripping means for anchoring the jack to an inner wall of the first casing and attachment means for connecting the jack to an end of a drill string;

a casing spear having gripping means for anchoring the casing spear to an inner wall of the stuck casing and attachment means for connecting the casing spear to an end of the first portion;

a sealing arrangement for closing fluid flow through the central bore and thereby increasing pressure in the jack to activate the jack; and

a measurement system to determine a distance of travel of the first portion relative to the second portion.

In this way, a standard jack and casing spear arrangement can be modified to determine the free point of the stuck casing. Preferably the first portion is a mandrel and the second portion is a cylindrical housing, the mandrel being moveable longitudinally in the cylindrical housing. Alternatively, the first portion is the housing moveable over a mandrel being the second portion. Preferably, the measurement system includes a plurality of shear elements arranged between the first portion and the second portion, the shear elements being spaced a fixed distance apart. In this way, as the first portion reaches and shears each shear element a pressure will be detectable at surface. The shear elements may be shear rings. Alternatively the shear elements may be one or more shear pins arranged around a circumference of the first portion. Preferably the jack is as described in US8365826 and incorporated herein by reference. The jack and casing spear may be a hydraulically powered fishing tool for retrieving another tool or a tubular from the well. The jack may comprise a tool housing being the cylindrical body supported in the well bore by a work string, which in turn supports a plurality of axially stacked pistons each movable in response to pressurized fluid transmitted downhole on the work string. Preferably the gripping means is a slip assembly which provides an anchor for axially fixing the position of the tool housing in the well bore, and is movable to a set position in response to axial movement of the plurality of pistons. Preferably, the mandrel axially moves relative to the tool housing when the anchor is set, and the casing spear at a lower end of the mandrel engages the stuck casing, such that axial movement of the mandrel in response to the plurality of pistons stretches the stuck casing and may release it to be pulled . Preferably the jack is a downhole power tool, being the DHPT of Ardyne AS, Norway.

Alternatively, the jack is as described in US20160076327 and incorporated herein by reference. This jack is a casing pulling tool in which the mandrel is attached to the work string above and the outer housing is attached to the fishing tool/casing spear and moves over the mandrel.

The casing spear may be any as are known in the art. Preferably, the casing spear is as described in WO2017182549 and incorporated herein by reference. Preferably, the casing spear is the FRM Spear of Ardyne AS, Norway. Preferably the sealing arrangement is a valve. The valve may be as is known in the art. More preferably the valve is as described in WO2015065196 and incorporated herein by reference. Preferably, the valve is the ALO valve of Ardyne AS, Norway. Alternatively, the sealing arrangement may comprise a drop ball and a ball seat.

The apparatus may include a casing cutter. In this way, when the free point is determined, the stuck casing can be cut and the free portion removed.

The apparatus may be based on the TITAN® system of Ardyne AS, Norway with the incorporation of the measurement means.

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 is a schematic illustration of the apparatus and method for determining a free point of stuck casing in a well bore, according to an embodiment of the present invention;

Figure 2(a) is a part sectional view of an upper portion of a prior art jack, Figure 2(b) is a part sectional view of a portion of a power section of the jack, and Figure 2(c) is a part sectional view of gripping means of the jack.

Figure 3 is a sectional view through a portion of a jack illustrating a modification for making measurements, according to an embodiment of the present invention; Figure 4 is a graph illustrating pressure against time measured at surface, according to an embodiment of the present invention; and

Figures 5(a) and 5(b) illustrate further steps in the method, according to further embodiments of the present invention.

Reference is initially made to Figure 1 of the drawings which illustrates apparatus and a method of determining the free point of stuck casing in a well bore, according to an embodiment of the present invention. In Figure 1 there is shown a cased well bore, generally indicated by reference numeral 10, in which 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, and a valve 21. Valve 21 is shown in the mandrel 30 of the jack but could be anywhere below the jack 18. 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 first casing 14. First casing 14 has a greater diameter than casing 12. In an embodiment, casing 12 is 9 5/8" diameter while the outer casing is 13 3/8" diameter.

Casing 12 requires to be recovered. 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 first 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. Thus casing 12 can be considered as stuck casing 12.

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 a modification of 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.

The jack of US 8,365,826 is shown in Figures 2(a)-(c). Referring now to Figure 2(a), the jack 18 includes an upper connector 24 for interconnection with a lower end of the workstring 16 via threads 34, thereby allowing the jack to be positioned within a well bore 10 and within a casing 14 or other tubular in the well. Connector 24 is releasably connected to mandrel 30 by L-shaped shear ring 35, is threadably connected at 40 to outer piston 36, is threaded at 41 to outer sleeve or housing 32, and is sealed to the O.D. of mandrel 30 by seal 43. Shear ring 35 is held in position by retainer sleeve 33. Ports 44 allow for venting, as required. In this arrangement the first portion is the mandrel 30 and the second portion is the housing 32, but this may be reversed for jacks in which the mandrel is connected to the lower end of the workstring. Figure 2(b) shows a lower portion of the tool, and a continuation of the mandrel 30 having a throughbore 46 therein. Inner piston 48 is sealed to the outer sleeve 32 by seals 51, and is threaded to the mandrel 30 by threads 50. The lower seal 51 may be a chevron-type seal held in place by retainer ring 53. Another outer piston 54 is threaded to the outer sleeve 32 by threads 56, with seals 58 sealing with the O.D. of the extended mandrel 30a. Each of seal packages 51 and 58 are preferably different seals, e.g., one seal may be an o-ring seal and the adjacent seal may be a chevron-type seal held in place by a retainer. While only a single pair of pistons 48,54 is illustrated, their will be an arrangement of stacked pistons, typically five pairs within the jack 18.

Referring now to Figure 2(c), the outer sleeve or housing 32 is connected to a top of actuator 68, and connector 59 threaded to both mandrel 30a and to mandrel extension 30b. Torque blocks 64 fit within pockets in the actuator 68, and slide within groove 65 so that the blocks rotate actuator 68, and thereby rotate the mandrel 30b. The torque blocks thus transfer torque between the housing 32 and the mandrel 30b. Actuator 68 includes a plurality of tapered actuator blocks 70 for engagement with a slip having similarly configured surfaces, so that when the actuator 68 moves downward relative to the slips 72, the teeth 73 are forced into biting engagement with the casing 14 to serve as an anchor 28 for the jack 18. The slip setting mechanism includes a cage 74 with radially positioned slip pockets or apertures. The lower end of cage 74 is threaded to cage block 78 having a collet mechanism 80 with a projection or other mechanism fitting within annular collet groove 82 in the O.D. of mandrel extension 30b. Picking up on the work string thus raises the actuator and releases the slips to an unset position within the well bore 10. The collet assembly maintains releasable relative between cage 74 and mandrel extension 30b as actuator 68 moves downward to set slips 72. The jack 18 is operated by fluid in throughbore 46. Pressure of fluid in the throughbore 46 is increased and operates the anchor 28 so as to engage the slips 72 with the inner wall 22 of the casing 14. The teeth 73 of the slips 72 is toothed to grip an inner surface 22 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. Pressurised fluid also enters ports 44 arranged circumferentially around the mandrel 30, at the upper end of each outer piston 54, when the inner piston 48 rests on the outer piston 54. A chamber is provided at this location so that fluid can enter the ports and will act on the lower end face of the inner piston 48. This will move the piston 48 upwards, crossing a vented space, until the upper end face of the inner piston 48 abuts the lower end face of the outer piston 54 above or upper piston 36. This movement constitutes a stroke of the jack 18. Movement of the inner mandrel 30 is driven by movement of the inner pistons 48. As there are multiple stacked pistons 48, the combined cross- sectional areas of the end faces when fluid pressure is applied generates a considerable lifting force via the inner mandrel 30. This tensile force is sufficient to stretch the stuck casing 12.

Of note is that the inner mandrel 30 will not move until the anchor 28 is set by virtue of the shear ring 35 in the upper portion of the jack 18 (see Figure 2(a)). When sufficient fluid pressure is applied to the inner pistons 48 and consequently the mandrel 30, the shear ring 35 will shear and allow the mandrel 30 to move upwards relative to the housing 32.

To provide the increase in fluid pressure in the throughbore 46, a valve 21 is located below the jack 18, in the mandrel 30. In Figure 1, the valve is illustrated as sitting directly below the jack 18, however the valve 21 may be positioned in any part of the mandrel below the jack 18, its position being determined by other tools on the string 16. A suitable valve 21 is described in US8,365,826 being a ball valve operated via a drop ball. In a preferred embodiment, an ALO valve as supplied by Ardyne AS is used. The ALO valve is designed to form part of mandrel 30, and in which a valve is provided with a passage for fluid, the passage including an opening and closing mechanism. In use: a first valve portion is connected to the mandrel 30a; second valve portion telescopic relative to the first valve portion is connected to a downhole object such as the casing spear 20; in an initial position, the first valve portion and the second valve portion are held together under the force of a spring and arranged so that the opening and closing mechanism is open; pulling the mandrel 30 moves the first valve portion relative to the second valve portion thereby extending the valve to close the opening and closing mechanism. Thus the valve 21 advantageously can be cycled between open and closed positions by applying tension to the mandrel 30 on which the upper portion of the valve is attached. The ALO valve is described in WO2015065196, the disclosure of which is incorporated herein in its entirety by reference.

Casing spear 20 operates by a similar principle as the anchor 28 of the jack 18 to grip the inner surface 23 of the length of stuck 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 23 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 25 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 25 thereof. This provides the ideal setting position of the spear 20. In a preferred embodiment the casing spear 20 is the Flow Release Mechanism (FRM) Spear as provided by the Ardyne AS. The FRM Spear is described in WO2017059345, the disclosure of which is incorporated herein in its entirety by reference. The casing spear 20 is attached to the mandrel 30 below the valve 21 via a standard complimentary male and female crew threads as are known the art as a box and pin section.

A measurement system, generally indicated by reference numeral 90, is provided in the apparatus 11. The measurement system 90 is used to determine the distance of travel of the mandrel 30 relative to the housing 32. Measurement system 90 is provided as a modification to the jack 18 of Figures 2(a)-2(c). Referring now to Figure 3, there is illustrated the upper portion of the jack 18 as shown in Figure 2(a) with the incorporation of features of the measurement system 90. Like parts to those of Figure 2(a) have been given the same reference numerals to aid clarity. Measurement system 90 comprises additional shear rings 92, 94 in a stacked arrangement between the mandrel 30 and connector 24 of the housing 32. While only two additional shear rings 92,94 are shown there may be any number as will be apparent by the description of measurement to follow. Each shear ring 35,92,94 is an L-shaped shear ring to provide a lip for staged engagement of a shoulder 96 of the mandrel 30. The shear rings 35,92,94 are held apart at a fixed distance A, by spacers 98a, b. While shear rings 35,92,94 are shown, these may be shear pins arranged circumferentially around mandrel 30. Alternatively, spaced apart grooves in the respective surfaces of the mandrel and housing could be used to sit shear rings within. Where it is known that a shorter length of casing is stuck and thus the stretch distance will be greater, the length of the spacers can be increased or, in the grooved arrangement, the shear rings need only be placed in some of the grooves so that the fixed distance is selectable.

In use, jack 18 is connected to string 16 with the valve 21 located on the mandrel 30 and the casing spear 20 located below. The string 16 may have further tools such as a casing cutter 100, motor 104 and taper mill 102 as are known in the art. Such an arrangement is currently supplied as TITAN® from Ardyne AS without modification of the jack 18 to include a measurement system.

The string 16 is run in well bore 10, to locate the apparatus 11 within casing 14. The casing spear 20 is set to engage the inner surface 23 of the stuck casing 12 with slips 19 to grip the stuck casing 12 at its upper end 25. Valve 21 is activated to close and create a pressure increase in the throughbore 46 of the jack 18. The anchor 18 of jack 18 is set so that the slips 72 engage the inner surface 22 of casing 14 to grip the casing 14. Continuous pumping of fluid from surface down the throughbore 46 of the string 16 will actuate the jack 18 by fluid pressure entering ports 44 and moving the pistons 48. This in turn moves the mandrel 30 upwards relative to the fixed housing 32. The first shear ring 35 will shear and a pressure response in the form of a spike will be detected at surface.

Referring to Figure 4 of the drawings, there is illustrated a graph of pressure 106 versus time 108 at surface as the jack is activated. The pressure response 110 is seen as an increase in pressure with an initial spike 112 when the first ring 35 is sheared. It is at this point that the mandrel 30 begins to move upwards, taking the valve 21 and spear 20 upwards too. As the casing spear 20 is fixed to the stuck casing 12, a tension will be applied to the stuck casing 12 as it is pulled by the jack 18. Continued increase in pressure will increase the tension and the stuck casing will begin to stretch between the anchor of the casing spear 20 and a free point 114 of the casing 12. Casing 12 will be stuck from the free point 114 and below. The mandrel 30 will move upwards by an equal amount to the stretch experienced by the casing 12. When the mandrel reaches the second ring 92, this will shear providing a second spike 116 in the pressure response 110. The point 118 were the gradient of the response 110 changes as the ring 92 is sheared indicates a first pressure value 120. Continued pressure increase is seen at surface and then a third spike 122 occurs when the shoulder 96 of the mandrel 30 reaches the third ring 94 and it is sheared. The point 124 were the gradient of the response 110 changes as the ring 94 is sheared indicates a second pressure value 126. Between the points 118,124 the mandrel 30 has travelled a distance A relative to the fixed housing 32. This distance A is also the amount of stretch experienced by the stuck casing 12 from the free point 114. The change in pressure, value 126 minus value 120, provides the pressure required to stretch the casing 12 over the distance A. Stretch in the mandrel 30 is negligible due to the increased amount of casing length to mandrel length. The change in pressure can be translated to tensile force applied by calculating the volume of fluid displaced and the surface area of the pistons. The prior art calculation of length of casing 12 to the free point 114, can then be made from the tensile force applied and the stretch length A. Thus the free point 114 of the stuck casing 12 has been determined.

It will be recognised that further shear rings may be arranged between the mandrel 30 and the housing 32 which will shear as the mandrel is moved and the casing stretched over further fixed distances between the shear rings. In this way, the calculation can be repeated over more than one fixed distance to provide greater accuracy on the determination of the free point 114. With this information, the apparatus 11 may be repositioned in the well bore 10, so that blades 101 of the casing cutter 100 are located above the free point 114, as illustrated in Figure 5(a). The casing 12 can be cut at this position in the knowledge that the cut casing section 128 will be free to be recovered. Figure 5(b) illustrates the casing spear 20 attached to the upper end 25 of the cut casing section 128 and the string 16 being pulled to retrieve the cut section of casing 12. If the cut section of casing 12 does not move when pulled by the spear 20, which may occur due to inaccuracies in calculating the free point 110 and/or the casing 12 being cut too close to the free point 110 were some material 26 may still be present, the jack 18 can be used to free the cut casing section 128 as is known in the art. Thus the determination of the free point, cutting the casing above the free point and recovering the cut casing section can be achieved on a single trip into the well bore.

In an alternative embodiment, the method may include the initial steps of cutting the casing 12 at a desired depth, to remove the maximum liftable cut casing section; using the casing spear to attempt to pull the cut casing section; if it can be pulled, recover the cut section of casing and begin the method again to recover a further cut section of casing; if it can't be pulled, engage the jack on upper casing and attempt to pull the cut casing section using the jack; if it can be pulled by the jack, pull until it is free to move and then use the casing spear to recover the cut section of casing; if the jack cannot pull the cut section of casing, then use the method of the present invention to determine the free point of the cut casing section; cut the cut casing section above the free point and recover the free cut casing section.

The principle advantage of the present invention is that it provides a method and apparatus for determining a free point of stuck casing in a well bore which is more accurate than prior art arrangements. A further advantage of the present invention is that it provides a method and apparatus for determining a free point of stuck casing in a well bore which allows the casing to be cut above the free point and recovered on the same trip as determining the free point. It will be apparent to those skilled in the art that modifications may be made to the invention herein described without departing from the scope thereof. For example, measurement system may be electronic and pass the signals to surface via telemetry through the string. The sealing arrangement may be a drop ball and ball seat located in the mandrel. While a casing spear is described any form of anchoring attachment to the stuck casing can be used. Additionally, where reference has been made to upper and lower positions, together with above and below 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.

Claims

1. A method of determining a free point of stuck casing in a well bore, the well bore including a first casing, the first casing located closer to surface than the stuck casing, comprising the steps:

(a) anchoring a jack to the first casing in a well bore;

(b) attaching a first portion of the jack to a casing spear;

(c) anchoring the casing spear in the stuck casing;

2. A method according to claim 1 wherein the first portion is a mandrel of the jack.

3. A method according to claim 1 or claim 2 wherein step (d) is achieved by pumping fluid down a drill string to the jack.

4. A method according to claim 2 or claim 3 wherein step (d) includes sealing a central bore below the jack to allow a fluid pressure increase in the jack and thereby activate the jack.

5. A method according to any preceding claim wherein the method includes creating a signal detectable at surface when the first portion has moved over a fixed distance of travel.

6. A method according to claim 5 wherein step (e) includes measuring a change in pressure over time for the first portion to move over the fixed distance.

7. A method according to claim 6 wherein the change in pressure is recorded for more than one fixed distance of travel of the first portion.

8. A method according to claim 7 wherein the change in pressure is recorded for successive fixed distances of travel of the first portion, with the signal being detectable at surface when each fixed distance of travel is achieved.

9. A method according to any one of claims 5 to 8 wherein the method includes shearing a shear ring between the first portion and a second portion of the jack to create the signal, and wherein at least two shear rings are spaced apart by the fixed distance along the first portion.

10. A method according to any preceding claim wherein the method includes the additional steps of cutting the stuck casing at a position above the free point and pulling the cut section of casing.

11. Apparatus for determining a free point of stuck casing in a well bore, the well bore including a first casing, the first casing located closer to surface than the stuck casing, the apparatus comprising : a jack having a first portion moveable longitudinally relative to a second portion and having a central bore therethrough, gripping means for anchoring the jack to an inner wall of the first casing and attachment means for connecting the jack to an end of a drill string;

a sealing arrangement for closing fluid flow through the central bore and thereby increase pressure in the jack to activate the jack; and a measurement system to determine a distance of travel of the first portion relative to the second portion.

12. Apparatus according to claim 11 wherein the first portion is a mandrel and the second portion is a cylindrical housing, the mandrel being moveable longitudinally in the cylindrical housing.

13. Apparatus according to claim 11 or claim 12 wherein the measurement system includes a plurality of shear elements arranged between the first portion and the second portion, the shear elements being spaced a fixed distance apart.

14. Apparatus according to claim 13 wherein the shear elements are shear rings.

15. Apparatus according to any one of claims 11 to 14 wherein the sealing arrangement is a valve.

16. Apparatus according to any one of claims 11 to 15 wherein the jack and the casing spear are a hydraulically powered fishing tool.

17. Apparatus according to any one of claims 11 to 16 further including a casing cutter.