Downhole tool and method of releasing slips on a downhole tool

Abstract

A downhole tool comprising a plurality of slips for engaging with a first surface in a well bore. The slips are configured to reversibly move between an unset position and a set position, where the sl

Description

The present invention relates to apparatus and methods for operating downhole tools and in particular, though not exclusively, to an emergency release mechanism to positively 5 unset slips on a tool to ensure their release so that the tool can be disengaged and removed from a wellbore. 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 10 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. 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 15 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 20 retract. In many wells there is debris present in the form of fine particulate matter entrained in the wellbore fluids or laying on the 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 25 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. 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 the work string on which the downhole tool is mounted is moved up or down the well. Inadvertent catching may result in the slip being 30 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 the location of interest in the wellbore. Ultimately there may come a time when the operator concludes that no amount of extra work will free the work string and remedial measures, 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. It is therefore an object of at least one embodiment of the present invention to provide a downhole tool with slips which obviates or mitigates one or more disadvantages of the prior art. It is therefore a further object of at least one embodiment of the present invention to provide a method of operating a downhole tool with slips which obviates or mitigates one or more disadvantages of the prior art. According to a first aspect of the present invention there is provided a downhole tool comprising: a plurality of slips for engaging with a first surface in a well bore, the slips configured 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 and engage the first surface; a first mechanism for moving the slips between the unset position and the set position; and an emergency release mechanism to move the slips to the unset position in the event that the slips fail to return to the unset position in the well bore, the emergency release mechanism comprising a drop ball seat in a bore of the downhole tool, and an emergency release piston operated by the action of a drop ball in the seat to create a release force against the slips and move them towards the unset position. 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. This may be considered as a secondary means of deactivation to positively retract the slips. Preferably, the downhole tool includes a spring arranged to be acted on by the first mechanism to provide an initial return force to move the slips from the set to the unset position and the release force is greater than the initial return force. In this way, the emergency release force is used when the initial return force fails to unset the slips and will provide a greater force which is more likely to be able to move the slips against obstructing debris. Preferably, the emergency release mechanism includes a valve opened by the action of the drop ball, the valve allowing fluid to pass from the bore above the drop ball seat to a first side of the emergency release piston. More preferably the fluid operates the emergency release piston. Preferably the first mechanism is a reversable 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. Preferably, there are 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. The first mechanism may include one or more cones which move under the slips to set the slips and the emergency release piston acts on the one or more cones to release the slips. The first mechanism may include one or more operating pistons whose movement is used to set the slips. More preferably, the slips are set by fluid acting on the operating pistons. In an embodiment, the emergency release pistons and the operating pistons are stacked and the fluid acts in a first direction on the operating pistons to set the slips and in a second opposite direction on the emergency release pistons to release the slips. In this way, there is 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. The downhole tool may be a packer and include a sealing element. Alternatively the downhole tool may be a spear with the slips anchoring on tubing to remove it from a well bore. In a further embodiment the downhole tool may be a jack with the slips anchoring on casing and acting in concert with other downhole tools to apply a force to a stuck object further down the wellbore. According to a second aspect of the present invention there is provided a method of releasing slips on a downhole tool when the slips have failed to unset, the downhole tool according to the first aspect and the method comprising the steps: (a) using the first mechanism to set the slips against the first surface; (b) unsetting the slips and on attempting to pull the downhole tool from the well bore noting that it has stuck; (c) dropping the drop ball down a through bore of a work string to which the downhole tool is attached; (d) seating the drop ball in the drop ball seat to seal the through bore; (e) pressurising up behind the drop ball to move a member in the emergency release mechanism; (f) using movement of the member to operate the emergency release piston to create a release force; (g) directing the release force against the slips to move the slips into the recess and the unset position; and (h) pulling the work string and the downhole tool from the well bore. In this way, a second activation method is provided to move the slips to the unset position. Preferably, at step (f) movement of the member opens a conduit and 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. 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. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figures l(a)-l(c) are part cross-sectional schematic illustrations of (a) upper section, (b) mid-section and (c) lower section of a downhole tool in an unset position according to an embodiment of the present invention; Figure 2 is a sectional view through the line A-A of Figure 1(c); Figure 3 is a part cross-sectional schematic illustrations of the lower section of the downhole tool of Figure 1(c) with the slips extended; Figures 4(a)-4(b) are part cross-sectional schematic illustrations of (a) upper section and (b) lower section of the downhole tool in a set position according to an embodiment of the present invention; and Figure 5 is a part cross-sectional schematic illustrations of the upper section of the downhole tool of Figure 1(a) with the emergency release mechanism activated. 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 between an unset position and the set position; and an emergency release mechanism 16 to move the slips from the set position to the unset position in the event that the slips fail to return to the unset position in the well bore, according to an embodiment of the present invention. At a lower end 18 of the tool 10, Figure 1(c), there is provided a connection 20a to locate the tool 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 outer body 24 provides a recess 26 and a set of cones 28a,b on a base of the recess 26. A 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. 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 of the slips 12 has a surface 48 complimentary to the outer surface 50 of the cones 28a,b. 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 l(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. 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 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. 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 so that 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 of the pistons 58 and an upperside 66 of the pistons 56. Conduits 70 between the upperside 72 of the pistons 58 and a chamber 76, formed between the upperside 72 of the pistons 58 and an underside 74 of the pistons 56, provide fluid passageways between adjacent chambers 76. Seals 78 are provided to otherwise seal the chambers 64,76 from each other while the pistons 56,58 move. An input conduit 80 through the outer body 24 delivers fluid to the first chamber 76a from a valve arrangement 82 forming part of the emergency release mechanism 16 at an upper end 84 of the tool 10. 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. 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. 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. In use, the tool 10 is configured in the unset position for run in. Shear pin 98 is holds the sleeve 88 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 surface 36 of the slips 12 lies 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 upperside 66 of pistons 56, so that chambers 64 are at their smallest dimensions. 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 upperside 66 of pistons 56. As chambers 64 get bigger, chambers 76 will reduce with the fluid in these chambers escaping through upper chambers 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. Continued fluid pressure is applied until the pistons 58 bottom-out, as illustrated in Figure 4(a). The upperside 72 of piston 58 now abuts the underside 74 of piston 56. Referring to Figure 4(b), with the slips 12 now gripping the object 93, the spring pin connector 52 disengages 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 well bore and the slips 12 may be part of a packer. Alternatively, the object 93 may be an object to 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. 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. 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 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 uppersides 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 re-engaged and the slip cage 30 is supported on the ledge 54 of the mandrel 22. 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 well bore 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. 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 the work string. 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; 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. 5 The principal advantage of the present invention is that it provides apparatus and method to release a slip which has become stuck in a well bore as a result of failure to unset when required. A further advantage of the present invention is that it provides an emergency release mechanism for a slip which can use parts of the mechanism used to initially set the slips. 10 07 02 25

Claims

1. A downhole tool comprising:a plurality of slips for engaging with a first surface in a well bore, the slips5 configured to reversibly move between an unset position wherein the slips locatewithin 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 and engage the first surface;10 a first mechanism for moving the slips between the unset position and theset position; andan emergency release mechanism to move the slips to the unset position in the event that the slips fail to return to the unset position in the well bore, the emergency release mechanism comprising a drop ball seat in a bore of the 15 downhole tool, and an emergency release piston operated by the action of a dropball in the drop ball seat to create a release force against the slips and move them towards the unset position,wherein the emergency release mechanism includes a valve opened by the action of the drop ball, the valve allowing fluid to pass from the bore above the 20 drop ball seat to a first side of the emergency release piston.

2. The downhole tool according to claim 1, wherein the downhole tool includes a spring arranged to be acted on by the first mechanism to provide an initial return force to move the slips from the set to the unset position and the release force is greater 25 than the initial return force.

3. The downhole tool according to claim 1 or claim 2, wherein the first mechanism is a reversable 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 30 position.

4. The downhole tool according to any preceding claim, wherein the fluid operates the emergency release piston.35 5. The downhole tool according to claim 4, wherein there are a plurality of emergencyrelease pistons and the fluid acts on the plurality of emergency release pistons to multiply up the force available to release the slips.07 02 256. The downhole tool according to any preceding claim, wherein the first mechanism includes one or more cones which move under the slips to set the slips and the emergency release piston acts on the one or more cones to release the slips.

7. The downhole tool according to any preceding claim, wherein the first mechanismincludes one or more operating pistons whose movement is used to set the slips.

8. The downhole tool according to claim 7, wherein the slips are set by fluid acting on the operating pistons.

9. The downhole tool according to claim 8, wherein the emergency release pistons and the operating pistons are the same pistons and the fluid acts in a first direction on the pistons to set the slips and in a second opposite direction on the pistons to release the slips.

10. The downhole tool according to claim 9, 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.

11. The downhole tool according to any preceding claim, wherein the downhole tool is a packer and includes a sealing element.

12. The downhole tool according to any preceding claim, wherein the downhole tool is a spear with the slips anchoring on tubing to remove it from a well bore.

13. The downhole tool according to any preceding claim, wherein the downhole tool is a jack anchoring in wellbore casing.

14. A method of releasing slips on a downhole tool when the slips have failed to unset, the downhole tool according to any of claims 1 to 13, comprising the steps:(a) dropping the drop ball down a through bore of a work string to which the downhole tool is attached;(b) seating the drop ball in the drop ball seat to seal the through bore;(c) pressurising up behind the drop ball to move a member in the valve of the emergency release mechanism;(d) using movement of the member to operate the emergency release piston to create a release force; and(e) directing the release force against the slips to move the slips into the recess and the unset position.

15. The method of releasing slips on a downhole tool according to claim 14, wherein at step (d) movement of the member opens a conduit and the method includes the step of pumping fluid against the drop ball, the fluid entering the conduit and being5 directed to act on the emergency release piston.LDCM

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