Downhole tool and method of releasing slips of a downhole tool

Abstract

A downhole tool having slips and including an emergency release mechanism as a secondary means of deactivation to unset the slips. In one embodiment, the downhole tool has hydraulically set slips and a spring to return the slips to the unset position when pressure is removed. However, if debris prevents the slips from completely unsetting, the emergency release mechanism is activated by dropping a ball from surface to land in a seat, diverting fluid to enter a hydraulic channel from the bore to act on a piston used to unset the slips. The force available via the release mechanism is many times that available from the spring and is thus much more likely to be able to move the slips against the obstructing debris.

Description

[0001] DOWNHOLE TOOL AND METHOD OF RELEASING SLIPS OF A DOWNHOLE TOOL

[0002] FIELD

[0003] This relates to a downhole tool and to a method for operating the downhole tool. In particular, though not exclusively, this relates to an emergency release mechanism for a downhole tool, a downhole tool comprising the emergency release mechanism and to a method to positively unset slips of a downhole tool to ensure their release so that the downhole tool can be disengaged and removed from a wellbore.

[0004] BACKGROUND

[0005] Certain downhole tools have slips that are designed to latch into objects such as the inner surface of a casing or liner within the wellbore. These tools are often used to apply high forces to those objects either to hold the tool against the object such as when used as part of a packer or when the object is being fished and the slips need to anchor to the object for it to be pulled free. Common features of slips are that they are made from hard materials and have sharp teeth designed to bite into the object in the wellbore.

[0006] Slips generally will have a recessed position where the teeth cannot contact downhole objects and an extended position where the teeth can bite. Slips can be set (moved from the recessed position to the extended position) either by application of hydraulic pressure or by mechanical manipulation, for instance by a J-slot mechanism. Slips can be unset (moved back to the recessed position) by reversing the setting operation i.e. by releasing hydraulic pressure or by mechanical manipulation as the case may be. Often there is a mechanism such as a return spring to encourage the slips to retract.

[0007] In many wells there is debris present in the form of fine particulate matter entrained in the wellbore fluids or lying on the low side of the wellbore. This debris can settle out and collect on or within the downhole tool. Often the build-up of debris can obstruct the operation of the downhole tool. In the case of tools with slips, this may mean that the slips cannot latch onto downhole objects, or more likely that the slips do not fully retract after latching, even with the aid of the return spring force.

[0008] Due to the nature of slips, it may be appreciated that a slip that is not fully retracted may inadvertently catch on wellbore objects when a work string on which the downhole tool is mounted is moved up or down the well. Inadvertent catching may result in the slips being pulled more fully into the extended position and gripping fast on the wellbore object. The work string is now stuck and repeated attempts to free it, for instance by reversing the direction of travel of the work string may be unsuccessful, resulting in a failure to pass a location of interest in the wellbore.

[0009] Ultimately, there may come a time when the operator concludes that no amount of extra work will free the work string and a remedial measure, such as parting the work string and leaving part of the work string in the wellbore, is the only way to recover at least some of the work string from the wellbore.

[0010] SUMMARY

[0011] Aspects of the present disclosure relate to a downhole tool and to a method for operating the downhole tool. In particular, though not exclusively, aspects of the present disclosure relate to an emergency release mechanism for a downhole tool, a downhole tool comprising the emergency release mechanism, and to a method to positively unset slips on a downhole tool to ensure their release so that the downhole tool can be disengaged and removed from a wellbore.

[0012] According to a first aspect there is provided a downhole tool comprising: a plurality of slips for engaging with a first surface in a wellbore, the slips configured and / or operable to reversibly move between an unset position wherein the slips locate within a recess of the downhole tool so that a gripping surface of the slips lies within an outer diameter of the downhole tool and a set position wherein the slips are displaced to extend the gripping surface beyond the outer diameter so as to engage the first surface; a first mechanism for moving the slips between the unset position and the set position; and an emergency release mechanism configured and / or operable to move the slips to the unset position, the emergency release mechanism comprising: a drop ball seat in a bore of the downhole tool; and one or more emergency release pistons operable by the action of a drop ball landing in the drop ball seat so as to create a release force against the slips and move them towards the unset position.

[0013] In this way, an additional force can be applied to assist in releasing and retracting the slips and move them back to the unset position in the event that the slips have not been able to return to the unset position, e.g. in the event that the first mechanism has failed to return the slips fully to the unset position. This may be considered as a secondary means of deactivation to positively retract the slips.

[0014] In use, the one or more emergency release pistons may be operated in the event that the slips fail to return to the unset position in the well bore, e.g. the first mechanism has failed to move the slips from the set position to the unset position. Beneficially, the downhole tool obviates or mitigates at least some of the disadvantages of the prior art.

[0015] The first mechanism may comprise a mechanism for moving the slips from the unset position to the set position and / or moving the slips from the set position to the unset position.

[0016] The downhole tool may include a spring. The spring may be arranged to act on the first mechanism. The spring may provide an initial return force to move the slips from the set to the unset position. The release force may be greater than the initial return force. In this way, the emergency release force may be used when the initial return force fails to unset the slips, e.g. return the slips fully to the unset position, and will provide a greater force which is more likely to be able to move the slips against obstructing debris.

[0017] The emergency release mechanism may include a valve. The valve may be configured and / or operable to be opened by the action of the drop ball. The valve may be configured and / or operable to allow fluid to pass from the bore above the drop ball seat to a first side of the one or more emergency release pistons. More particularly, the fluid may operate the one or more emergency release pistons.

[0018] The first mechanism may be a reversible mechanism. The emergency release mechanism may be used in the event that reversal of the first mechanism fails to move the slips to the unset position.

[0019] There may be a plurality of emergency release pistons. The plurality of emergency release pistons may be arranged in series. The plurality of emergency release pistons may be arranged axially.

[0020] The plurality of emergency release pistons may be fluidly connected. The fluid may act on the plurality of emergency release pistons to multiply up the force available to release the slips.

[0021] The first mechanism may include one or more cones. The one or more cones may be configured and / or operable to move under the slips to set the slips. The one or more emergency release pistons may be configured and / or operable to act on the one or more cones to release the slips.

[0022] The first mechanism may include one or more operating pistons. Movement of the one or more operating pistons may be used to set the slips. More particularly, the slips may be set by fluid acting on the one or more operating pistons.

[0023] The one or more emergency release pistons and the one or more operating pistons may be stacked. The fluid may act in a first direction on the one or more operating pistons to set the slips, e.g. move the slips from the unset position to the set position. The fluid may act in a second opposite direction on the one or more emergency release pistons to release the slips, e.g. move the slips to the unset position.

[0024] In this way, there may be a first fluid flow path through the tool to set the slips and a second fluid flow path through the tool to operate the emergency release mechanism and release the slips. The valve of the emergency release mechanism may control fluid flow through the second fluid flow path.

[0025] The one or more emergency release pistons and the one or more operating pistons may be the same one or more pistons. The fluid may act in a first direction on the one or more pistons to set the slips, e.g. move the slips from the unset position to the set position. The fluid may act in a second, opposite, direction on the one or more pistons to release the slips, e.g. move the slips to the unset position.

[0026] The first mechanism may comprise a first fluid flow path through the tool to set the slips, e.g. move the slips from the unset position to the set position. The first fluid flow path may comprise chambers on second sides of the one or more pistons. The second sides may be undersides of the one or more piston, e.g. a lower or downhole side in use. The first fluid flow path may comprise ports providing a fluid passageway between a central through bore of the tool and the second side chambers.

[0027] The emergency release mechanism may comprise a second fluid flow path through the tool to release the slips, e.g. move the slips to the unset position. The second fluid flow path may comprise chambers on first sides of the one or more pistons. The first sides may be upper sides of the one or more pistons, e.g. an upper or uphole side in use. The second fluid flow path may comprise conduits between the bore above the drop ball seat and the first side chambers. The valve of the emergency release mechanism may control fluid flow through the second fluid flow path. The second fluid flow path may comprise conduits between the first side chambers, to fluidly connect the first side chambers in series.

[0028] The emergency release mechanism may further comprise a third fluid flow path through the tool. The third fluid flow path may be a vent flow path. The third fluid flow path may comprise conduits between the first side chambers and outside of the tool. The valve of the emergency release mechanism may control fluid flow through the third fluid flow path. The valve may have an initial position in which fluid flow along the third fluid flow path is permitted and the second fluid flow path is blocked. The valve may have an activated position in which fluid flow along the second fluid flow path is permitted and the third fluid flow path is blocked.

[0029] The first direction may be upwards in the tool, e.g. an uphole direction in user. The second direction may be downwards in the tool, e.g. a dowhole direction in use.

[0030] The downhole tool may comprise or take the form of a packer and may include one or more sealing elements for engaging the wellbore.

[0031] Alternatively, the downhole tool may comprise or take the form of a spear, with the slips anchoring on tubing to facilitate removal of the tubing from a wellbore.

[0032] In a further alternative, the downhole tool may comprise or take the form of a jack, with the slips anchoring on casing and e.g. acting in concert with other downhole tools to apply a force to a stuck object further down the wellbore.

[0033] According to a second aspect there is provided a method of releasing slips of a downhole tool, the downhole tool according to the first aspect, and the method comprising the steps: dropping the drop ball into a throughbore of the downhole tool to seat the drop ball in the drop ball seat and seal the throughbore; and pressurising up behind the drop ball to operate the one or more emergency release pistons so as to create a release force directed against the slips and move the slips into the unset position.

[0034] In this way, a second activation method is provided to move the slips to the unset position.

[0035] The method may release slips of a downhole tool when the slips have failed to unset, e.g. via a first mechanism.

[0036] Beneficially, the method obviates or mitigates one or more disadvantages of the prior art.

[0037] Pressuring up behind the drop ball may move a member in the emergency release mechanism. Movement of the member may operate the one or more emergency release pistons.

[0038] Movement of the member may open a conduit. The method may include the step of pumping fluid against the drop ball, the fluid entering the conduit and being directed to act on the one or more emergency release pistons.

[0039] The method may further comprise the initial steps of using the first mechanism to set the slips against the first surface, and unsetting the slips and on attempting to pull the downhole tool from the wellbore noting that it has stuck.

[0040] The method may further comprise the final step of pulling the work string and the downhole tool from the wellbore.

[0041] According to a third aspect, there is provided an emergency release mechanism for a downhole tool comprising slips configured and / or operable to reversibly move between an unset position and a set position, the emergency release mechanism comprising: a drop ball seat; and one or more emergency release pistons operable by the action of a drop ball landing in the drop ball seat so as to create a release force against the slips and move them towards the unset position.

[0042] 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 and elements of the embodiments discussed below may be employed separately or in any suitable combination to produce the desired results. Additionally, while relative terms such as ‘upper’ and ‘lower’ are used and the drawings indicate vertical wells, the invention finds application in deviated wells.

[0043] The invention is defined by the appended claims. However, for the purposes of the present disclosure it will be understood that any of the features defined above or described below may be utilised in isolation or in combination. For example, features described above in relation to one of the above aspects or below in relation to the detailed description may be utilised in any other aspect, or together form a new aspect.

[0044] BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which:

[0046] Figure 1(a) is a part cross-sectional schematic illustration of an upper section of a downhole tool in an unset position;

[0047] Figure 1(b) is a part cross-sectional schematic illustration of a mid-section of the downhole tool in the unset position;

[0048] Figure 1(c) is a part cross-sectional schematic illustration of a lower section of the downhole tool in the unset position;

[0049] Figure 2 is a sectional view through the line A-A of Figure 1(c);

[0050] Figure 3 is a part cross-sectional schematic illustration of the lower section of the downhole tool of Figure 1(c) with the slips extended, e.g. in a set position;

[0051] Figures 4(a) is a part cross-sectional schematic illustration of the mid-section of the downhole tool in a set position;

[0052] Figure 4(b) is a part cross-sectional schematic illustration of the lower section of the downhole tool in the set position; and

[0053] Figure 5 is a part cross-sectional schematic illustration of the upper section of the downhole tool of Figure 1(a) with the emergency release mechanism activated.

[0054] DETAILED DESCRIPTION OF THE DRAWINGS

[0055] Referring initially to Figures 1(a) to 1(c) there is illustrated a downhole tool, generally indicated by reference numeral 10, which includes slips 12, a first mechanism 14 for moving the slips 12 between an unset position and a set position; and an emergency release mechanism 16 to move the slips 12 from the set position to the unset position in the event that the slips 12 fail to return to the unset position in the wellbore, according to an embodiment of the present invention.

[0056] At a lower end 18 of the tool 10, Figure 1(c), there is provided a connection 20a to locate the tool 10 in a work string or pipe string (not shown). Connection 20a may be one part of a standard box and pin section as is known in the art. The connection 20a is formed on a mandrel 22, around which is mounted an outer body 24. The mandrel 22 and the outer body 24 are configured to move relative to each other. The outer body 24 provides a recess 26 and a set of cones 28a, b on a base of the recess 26. The recess 26 is an outer recess. A slip cage 30 is mounted around the outer body 24. The slip cage 30 is arranged across the recess 26, providing pockets 32 in each of which a slip 12 is located and a portion retained in rails 34 on the outer body 24. This is best seen with the aid of Figure 2. While four slips 12 are illustrated equally spaced around the circumference of the tool 10, there may be any plurality chosen.

[0057] Each slip 12 is substantially rectangular in shape with a curved surface 36 matching the curvature of the outer diameter of the slip cage 30. The surface 36 has grooves or teeth designed to bite into a metal surface in a wellbore and may be considered as a gripping surface 36. An underside, e.g. inner side, of the slips 12 has a surface 48 complimentary to the outer surface 50 of the cones 28a, b.

[0058] The slip cage 30 has a ledge 38 which abuts the end face 40 of a cone 28b and, with an end piece 42 of the outer body 24, forms a housing 44 containing a spring 46. Spring 46 is biased to separate the ledge 38 and end piece 42, so that the slips 12 rest upon the cones 28a, b and the surfaces 48,50 abut. In this position the slips 12 sit within the recess 26 and lie within an outer diameter of the tool 10, defined by the outer diameter of the slip cage 30. This position, as illustrated in Figures 1(a)-(c), may be considered as an unset position since the gripping surface 36 is prevented from contacting any surfaces or objects inside the wellbore.

[0059] The slips 12 are initially held in the unset position by virtue of a spring pin connector 52 between the mandrel 22 and the slip cage 30 located below the slips 12. The spring pin connector 52 initially holds the slip cage 30 to the mandrel 22. The spring pin connector 52 initially prevents movement of the slips 12 relative to the mandrel 22. The mandrel 22 also has a shoulder 54 at the lower end 18, which can support the slip cage 30 and limit its movement down the tool 10. Thus, the shoulder 54 prevents movement over the slips 12 down the tool 10 relative to the mandrel 22.

[0060] Continuing to the mid-section of the tool 10, shown in Figure 1(b), the mandrel 22 provides a series of stacked pistons 56. Although only three are shown there may be any number. A complimentary set of pistons 58 are attached to the outer body 24. Of each complimentary pair of pistons 56, 58, the piston 56 is towards an upper end 84 of the tool 10 and the piston 58 is towards a lower end 18 of the tool 10. The pistons 56, 58 of each complimentary pair of pistons 56, 58 are initially spaced apart, e.g. when the slips 12 are in the unset position. The pistons 56, 58 are configured to move relative to each other. The pistons 56,58 interlink and provide piston and end stops depending upon the relative movement of the outer body 24 and the mandrel 22. The mandrel 22 includes ports 60, providing a fluid passageway between a central through bore 62 of the mandrel 22 and a chamber 64 formed between the underside 68, e.g. lower or downhole side in use, of the pistons 58 and an upper side 66, e.g. upper or uphole side in use, of the pistons 56. The chambers 64 may be referred to as underside chambers. Conduits 70 between the upper side 72 of the pistons 58 and a chamber 76, formed between the upper side 72 of the pistons 58 and an underside 74 of the pistons 56, provide fluid passageways between adjacent chambers 76. The chambers 76 may be referred to as upperside chambers. The conduits 70 are provided through the pistons 58 and the outer body 24. Seals 78 are provided to otherwise seal the chambers 64,76 from each other while the pistons 56,58 move. Seals 78 are provided between the pistons 56 and the outer body 24, and between the pistons 58 and the mandrel 22. An input conduit 80 through the outer body 24 delivers fluid to the first chamber 76a, e.g. the uppermost chamber 76a, from a valve arrangement 82 forming part of the emergency release mechanism 16 at an upper end 84 of the tool 10.

[0061] Valve arrangement 82 is shown in Figure 1(a), illustrating the upper end 84 of the tool 10, in the unset position. Within the inner bore 86 of the tool 10 is a sleeve 88 including a drop ball seat 90 at a lower end thereof located coaxially with the tool bore 86 and mandrel bore 62. Sleeve 88 is fixed to the outer body 24 at an upper end. Sleeve 88 is provided with two sets of ports 92,94 through the sleeve body. Between the sleeve 88 and the outer body 24 there is located a valve member 96. Valve member 96 is initially held to the sleeve 88 by a shear pin 98. Valve member 96 includes a port 99 which leads to a recess 97 on an outer radial surface of the valve member 96. A further port 95 is located through the outer body 24 to radially exit the tool 10 from the valve member 96. Input conduit 80 is directed through the outer body 24 to the valve member 96. The input conduit 80 extends between the recess 97 and the first chamber 76a. In the unset position, the shear pin 98 holds the valve member 96 in a position such that two sets of ports 92,94 through the sleeve 88 are blocked by the body of the valve member 96. Additionally, a fluid flow path is set-up from the input conduit 80 along the recess 97 in the valve member 96 and into the port 95 to radially exit the tool 10. In the unset position, the valve member 96 prevents fluid flow between the inner bore 86 and the first chamber 76a, and permits fluid flow between the first chamber 76a and outside of the tool 10. At the upper end 84, the outer body 24 forms a connection 20b to locate the tool 10 in a work string or pipe string (not shown). Connection 20b may be one part of a standard box and pin section as is known in the art being complimentary to the connection 20a at the lower end 18 of the tool 10.

[0062] In use, the tool 10 is configured in the unset position for run in. Shear pin 98 holds the valve member 96 to the outer body 24 and the valve member 96 blocks the passage of fluid from the inner tool bore 86 entering the valve arrangement 82. The slips 12 are lying in the recess 26 against the cones 28 so that the gripping surfaces 36 of the slips 12 lie within an outer diameter of the tool 10. The slip cage 30 abuts the shoulder 54 and the spring pin connector 52 is engaged to hold the slip cage 30 to the mandrel 22. The underside 68 of pistons 58 will abut the upper side 66 of pistons 56, so that chambers 64 are at their smallest dimensions. The chambers 76 are at their largest dimensions.

[0063] Once the tool 10 is run into the wellbore and positioned at the object 93 (shown on Figure 3), a valve (not shown) is operated further down the work string to close the inner bore 86 which passes through the tool 10. Fluid is then pumped from surface through the inner bore 86 to a first mechanism 14 used to set the slips 12. Fluid will enter the ports 60 in the mandrel 22 at the pistons 56,58. Chambers 64 will expand separating the underside 68 of pistons 58 from the upper side 66 of pistons 56. Pistons 58 will move upwards relative to the complimentary pistons 56. As chambers 64 get bigger, chambers 76 will reduce with the fluid in these chambers escaping through upper chambers 76 and through the input conduit 80, along the recess 97 to vent out of the tool via port 95. Accordingly, the fluid will act on the area of the undersides 68 over the multiple pistons 58 creating a significant force to move the outer body 24 relative to the mandrel 22. This force will pull the cones 28a, b up under the slips 12, forcing them radially outwards until the gripping surface 36 engages the surface 91 of the object 93. This can be seen in Figure 3 where it shows that the ledge 38 and end piece 42 have come together thereby reducing the size of the housing 44 and contracting the spring 46.

[0064] Continued fluid pressure is applied until the pistons 58 bottom-out, as illustrated in Figure 4(a). In each complimentary pair of pistons 6, 58, the upper side 72 of piston 58 now abuts the underside 74 of piston 56. Chambers 64 are at their largest dimensions. The chambers 76 are at their smallest dimensions. Referring to Figure 4(b), with the slips 12 now gripping the object 93, the spring pin connector 52 is disengaged so that the mandrel 22 is free from the slip cage 30 and other tools on the work string can perform tasks with the slips 12 anchored to the object 93. The tool 10 is now in the set position. The object 93 may be tubing such as casing or liner in a wellbore and the slips 12 may be part of a packer. Alternatively, the object 93 may be an object to be fished from a wellbore such as a section of casing or liner. In this arrangement the slips 12 will act as a spear. Still further, the object may be an outer casing and the slips 12 form the anchor arrangement for a downhole pulling tool.

[0065] When it is time to unset the slips 12, pump pressure is first reduced and the weight is set down from surface. The contracted spring 46 will expand and separate the ledge 38 from end piece 42, thereby pulling the cones 28a, b back along the slips 12 and drawing them into the recess 26. Ideally, this would be as shown in Figure 1(c) and the tool 10 can be pulled out of the hole as the slips 12 are free of the surface 91 of the object 93. However, in the set position (Figure 4(b)) there is space 89 behind the slips 12 in which debris in the form of fine particulate matter entrained in the wellbore fluids or laying on the side of the wellbore can settle out and collect during operation of the work string in the wellbore. As a result, when the spring 46 expands, there is insufficient return spring force to move the slips 12 back into the recess 26 and they will only partially or possibly not retract. If the slips 12 are not fully retracted, the gripping surface 36 still sits proud from the outer diameter of the slip cage 30 and tool 10. When the work string is pulled from the well, the slips 12 may catch on other objects in the well. Inadvertent catching may result on the slip 12 being pulled more fully into the extended position and gripping fast on the other object. The work string is now stuck and repeated attempts to free it, for instance by reversing the direction of travel of the work string may be unsuccessful, and at worst resulting in the work string becoming stuck in the wellbore.

[0066] If this occurs, the present invention provides use of an emergency release mechanism 16. The upper end 84 of the tool 10 will be arranged as shown in Figure 1(a). An activation drop ball 87 is pumped from surface down the tool inner bore 86 to land in the drop ball seat 90. This blocks the bore 86 and forces fluid through the ports 92, 94. Continued pumping causes the pin 98 to shear and release the valve member 96 which moves downwards relative to the sleeve 88 until ledge 85 on the valve member 96 abuts lip 83 on the sleeve 88 whereupon its position is fixed. This is as illustrated in Figure 5. Movement of the valve member 96 has blocked port 95. Port 94 is now aligned with port 99, recess 97 and conduit 80 to provide a fluid passageway from the inner tool bore 86 above the drop ball 87 into the conduit 80 towards the pistons 56,58. The pistons 56,58 will be in the configuration shown in Figure 4(a). Fluid will now enter chamber 76a and via conduits 70 into all the chambers 76. The pressure in the chambers 76 will increase and move the pistons 58 downwards relative to the complimentary pistons 56. Fluid in the opposite chambers 64 will exhaust through ports 60 downhole in the throughbore 62. As the pistons 58 are attached to the outer body 24 and the cones 28a, b, these will be forced downwards relative to the static slips 12 from the position shown in Figure 4(b). Due to a portion of the slips 12 being contained in rails 34 on the outer body 24 (see Figure 2), the slips 12 will be pulled away from the surface 91 of the object 93. This will be achieved with significant force due to the combined surface areas of the upper sides 72 of the pistons 58. This force is greater than the return force of the spring 46. The cone 28b will contact ledge 38 of the slip cage 30 and force it downwards so that the sprung pin connector 52 is reengaged and the slip cage 30 is supported on the ledge 54 of the mandrel 22. The ledge 54 of the mandrel 22 can resist further downward movement of the slip cage 30 and the slips 12. This can facilitate any further relative movement between the cones 28a, b and the slips 12 to fully return the slips 12 to the unset position. This is as illustrated in Figure 1 (c). Accordingly, the slips 12 are now free from the object 93 and their surfaces 36 sit within the pockets 32 of the slip cage 12. The tool 10 can now be safely pulled from the wellbore without risk of sticking. It is noted that the force available via the hydraulic emergency release mechanism 16 is many times that available from the spring 46 and is thus much more likely to be able to move the slips against the obstructing debris.

[0067] The emergency release mechanism 16 is considered for use in emergencies only as it necessitates permanent blocking of the main throughbore 86 of the tool 10. Such action is likely to render any other functions unable to be performed except retrieval of the work string.

[0068] The first mechanism 14 used to initially set and unset the slips 12 need not be as described with respect to the figures herein as this is only one example of a mechanism to set slips. Those skilled in the art will recognise that other arrangements can be used. Other embodiments can also be realised, for example, there may be: a plurality of conduits 80 arranged to deliver fluid to the piston 58 at chamber 76; an odd or even number of slips 12; any number of pistons 58 but it is advantageous to have multiple pairs of pistons to deliver a greater emergency release force if required; a piston 58 only used for the emergency release mechanism though it may be advantageous to use the same piston 58 to assist in setting the slips initially. In embodiments for which the action of pulling on the work string will increase setting of the slips, the present invention provides most benefit.

[0069] An advantage of the apparatus and method is to release a slip which has become stuck in a wellbore as a result of failure to unset when required.

[0070] A further advantage is to provide an emergency release mechanism for a slip which can use parts of the mechanism used to initially set the slips.

Claims

CLAIMS1. A downhole tool comprising: a plurality of slips for engaging with a first surface in a wellbore, the slips configured and / or operable to reversibly move between an unset position wherein the slips locate within a recess of the downhole tool so that a gripping surface of the slips lies within an outer diameter of the downhole tool and a set position wherein the slips are displaced to extend the gripping surface beyond the outer diameter so as to engage the first surface; a first mechanism for moving the slips between the unset position and the set position; and an emergency release mechanism configured and / or operable to move the slips to the unset position, the emergency release mechanism comprising: a drop ball seat in a bore of the downhole tool; and one or more emergency release pistons operable by the action of a drop ball landing in the drop ball seat so as to create a release force against the slips and move them towards the unset position.

2. The downhole tool according to claim 1 , wherein the downhole tool includes a spring arranged to act on the first mechanism to provide an initial return force to move the slips from the set to the unset position, wherein the release force is greater than the initial return force.

3. The downhole tool according to claim 1 or claim 2, wherein the first mechanism is a reversible mechanism and the emergency release mechanism is used in the event that reversal of the first mechanism fails to move the slips to the unset position.

4. The downhole tool according to claim 1 , 2 or 3, wherein the emergency release mechanism includes a valve configured and / or operable to be opened by the action of the drop ball, the valve configured and / or operable to allow fluid to pass from the bore above the drop ball seat to a first side of the one or more emergency release pistons.

5. The downhole tool according to any preceding claim, wherein the fluid operates the one or more emergency release pistons.

6. The downhole tool according to claim 5, wherein there is a plurality of emergency release pistons and the fluid acts on the plurality of emergency release pistons to multiply up the force available to release the slips.

7. The downhole tool according to any preceding claim, wherein the first mechanism includes one or more cones which are configured and / or operable to move under the slips to set the slips, and wherein the one or more emergency release pistons are configured and / or operable to act on the one or more cones to release the slips.

8. The downhole tool according to any preceding claim, wherein the first mechanism includes one or more operating pistons configured and / or operable such that movement of the one or more operating pistons sets the slips.

9. The downhole tool according to claim 8, wherein the slips are configured and / or operable to be set by fluid acting on the one or more operating pistons.

10. The downhole tool according to claim 9, wherein the one or more emergency release pistons and the one or more operating pistons are stacked and the fluid acts in a first direction on the one or more operating pistons to set the slips and in a second opposite direction on the one or more emergency release pistons to release the slips.

11. The downhole tool according to claim 10, wherein the downhole tool includes a first fluid flow path through the tool to set the slips and a second fluid flow path through the tool to operate the emergency release mechanism to release the slips.

12. The downhole tool according to claim 9, wherein the one or more emergency release pistons and the one or more operating pistons are the same one or more pistons, and wherein the fluid acts in a first direction on the one or more pistons to set the slips and in a second opposite direction on the one or more pistons to release the slips.

13. The downhole tool according to claim 12, wherein the first mechanism comprises a first fluid flow path through the tool to set the slips and the emergencyrelease mechanism comprises a second fluid flow path through the tool to release the slips.

14. The downhole tool according to claim 13, wherein the emergency release mechanism comprises a third fluid flow path, wherein the third fluid flow path is a vent flow path.

15. The downhole tool according to any preceding claim, wherein the downhole tool comprises or takes the form of a packer and includes one or more sealing elements for engaging the wellbore.

16. The downhole tool according to any preceding claim, wherein the downhole tool comprises or takes the form of a spear with the slips anchoring on tubing to facilitate removal of the tubing from a wellbore.

17. The downhole tool according to any preceding claim, wherein the downhole tool comprises or takes the form of a jack configured and / or operable to anchor in a wellbore casing.

18. A method of releasing slips on a downhole tool, the downhole tool according to any of claims 1 to 17, the method comprising the steps: dropping the drop ball into a through bore of the downhole tool to seat the drop ball in the drop ball seat and seal the through bore; and pressurising up behind the drop ball to operate the one or more emergency release pistons so as to create a release force against the slips and move the slips into the unset position.

19. The method according to claim 18, wherein pressuring up behind the drop ball moves a member in the emergency release mechanism, wherein movement of the member opens a conduit, and wherein the method includes the step of pumping fluid against the drop ball, the fluid entering the conduit and being directed to act on the emergency release piston.

20. An emergency release mechanism for a downhole tool comprising slips configured and / or operable to reversibly move between an unset position and a set position, the emergency release mechanism comprising: a drop ball seat; and one or more emergency release pistons operable by the action of a drop ball landing in the drop ball seat so as to create a release force against the slips and move them towards the unset position.

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