WELL ABANDONMENT AND SLOT RECOVERY

The present invention relates to a method and apparatus for well abandonment and in particular, though not exclusively, to a method and apparatus for severing a casing of an inner casing string located within an outer casing string in a wellbore.

During ongoing operations on oil and gas wells it is sometimes necessary to remove a section of casing from the wellbore such as in well abandonment and slot recovery operations. This is done by severing the casing at a pre-determined location, latching a casing spear to the top of the severed casing section and lifting it out of the well. There are many methods of severing casing; explosives and rotary cutting are examples.

When a well is constructed, casing strings are run into the well. Each casing string is made up of tubular sections, or joints, that are usually about 40 feet (12.2 m) long and screwed together to form longer lengths. Each end of the casing joint has male threads and the casing joints are connected together using a collar or coupling, composed of a short cylindrical steel pipe that is larger in diameter than the joints and has female threads. Multiple casing strings are run into the well, with increasingly narrower diameters as the depth of the well increases. Consequently, the casing strings are nested, with a smaller diameter casing string being located inside previously installed casing strings of a larger size. Due to the nature of well construction, particularly for horizontal or inclined wells, it is not possible to guarantee that the narrower casing string will be centralised within the previously installed string, and thus the outside of a smaller casing string may be close to, or even in contact with, the inside of the larger casing string. A disadvantage of this well construction is that when casing cutting of the inner casing string is carried out, there is a risk that the outer casing may also be severed. If the outer casing string is impacted, damaged or punctured during the severing operation, well integrity is lost.

It is an object of the present invention to provide a method of severing casing which obviates or mitigates the disadvantages of the prior art.

According to a first aspect of the present invention there is provided a method of severing casing of an inner casing string located inside an outer casing string in a well bore, comprising the steps:

(a) running a tool string, including a casing severing tool, into the inner casing string, to a first cutting depth;

(b) determining a location of a coupling on the inner casing string close to the first cutting depth;

(c) locating the casing severing tool within a first separation distance from the located coupling; and

(d) severing casing of the inner casing string. As the coupling has a greater diameter than the casing joints of the inner casing string, there will be a region close to the coupling where the inner and outer casing strings are separated by a gap. By severing the casing of the inner casing string at the gap, damage or puncturing of the outer casing string can be avoided.

Preferably, the first separation distance is five feet (152.4cm). More preferably, the first separation distance is three feet (91.4cm). Due to the stiffness of the tubular to be severed, the gap between the two casing strings is substantially maintained over a short distance from the located coupling. In critical applications, the first separation distance may be one foot (30.5cm) so that the severing takes place within one foot of the casing. Preferably, the severing tool has an active diameter between an outer diameter of the inner casing string and an outer diameter of the coupling. In this way, the gap provides a level of comfort so that the inner casing string is completely severed while the outer casing string is not impacted to maintain its integrity.

Preferably, the severing tool is a casing cutter and step (d) is performed by a cutter knife being rotated about a centre axis of the tool string. Preferably, the cutter knife has a maximum sweep diameter between an outer diameter of the inner casing string and an inner diameter of the outer casing string. In this way, the distal end or blade tip of the cutter knife will pass through the inner casing string wall but not reach the outer casing string. This ensures the integrity of the well after the casing is severed. Preferably, the method includes the step of centralising the severing tool in the inner casing string. In this way, the active diameter of severing tool with be co-linear with the diameter of the inner casing string and the coupling, so that an eccentric sever does not occur which could risk damage to the outer casing string. For the severing tool comprising a cutter knife, this may be achieved by providing an odd number of knives being greater than or equal to three. As the knives extend from the tool they will centralise the tool on contact with the inner casing string.

Preferably, the method includes mounting a coupling locator on the tool string. More preferably, the coupling locator sends a first signal to surface when a coupling is detected. Alternatively, the coupling locator activates an anchoring device on the tool string to anchor the tool string against the inner casing string when a coupling is located. Preferably, the coupling locator locates the position of a coupling by logging a change in a second signal as the tool string is moved in the inner casing string. In an embodiment, the coupling locator is a CCL (Casing Collar Locator). Such tools are known for determining the position of a coupling. This is done by moving magnets, incorporated in the tool, past a coupling which changes the magnetic field due to the increased volume of metal of the coupling. This information together with a knowledge of the CCL's depth in the well locates the coupling depth.

The tool string may be a pipe string. Optionally, the coupling locator may be run on wireline, through the drill pipe, and the tool string moved to position the severing tool at within the first separation distance. A suitable tool that can detect couplings on the casing string surrounding the drill pipe may be an MTD (Metal Thickness Detector). MTD uses eddy currents to detect changes in wall thickness of the tubular surrounding the MTD and can identify changes in thickness of more than one tubular at a time.

Alternatively, the severing tool may be used to locate a coupling. By extending the knives on the severing tool and moving the tool string past a coupling, changes in tool string weight may be seen at surface as the knives enter and are then pushed back by a recess where the casing string joints are screwed together. At step (c) the severing tool may be located within the first separation distance by movement of the tool string. Alternatively, at step (c) the severing tool may be located within the first separation distance by being arranged on the tool string at a distance less than the first separation distance from the coupling locator. In this arrangement the movement of the tool string is stopped when the coupling is located.

The method may include the additional step of removing the severed casing from the well bore. Thus, the method may be considered as a method of well abandonment. Alternatively, the method may be considered as a method of slot recovery. According to a second aspect of the present invention there is provided a bottom hole assembly for severing casing of an inner casing string located inside an outer casing string in a well bore, the assembly comprising :

on a tool string, a coupling locator to determine the position of a coupling on the inner casing string, and a severing tool to sever the inner casing string.

The coupling locator may be any device which can determine the position of a casing coupling in a well bore. The coupling locator may be a CCL (Casing Collar Locator). Such tools are known for determining the position of a coupling by moving magnets, incorporated in the tool, past a coupling which changes the magnetic field due to the increased volume of metal of the coupling. This information together with a knowledge of the CCL's depth in the well can locate the coupling depth. The coupling locator may be an MTD (Metal Thickness Detector). An MTD uses eddy currents to detect changes in wall thickness of the tubular surrounding the MTD and so detects the increased wall thickness of the casing coupling as it moves through the inner casing string.

The severing tool may be any device which can part casing. Preferably, the severing tool has an active diameter between an outer diameter of the inner casing string and an outer diameter of the coupling. In this way, the inner casing string is severed while the outer casing string is not impacted if the inner casing string is severed near the coupling.

Preferably, the severing tool is a casing cutter having at least one cutter knife with a maximum sweep diameter between an outer diameter of the inner casing string and an inner diameter of the outer casing string. In this way, the distal end or blade tip of the cutter knife will pass through the inner casing string wall but not reach the outer casing string. The tool string may be a tubular string allowing fluid flow therethrough. The tool string may by drill pipe or coiled tubing. Preferably, the casing cutter is activated by fluid flow. More preferably, the casing cutter is operated by rotation of the tool string. The tool string may include a motor to rotate the casing cutter.

The coupling locator and the severing tool may be arranged on the tool string at less than or equal to a first separation distance apart. The first separation distance may be five feet (152.4cm). More preferably, the first separation distance is three feet (91.4cm). Alternatively, the first separation distance may be one foot (30.5cm). In this way, when the coupling locator determines the position of the coupling, the tool string may be stopped and the severing tool operated to sever the casing at a the first separation distance from the coupling.

The bottom hole assembly may include an anchoring device. In this way, the tool string can be anchored to the inner casing string when the casing is severed. This assists in stabilising the severing tool. The anchoring device may be used to hold the inner casing string in tension during the severing procedure. Alternatively or additionally, the anchoring device may be used as a casing spear to remove the severed casing from the well bore.

The bottom hole assembly may include a casing spear. In this way, the severed casing can be removed from the well bore. Alternatively or additionally, the anchoring device may be used as the casing spear.

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:

Fig 1(a) is a schematic illustration of apparatus in wellbore in a method of severing casing of an inner casing string located inside an outer casing string according to an embodiment of the present invention, with Figs 1(b), 1(c) and 1(d) being cross-sectional views through the wellbore at three locations;

Fig. 2 is a schematic illustration of casing being severed according to an embodiment of the present invention; Fig. 3 is a schematic illustration of a casing severing tool being used to determine the location of a coupling according to an embodiment of the present invention; and Fig. 4 is a schematic illustration of apparatus in wellbore in a method of severing casing of an inner casing string located inside an outer casing string according to a further embodiment of the present invention.

Reference is initially made to Fig. 1(a) of the drawings which illustrates a bottom hole assembly, generally indicated by reference numeral 10, including a coupling locator 12 and a casing severing tool 14, run on a tool string 16 into an inner casing string 18 located inside an outer casing string 20, according to an embodiment of the present invention. The inner casing string 18 is formed of casing sections or joints 22a, b,c which are fixed together using coupling 24a, b. Coupling 24a, b may be referred to as a collar. Tool string 16 is preferably a tubular string such as drill pipe or coiled tubing through which fluid can flow.

Well construction inevitably requires multiple casing strings to be used, smaller diameter casing will be located inside a previously installed larger size. Due to the nature of well construction it is not possible to guarantee that casing strings will be centralised within the previously installed string, and it is sometimes the case that the outside of a smaller string is close to, or even in contact with, the inside of the larger string. This is as illustrated in Fig. 1(c). Due to the nature of well construction, in general it is not known how close the strings are to each other. While two casing strings 18,20 are shown in Figs l(a)-(d), it will be appreciated that multiple casing strings can be constructed in a nested arrangement. When casing needs to be removed from the well such as in well abandonment or slot recovery application, a severing tool is typically run into the well and by severing the casing at a pre-determined location, latching a casing spear to the top of the casing piece, it can be lifted out of the well. To ensure well integrity it is important not to damage or puncture the outer casing string during the severing operation. It may be appreciated that if the two strings are close to or in contact with each other, ensuring that the outer casing is not damaged may not be possible.

Casing strings 18,20 are supplied in short sections or joints 22 which are connected by couplings 24. Most couplings are larger in diameter than the casing itself. During the well construction, a 'tally' of the length of each casing joint that is run into the well will be taken. This means that in principle, the position of each coupling is known. As the thickness of the wall of casing between the couplings is less than the thickness at the couplings, this tally is used to try and locate the severing tool away from the couplings to a position mid-way between the couplings, so that the casing string can be severed more easily. However, as shown in Fig. 1(c) this position is more likely to be where the casing strings are closest together.

The present invention recognises that that close to each coupling, there is a region where the two casing strings are separated by a gap 26, seen in Figs 1(b) and 1(d). The gap 26 will typically be at least half the increased diameter of the coupling over the diameter of the inner casing string. At larger distances from the coupling it is possible that the gap 26 will decrease as illustrated in Figs 1(a) and 1(c). If the casing can be severed in the region of the gap 26, then a severing tool 14 with an active diameter i.e. radial extent of cutter knives for example, being greater than the inner casing diameter but less than the diameter of the gap 26, will not contact the outer casing string during severing. Experience has shown that the system of using a tally of casing joints is not accurate enough to ensure that the severing tool 14 is located close enough to a coupling to lie within the region of the gap. The present invention therefore uses a coupling locator 12 as an additional means of determining the location of each coupling. The present invention provides equipment and a process that can sever a tubular within a short distance of a coupling. This ensures that the outer casing string is left undamaged when a portion of the inner casing string is removed.

In an embodiment, the method consists of the following steps:

• Run a tubular string 16 with a severing tool 14 into the well down to the region of the cutting depth

· Locate the position of a coupling 24 close to the desired cutting depth

• Move the string 16 until the severing tool 14 is within a specified distance of the located coupling 24

• Perform the severing operation Due to the stiffness of the tubular to be severed, the gap between the two casing strings 18,20 is substantially maintained over a short distance from the coupling 24. In order to ensure that the severing tool 14 does not affect the outer casing, severing should be done no more than five feet (152.4cm) from the coupling. This represents a distance of 12.5 percent of the full length of a typical casing joint 22. Typically, severing should occur within three feet (91.4cm) of the coupling, representing a distance of 7.5 percent of the casing joint 22. In critical applications, severing should be done within 1 foot (30.5cm) of the coupling. This represents a distance of 2.5 percent of the length of a casing joint 22. The distance selected can be considered as a first separation distance 30 from the coupling 24 and can be set on a per job basis dependent on the operating conditions.

The method can be seen carried out in Figure 1(a). A coupler locator 12 and severing tool 14 are mounted on a tool string 16 as a bottom hole assembly 10 and run into an inner casing string 18 of a well bore 28. This bottom hole assembly 10 is run to a depth representing a desired length of the inner casing string 18 which requires to be removed. This may be considered as a first cutting depth. Once at the first cutting depth, within a calculated casing joint length of the first cutting depth or continuously as the string 10 is run in, the coupling locator 12 is activated and as it moves in the inner casing string 18, a signal is sent to surface on detection of a coupling 24. In an alternative embodiment, the coupling locator 12 logs a distance of the inner casing string 18 being greater than a casing joint 22 length, the BHA 10 is pulled and the data analysed from the casing locator 12 to determine the position of the coupling 24 closest to the first cutting depth.

The tool string 16 is then moved to position the severing tool 14 near the coupling 24. The severing tool 14 will be positioned within the first separation distance 30 from the coupling 24. In the embodiment where a signal is sent to surface, movement of the tool string 16 can be stopped when the signal is detected and, if the severing tool 14 and coupling locator 12 are separated on the tool string 16 by a distance of less than the first separation distance 30, the severing tool 14 will be at the correct position for severing the inner casing string 18 when the tool string 16 is stopped. The severing tool 14 is then activated to sever the inner casing string 18. The severing tool 14 will have been selected such that the active area of severing from the tool has a diameter greater than the diameter of the inner casing string 18 to ensure that the inner casing string 18 is severed while extending no greater than the diameter of the coupling 24 to ensure that the outer casing string 20 is not severed or contacted by the action of the severing tool 14.

The bottom hole assembly 10 may include a casing spear 32, so that once the casing joint 22 of the inner casing string 18 has been severed, the casing spear 32 can latch or anchor to the cut section of casing 34 and by pulling the tool string 16 the cut section of casing 34 will be removed from the well bore 28. The bottom hole assembly may include an anchoring device 36, the anchoring device 36 being set to hold the tool string 16 to the inner casing string 18. This may be done when the severing tool 14 is operated to assist in stabilising the severing tool 14. The anchoring device 36 may be engaged and the tool string 16 raised so that the inner casing string 18 is held in tension during the severing process. By holding the inner casing string 18 in tension during severing a signal will be noted at surface when the casing joint 22 has been severed. In an embodiment, the anchoring device 36 may act as the casing spear 32, so that the anchoring device 36 grips the cut section of casing 34 and pulls it to the surface of the well bore 28.

The bottom hole assembly 10 may include a centralising device 38. The centralising device 38 will locate the severing tool 14 in the centre of the inner casing string 18 and thereby ensure that the diameter of active area of the severing tool 14 is centred with respect to the coupling 24. In this regard the centralising device will prevent the severing tool 14 being operated in an eccentric position with respect to the couplings. The centralising device 38may be a plurality of stabilising blocks mounted on an outer surface of the string 16 or severing device 14. Alternatively, the centralising device may be the anchoring device 36 in which slips are arranged to extend radially uniformly from the device 36.

The severing tool 14 may be any tool capable of parting the inner casing string 18 to provide a cut section of casing 34. Known tools can use blades or knives to cut the casing and are referred to as casing cutters; grinding elements to mill away the casing; chemicals to dissolve the casing; lasers to ablate the casing; water jets cut the casing; and controlled explosives to obliterate the casing. Other severing tools may also be used.

An example of a severing tool 14 is a casing cutter activated by fluid flow through and by rotation of the tubular string 16. This is as illustrated in Fig. 2, showing a casing cutter, generally indicated by reference numeral 114. Like parts to those in Fig. 1(a) have been given the same reference numeral with the addition of Ί00' to aid clarity. Casing cutter 114 has a through bore 40 which includes a piston 42 operated by fluid flow in the through bore 40. As flow is applied, movement of the piston 42 causes cutter knives 44 are deployed which engage the inner wall 64 of the casing joint 122 to be severed. Rotation of the tubular string 16 allows the knives 44 to cut through the casing joint 122 until it is completely severed. Though a single knife 44 is shown in Fig. 2 it will be appreciated that a number of knives, typically three, arranged equidistantly around the tool 114 will be deployed. It will be appreciated that the maximum sweep of the knives 44 is restricted so that the knife tips 46 cannot come into contact with the outer casing string 120. In Fig. 2 an example is shown with an inner casing string 118 of 13 3/8 ^" (340mm) and an outer casing string 120 of 20" (508mm). However, casing in a well is known to be supplied in standard diameters e.g. 5", 5 1/2", 6", 6 5/8" 7", 7 5/8", 8 5/8", 9 5/8", 10 3/4", 11 3/4", 13 3/8", 14", 16", 18 5/8" and 20" (127mm, 140mm, 152mm, 168mm, 179mm, 193mm, 219mm, 244mm, 273mm, 298mm, 340mm, 356mm, 406mm, 473mm and

508mm). Any two casing diameters may be used including non-standard diameters. Each casing joint 122a, b is held together by a casing coupling 124. Though not shown the casing joint 122a, b will have male threads which mate with female threads on the coupling 124. The coupling 124 is seen to be thicker walled than the casing 122a, b and thus a gap 126 exists close to the coupling 122a, b at the outer surface 48 of the casing joint 122a, b. In Fig. 2 the inner casing string 118 is eccentrically positioned in the outer casing string 120 so that the coupling 124 contacts the inner wall 50 of the outer casing string 120. Thus the coupling 124 prevents the walls 48,50 coming into contact and ensures there is at least a radial gap 126 therebetween. The casing joint 122a, is severed within a first separation distance 130 from the coupling 124 so that the knives 44 will exit the outer surface 48 in the gap 126 and be prevented from reaching the inner surface 50. A stop 52 is used in the tool 114 to limit the sweep i.e. maximum active diameter, shown as radius 66 (being half the maximum active diameter), of the knives 44, so that they provide a complete cut 54 through the casing joint 122a to sever it. This provides a cut section of casing 134 to be removed and the remaining casing 56 in the well bore 128. By restricting the sweep that the cutter knives 44 can extend out to, guarantees no damage to the outer casing 120. The severing tool 114 is centralised in the inner casing string 118 using stabiliser 58 so that the knives 44 will cut through the casing joint 122 symmetrically and thus extend by equal amounts into the gap 126.

An example of tool 12 that can determine the position of a coupling is a CCL (Casing Collar Locator). Such tools incorporate magnets. Moving the magnets past a coupling changes the magnetic field due to the increased volume of metal of the coupling. This information together with a knowledge of the CCL's depth in the well locates the coupling depth. It will be appreciated that any suitable coupling locator could be used together with any severing tool.

On some casing joints (API Buttress for instance), there is a noticeable recess in the inner diameter of the inner casing string 18 where adjacent casing joints 22a, b are screwed together. Such an arrangement is illustrated in Fig. 3. If such a recess 70 is present, this can be used to detect the location of the coupling 24. The coupling locator 12 may then operate on the same principle as caliper tools which monitor changes in the inner diameter of a tubular. Alternatively, the casing severing tool 14 may be used to locate a coupling 24 and a separate coupling locator 12 does not need to be provided on the tool string 16. Referring to Fig. 3 of the drawings there is illustrated a casing severing tool, generally indicated by reference numeral 214, used to determine the location of a coupling 224. In Fig. 3, like parts to those of Fig. 1(a) have been given the same reference numeral with the addition of 200 to aid clarity. The casing severing tool 214 may operate as for the tool 114 in Fig. 2. The illustration in Fig. 3 is simplified by just indicating a single cutter knife 244 exiting the body of the tool 214 and it will be recognised that a plurality of knives acting in the same manner as knife 244 will be arranged around the tool 214. Without rotating the tool string (not shown) or the severing tool 214, the knives 244 are actuated. This may be by fluid flow through the tool string. The knives 244 extend and initially contact the inner wall 264 of a casing joint 222b, for example. The tool string is then moved longitudinally and when it reaches a coupling 224, the tip 246 of the knife 244 will pass an end 68 of the casing joint 222b and the knife 244 will extend into a recess 70 formed at the coupling 224 between adjacent coupling joints 222a, b. On exiting the recess 70 the knife 44 will be pushed back over the end 69 of the adjacent casing joint 222a to return to moving along the inner surface 264 of the casing joint 222a. A hook load change is seen as the knives 244 hang up in the recess 70. The change is noted when the tool string 216 is moved in either direction.

Thus by extending the knives 244 on the severing tool 214 and moving the tool string past a coupling 224, changes in tool string weight will be seen at surface as the knives 244 enter and are then pushed back by a recess 70 where the casing string joints 222a, b are screwed together. Detecting the change in tool string weight at surface will indicate that the coupling 224 has been reached and its location is then known. The tool string can then be moved a distance less than or equal to the first separation distance as indicated in Figure 1(a) and the tool string rotated to cut the casing joint 222a, b with the knives 244. The inner casing string 218 is thus severed. Another example of a tool 12 that can identify a coupling is an MTD (Metal Thickness Detector). MTD uses eddy currents to detect changes in wall thickness of the tubular surrounding the MTD. MTD can identify changes in thickness of more than one tubular at a time.

Reference is now made to Fig. 4 of the drawings which illustrates a method of severing casing of an inner casing string located inside an outer casing string which uses an MTD, generally indicated by reference numeral 312, or like coupling locator according to a further embodiment of the present invention. Like parts to those of Fig. 1(a) have been given the same reference numeral with the addition of 300 to aid clarity. In Fig.3 the severing tool 314 is run on a tubular string 316 such as drill pipe. In this example the severing tool 314 is shown as a casing cutter operated by a motor 60 driven by fluid flow through the tubular string 316. An anchoring device 136 is also shown to hold the inner casing string 318 in tension while the casing joint 322 is severed by cutting with the casing cutter 314. In this method, the tubular string 316 is run to a depth below the intended cutting depth 62 for the desired length of casing 334 to be removed. The coupling locator 312 is run into the well bore 328 on wireline 60, through the drill pipe of the tubular string 316. As the coupling locator 312 is an MTD, it can detect couplings on the casing string 318 surrounding the drill pipe. When a coupling 324a is detected close to the cutting depth 62 its position is logged by the MTD. The wireline 60 and coupling locator 312 can be POOH, and the severing tool 314 is position within the separation distance 330 to make the cut 354 and provide the cut casing section 334 for removal. The anchoring device 336, if used, can grip the inner wall 364 of the inner casing string 318 and the tubular string 316 raised so as to hold the casing string 318 in tension while the casing joint 322 is severed. The cut 354 will be made at the location of the gap 326 so that the outer casing string 320 is not damaged or punctured when the casing joint 322 is severed. If desired the anchoring device 336 can be repositioned to grip an upper end of the cut section of casing 334 and used as a casing spear for retrieval of the cut section of casing 334 by pulling the tubular string 316 from the well bore 328.

The principle advantage of the present invention is that it provides a method of severing casing of an inner casing string located inside an outer casing string in a well bore which maintains the integrity of the outer casing string.

A further advantage of the present invention is that it provides a method of severing casing of an inner casing string located inside an outer casing string in a well bore in which the casing is severed near to a coupling of the inner casing string.