RETRIEVAL TOOL STRING

Description

The present invention relates to a retrieval tool string for retrieving a fish with a profile facing a top of a well having a well tubular, where the fish is a well component intentionally or unintentionally left in a wellbore such as a plug, a straddle or a tool. The invention also relates to a retrieval method for retrieving a fish in a well.

A well component intentionally or unintentionally left in a wellbore such as a plug, a straddle or an intervention tool is called a fish. Such fish can be more or less stuck in the well. Plugs are intentionally firmly fastened in the well and can require high force to pull and release again. Lost tools can be much easier to fish as they merely require “collecting”, but such lost tools may be quite long. When run on wireline, known fishing tools have limited use when fishing either a long fish or fishing a highly stuck fish in the horizontal part of the well since the tool string is restricted in length as it needs to fit into a lubricator which is a long, high-pressure pipe fitted to the top of a wellhead or Christmas tree so that tools may be put into a high-pressure well.

In order to provide sufficient pulling force for releasing the stuck fish, the fishing tool needs to be connected with a powerful pulling tool which itself is quite long as the pulling has its own motor, pump and anchor section besides the hydraulic section in order to be able to provide a high pulling force on wireline. Furthermore, the fishing tool also needs to be connected with a conveyance tool to be able to propel itself in the horizontal part of the well. Thus, wireline fishing tools either cannot fish very long fish or cannot fish highly stuck fish in the horizontal part of the well. The reason is that wireline tools need to fit into the lubricator together with the long fish or the tool section for conveyance and pulling.

It is an object of the present invention to wholly or partly overcome the above disadvantages and drawbacks of the prior art. More specifically, it is an object to provide an improved wireline fishing tool string able to fish long fish and/or highly stuck fish in a horizontal part of the well.

The above objects, together with numerous other objects, advantages and features, which will become evident from the below description, are accomplished by a solution in accordance with the present invention by a retrieval tool string for retrieving a fish with a profile facing a top of a well having a well tubular, where the fish is a well component intentionally or unintentionally left in a wellbore such as a plug, a straddle or a tool, the retrieval tool string comprising:

• • a wireline fishing tool having a tool extension and comprising:
    • a wireline connection for connection to a wireline,
    • an electric motor powered by the wireline,
    • a first tool part and a second tool part, the first tool part being nearest the wireline connection and comprising the electric motor, and the second tool part comprising a fish profile engagement device for engaging the profile of the fish, wherein the first tool part and the second tool part have a disconnection arrangement for disconnecting the second tool part in the well.

By having this disconnectability, the tool string needed for pulling the stuck fish in the horizontal part of the well can be longer than the tool for just collecting the hung-off fishing tool part and bringing it out of the well. When a fish is stuck in a horizontal part of the well, a conveyance tool is needed to transport the tool string to the location of the fish. When run on wireline, such conveyance tool, e.g. a downhole tractor, is quite long as it needs to bring its own motor and pump to drive the conveyance part, such as the wheels and wheel arms. Furthermore, in order to pull the stuck fish, a pulling tool, such as a stroking tool generating an axial force, and an anchoring unit are needed, and when run on wireline such pulling tool and anchoring unit are quite long as they need to bring their own motor and pump to deliver the pulling force. Thus, the total length of the tool string is near the maximum length of the lubricator, which is a long, high-pressure pipe fitted to the top of a wellhead or Christmas tree so that tools may be put into a high-pressure well. When having pulled the fish, the tool string may be too long to re-enter the lubricator, and in prior art solutions, such operations then cannot be performed on wireline. When having the solution provided by the retrieval tool string, the lower part of the primary tool in the form of the wireline fishing tool can be disconnected, the secondary tool being the pulling tool, etc., and then a shorter secondary tool can be run in the well and connected to the wireline fishing tool, after which the fish can be brough out of the well as the tool string with the shorter secondary tool is shorter so that the whole tool string and the fish can fit into the lubricator. In that way, the fishing of long fish or very stuck fish is possible on wireline.

Also, the second tool part may further comprise an anchoring part movable from a retracted position to a projected position in which the anchoring part abuts an inner face of the tubular for anchoring the second tool part.

In addition, the second tool part may further comprise an activation element movable axially along the tool extension for activating the anchoring part to move from the retracted position to the projected position.

Furthermore, in the retracted position of the anchoring part, the anchoring part may be retracted in the second tool part.

Moreover, the anchoring part may not extend beyond the outer boundary of the second tool part.

Additionally, the anchoring part may be radially projectable.

Further, the anchoring part may have an inclining face facing an outer face of the activation element.

Also, the movement of the activation element may be performed by an electrical actuator or a pulling tool.

Furthermore, the pulling tool may comprise the motor driving a pump for providing pressurised fluid to move a piston in a piston chamber for driving a stroking shaft back and forth for pulling or pushing.

In addition, the electrical actuator may comprise the electric motor rotating a shaft.

Further, the shaft may comprise a threaded section that engages a nut.

Additionally, the nut may engage the first part, which may be connected with the second part, so that when rotating the shaft, the motor moves the second part of the fish profile engagement device in relation to the first part to engage the fish profile engagement device with the profile of the fish.

Moreover. the activation element may be connected to or form part of the second part.

Also, the second tool part may further comprise a control part for controlling a position of the activation element.

Furthermore, the control part may comprise a shear pin, a spring or a ratchet mechanism.

In addition, the fish profile engagement device may comprise a first portion and a second portion, the first portion being movable in relation to the second portion or vice versa to fasten the fish profile engagement device in or around the profile.

Further, the activation element may be connected to the second portion by means of a shear pin, a thread or a ratchet mechanism.

Moreover, the disconnection arrangement may comprise a first part releasably attached to a second part, where the first part is part of the first tool part, and the second part is part of the second tool part.

Also, the nut may engage the first part, which may be connected with the second part, so that when rotating the shaft, the motor moves the activation element via the nut to project the anchoring part.

Furthermore, the first part of the disconnection arrangement may comprise a first engagement device that is part of the first tool part and engages a second engagement device that is part of the second tool part.

Additionally, the first engagement device may be a projectable element, and the second engagement device may be a profile, or the second engagement device may be a projectable element, and the first engagement device may be a profile.

In that way, the first portion of the fish profile engagement device can be moved in relation to the second portion to engage the fish profile engagement device with the fish or release the fish profile engagement device from the fish.

Also, the second portion of the fish profile engagement device may comprise an engagement sleeve and a plurality of latching dogs.

In addition, the anchoring part may extend through an opening in the engagement sleeve.

Moreover, the engagement sleeve may abut the fish when the fish profile engagement device is connected with the fish, and the latching dogs engage the profile of the fish.

Furthermore, the second part of the disconnection arrangement may comprise a third engagement device that is part of the first tool part and engages a fourth engagement device that is part of the second tool part.

Additionally, the third engagement device may be a projectable element, and the fourth engagement device may be a profile, or the fourth engagement device may be a projectable element, and the third engagement device may be a profile.

Also, the first engagement device may engage the second engagement device by means of a shearing element, such as a shear pin.

Further, the second part may comprise a first sleeve and the activation element, the first sleeve being connected to the activation element by means of a shear element, such as a shear pin.

In that way, the motor can move the first sleeve for breaking the shear element in order to release the first tool part from the second tool part and hang off the second tool part of the wireline fishing tool in the vertical part of the well. By having this hang-off ability, the tool string needed for pulling the stuck fish in the horizontal part of the well can be longer than the tool for just collecting the hung-off fishing tool part and bringing it out of the well. The fish and the second tool part can be anchored in the vertical part of the well and collected with the shorter tool string. When having the solution provided by the retrieval tool string, the lower part of the primary tool in the form of the second tool part of the wireline fishing tool can be disconnected after being anchored inside the well, and the secondary tool can then be pulled out and exchanged with a shorter secondary tool that can be run in the well and connected to the wireline fishing tool, after which the fish can be brough out of the well as the tool string with the shorter secondary tool is shorter so that the whole tool string and the fish can fit into the lubricator.

Furthermore, the second portion and the activation element may be fastened by means of a shearing element until reaching a predetermined pulling force away from the fish shearing the shearing element, and the activation element is allowed to move away from the anchoring part, releasing the anchoring part and the second tool part from the tubular.

Moreover, the wireline fishing tool may be a primary tool, and the retrieval tool string may further comprise a secondary tool connecting the wireline fishing tool to the wireline.

Also, the secondary tool may comprise a driving unit for conveying the retrieval tool string down the well, an anchor unit for anchoring the retrieval tool string at a location in the well and/or a stroking unit for providing an axial pulling force on at least the wireline fishing tool.

Finally, the invention relates to a retrieval method for retrieving a fish in a well comprising:

• • connecting a retrieval tool string to a wireline,
  • submerging the retrieval tool string down the well,
  • reaching a fish at a first location in the well,
  • engaging the fish profile engagement device to the profile of the fish,
  • pulling the fish for releasing the fish from the well tubular,
  • moving the retrieval tool string and the fish closer to the top of the well to a second location,
  • anchoring the second tool part in the well tubular at the second location,
  • disconnecting the first tool part from the second tool part,
  • moving the first tool part closer to the top and out of the well, leaving the second tool part at the second location,
  • entering the well,
  • connecting the first tool part to the second tool part,
  • releasing the second tool part from the second location, and
  • pulling the retrieval tool string and the fish out of the well.

A fish may be anything left in a wellbore. It does not matter whether the fish consists of junk metal, a hand tool, a length/part of a drillpipe or drill collars, an intervention tool, a straddle, a plug, or an expensive MWD and directional drilling package. Once the component is lost, it is properly referred to as simply “the fish”.

A fish is thus anything put into the hole, which is accurately measured and sketched, so that appropriate fishing tools can be selected if the item must be fished out of the hole.

The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments and in which:

FIG. 1 is a cross-sectional view of a retrieval tool string having a wireline fishing tool approaching a fish,

FIG. 2 is a cross-sectional view of the retrieval tool string of FIG. 1 in which the wireline fishing tool has landed on the fish,

FIG. 3 is a cross-sectional view of the retrieval tool string of FIG. 2 in which the wireline fishing tool has fastened itself to the fish,

FIG. 4 is a cross-sectional view of the retrieval tool string of FIG. 3 in which the second tool part has been anchored in the well,

FIG. 5 is a cross-sectional view of the retrieval tool string of FIG. 4 in which the first tool part has been moved in relation to the second tool part to disengage the first tool part from the second tool part,

FIG. 6 is a cross-sectional view of the retrieval tool string of FIG. 5 in which the first tool part has been released from the second tool part, leaving the second tool part in the well as a “hung-off” fishing tool part,

FIG. 7 is a cross-sectional view of another retrieval tool string having a wireline fishing tool approaching a fish,

FIG. 8 is a cross-sectional view of the retrieval tool string of FIG. 7 in which the wireline fishing tool has landed on the fish,

FIG. 9 is a cross-sectional view of the retrieval tool string of FIG. 8 in which latching dogs have been moved radially outwards, and the wireline fishing tool is fastened to the fish as shown in FIG. 10,

FIG. 11 is a cross-sectional view of the retrieval tool string of FIG. 10 in which the second tool part has been anchored in the well, and

FIG. 12 is a cross-sectional view of the retrieval tool string of FIG. 10 in which the first tool part has been moved downwards in the well to disengage the first tool part from the second tool part so that the second tool part can be left in the well.

All the figures are highly schematic and not necessarily to scale, and they show only those parts which are necessary in order to elucidate the invention, other parts being omitted or merely suggested.

FIG. 1 shows a retrieval tool string 100 for retrieving a fish 2 with a profile 3 facing a top 51 of a well having a well tubular 101. The fish 2 may be a well component intentionally or unintentionally left in a wellbore such as a plug, a straddle or a tool. The retrieval tool string 100 comprises a wireline fishing tool 1 having a tool extension L. The wireline fishing tool 1 comprises a wireline connection 4 for connection to a wireline 5, an electric motor 6 powered by the wireline 5, a first tool part 7 and a second tool part 8. The first tool part 7 is nearest the wireline connection 4 and comprises the electric motor 6, and the second tool part 8 comprises a fish profile engagement device 9 for engaging the profile of the fish. The first tool part 7 and the second tool part 8 have a disconnection arrangement 10 for disconnecting the second tool part 8 in the well for leaving the second tool part 8 in the well as a “hung-off” fishing tool part, and thus, like a liner hanger, the second tool part 8 forms a fish hanger.

By having this disconnectability, the tool string needed for pulling the stuck fish in the horizontal part of the well can be longer than the tool for just collecting the hung-off fishing tool part and bringing it out of the well. When a fish is stuck in a horizontal part of the well, a conveyance tool is needed to transport the tool string to the location of the fish. When run on wireline, such conveyance tool, e.g. a downhole tractor, is quite long as it needs to bring its own motor and pump to drive the conveyance part, such as the wheels and wheel arms. Furthermore, in order to pull the stuck fish, a pulling tool, such as a stroking tool generating an axial force, and an anchoring unit are needed, and when run on wireline such pulling tool and anchoring unit are quite long as they need to bring their own motor and pump to deliver the pulling force. Thus, the total length of the tool string is near the maximum length of the lubricator, which is a long, high-pressure pipe fitted to the top of a wellhead or Christmas tree so that tools may be put into a high-pressure well. When having pulled the fish, the tool string may be too long to re-enter the lubricator, and in prior art solutions, such operations then cannot be performed on wireline. When having the solution provided by the retrieval tool string 100, the lower part of the primary tool in the form of the wireline fishing tool 1 can be disconnected, the secondary tool being the pulling tool, etc., and then a shorter secondary tool can be run in the well and connected to the wireline fishing tool 1, after which the fish can be brough out of the well as the tool string 1 with the shorter secondary tool is shorter so that the whole tool string 100 and the fish can fit into the lubricator. In that way, the fishing of long fish or highly stuck fish is possible on wireline.

In order to not just leave the disconnected second tool part 8 in the horizontal part of the well, the second tool part 8 further comprises an anchoring part 11 for being able to anchor the second tool part 8 in the well tubular 101. The anchoring part 11 is thus movable from a retracted position, shown in FIG. 2, to a projected position, shown in FIG. 4, in which the anchoring part 11 abuts an inner face of the tubular 101 for anchoring the second tool part 8. In the retracted position of the anchoring part 11, the anchoring part 11 is retracted in the second tool part 8.

Thus, the anchoring part 11 does not extend beyond the outer boundary of the second tool part 8. The second tool part 8 further comprises an activation element 12 movable axially along the tool extension L for activating the anchoring part 11 to move from the retracted position to the projected position, shown in FIG. 4. As shown in FIG. 1, the activation element 12 is a wedging element wedging underneath the anchoring part 11 for pressing the anchoring part 11 radially outwards, and the anchoring part 11 may thus be radially projectable. The anchoring part 11 has an inclining face 42 facing an outer face 43 of the activation element. The movement of the activation element 12 may be performed by an electrical actuator 44. In another embodiment, the activation element may be moved by a pulling tool (not shown). The pulling tool comprises a motor 6 driving a pump for providing pressurised fluid to move a piston in a piston chamber for driving a stroking shaft back and forth for pulling or pushing. The electrical actuator 44 comprises the electric motor 6 rotating a shaft 17B. The shaft 17B comprises a threaded section that engages a nut 25 for converting the rotational energy of the motor 6 to an axial movement along the tool extension L. Keys 27 are positioned in two slots 47 and thus prevent the nut 25 from rotating along with the rotating shaft 17B. The disconnection arrangement 10 comprises a first part 14 releasably attached to a second part 15, where the first part 14 is part of the first tool part 7, and the second part 15 is part of the second tool part 8. The nut 25 engages the first part 14, which is connected with the second part 15, so that when rotating the shaft 17B the motor 6 moves a second portion 17 of the fish profile engagement device 9 in relation to a first portion 16 to engage the fish profile engagement device 9 with the profile of the fish, as shown in FIG. 3. The activation element 12 is connected to or forms part of the second part 15.

After having activated the anchoring part 11 by moving the activation element 12 and pressing the anchoring part 11 radially outwards as shown in FIG. 4, the anchoring part 11 needs to be maintained in this position, and therefore the second tool part 8 further comprises a control part 18 for controlling a position of the activation element 12. The control part 18 comprises a shear pin 48, a spring and/or a ratchet mechanism. The fish profile engagement device 9 comprises the first portion 16 and the second portion 17, the first portion 16 being movable in relation to the second portion 17 or vice versa to fasten the fish profile engagement device 9 in or around the profile, as shown in FIGS. 2 and 3 and FIGS. 9 and 10. The activation element 12 may thus be connected to the second portion 17 by means of a shear pin, a thread or a ratchet mechanism. The nut 25 engages the first tool part 7, which is connected with the second tool part 8, so that when rotating the shaft 17B the motor 6 moves the activation element 12 towards the fish via the nut 25 to project the anchoring part 11, as shown in FIGS. 4 and 11.

The first part 14 of the disconnection arrangement 10 comprises a first engagement device 31 that is part of the first tool part 7 and engages a second engagement device 32 that is part of the second tool part 8. The first engagement device 31 is a projectable element, and the second engagement device 32 is a profile, as shown in FIGS. 1-6. In FIGS. 7-12, the second engagement device 32 is a projectable element, and the first engagement device is a profile.

In that way, the first portion 16 of the fish profile engagement device 9 can be moved in relation to the second portion 17 to engage the fish profile engagement device 9 with the fish or release the fish profile engagement device 9 from the fish if needed, should the fishing operation fail and the fishing tool 1 need to be pulled out of the well.

In FIG. 1, the second portion 17 of the fish profile engagement device 9 comprises an engagement sleeve 28 and a plurality of latching dogs 9. The anchoring part 11 extends through an opening 30 in the engagement sleeve 28 as shown in FIG. 4. The engagement sleeve 28 abuts the fish when the fish profile engagement device 9 is connected with the fish, and the latching dogs 9 engage the profile of the fish.

As shown in FIGS. 1-6, the second part 15 of the disconnection arrangement 10 comprises a third engagement device 31b that is part of the first tool part 7 and engages a fourth engagement device 32b that is part of the second tool part 8. The third engagement device 31b may be a projectable element, and the fourth engagement device 32b may be a profile, or the fourth engagement device 32b may be a projectable element, and the third engagement device 31b may be a profile.

In FIG. 1, the retrieval tool string 100 is approaching a fish. The first tool part 7 and the second tool part 8 are connected from surface by means of the disconnection arrangement 10 with a double engagement in the form of the first engagement device 31 that is part of the first tool part 7 and engages a second engagement device 32, and a second engagement where the third engagement device 31b engages the fourth engagement device 32b. The first engagement device 31 and the second engagement device 32 are thus able to hold the first portion 16 while the third engagement device 31b and the fourth engagement device 32b engage the outer part of the tool 1 for moving the second portion 17 in order to move the latching dogs 9 radially outwards as shown in FIG. 3. This movement may be performed by the motor 6, the shaft 17B and the nut 25. By rotating the shaft 17B even more, the nut 25 moves the first part 14 and the second part 15, thereby moving the activation element 12 along the axial extension L of the tool 1, pressing the anchoring part 11 radially outwards as shown in FIG. 4. In FIG. 5, a continuous movement moves a sleeve 41 of the second part 15 in relation to the activation element 12, shearing a shear pin 46 to disengage the first tool part 7 from the second tool part 8. In FIG. 6 the first tool part 7 has been released from the second tool part 8, leaving the second tool part 8 in the well as a “hung-off” fishing tool part. Thus, by a very simple axial movement, the second tool part 8 is hung off as the anchoring part 11 is activated, the second tool part 8 is disconnected and left in the well until a shorter tool string (second tool not shown) enters and collects the second tool part 8 by releasing the anchoring parts 11 and pulling the tool string 100 out of the well.

In FIG. 7, the retrieval tool string 100 is approaching a fish. At the surface, the second tool part 8 is connected by means of the disconnection arrangement 10 with a double engagement in the form of the first engagement device 31, which engages the second engagement device 32, and a second engagement where the third engagement device 31b engages the fourth engagement device 32b. The second engagement device 32 is dogs engaging the first engagement device 31, which is a profile. The first engagement device 31 and the second engagement device 32 are thus able to hold the first portion 16 while the third engagement device 31b and the fourth engagement device 32b engage the outer part of the tool 100 for moving the second portion 17 to move the latching dogs 9 radially outwards as shown in FIG. 9. Again, the motor 6 moves the latching dogs 9 from the position in FIG. 8 to the position in FIG. 9, fastening the fish 2 as shown in FIG. 10 in that the outer diameter of the latching dogs 9 is larger than the inner diameter of the opening in the fish. By further rotation of the shaft 17B which is not visible as it is inside the housing, the sleeve 41 is moved towards the fish 2, the shear pin 18 breaks, and the activation element 12 moves the anchoring part 11 into engagement with the well tubular 101 so that the second tool part 8 has been anchored in the well, as shown in FIG. 11. In FIG. 12, the first tool part 7 has been moved downwards in the well by a stroking tool (not shown), and the second tool part 8 is held stationary by the anchoring part 11 to disengage the first tool part 7 from the second tool part 8 so that the second tool part 8 can be left in the well and e.g. be fetched by a second tool (not shown). Thus in FIG. 12, the second engagement device 32 has moved to be opposite a cavity 33 so that the second engagement device 32 is free to move radially outwards and free of the first engagement device 31, which is a groove or profile. Thus, the first part 14 is released and free to move away from the second part 15. The part 17c of the second portion 17 is fastened to the part 17d of the latching dogs 9 by means of a thread, and the second portion 17 is held together with the first portion 16 by means of a friction element such as a snap ring. In FIG. 7, the parts having no. 16, 16b and 16a is always moving together as they are fixedly connected.

In another solution not shown, the first engagement device 31 engages the second engagement device 32 by means of a shearing element, such as a shear pin, instead of a projectable element engaging a profile.

In FIGS. 7-12, the second part 15 comprises the first sleeve 41 and the activation element 12, where the first sleeve 41 is connected to the activation element 12 by means of a shear element 18, such as a shear pin. In that way, the motor 6 can move the first sleeve 41 for breaking the shear element in order to release the first tool part 7 from the second tool part 8 and hang off the second tool part 8 of the wireline fishing tool 1 in the vertical part of the well. By having this hang-off ability, the tool string needed for pulling the stuck fish, in a first run, in the horizontal part of the well can be longer than the tool for just collecting the hung-off fishing tool part and bringing it out of the well in a second run. The fish and the second tool part 8 can be anchored in the vertical part of the well and collected with the shorter tool string 100, i.e. a secondary tool (not shown). When having the solution provided by the retrieval tool string 100, the lower part of the primary tool in the form of the second tool part 8 of the wireline fishing tool 1 can be disconnected after being anchored inside the well, and the first tool part can then be pulled out and exchanged with a shorter secondary tool that can be run in the well and connected to the wireline fishing tool 1, after which the fish can be brough out of the well as the tool string 100 with the shorter secondary tool is shorter so that the whole tool string 100 and the fish can fit into the lubricator.

As shown in FIGS. 1-6, the second portion 17 and the activation element 12 are fastened by means of a shearing element 48B until reaching a predetermined pulling force away from the fish shearing the shearing element 48B, and the activation element 12 is allowed to move away from the anchoring part 11, releasing the anchoring part and the second tool part 8 from the tubular 101, i.e. when the longer tool string has been pulled out of the well, and the shorter tool string has been entered and connected to the second tool part 8 in order to release the second tool part 8 from the well and bring the fish to surface.

The wireline fishing tool 1 can be considered a primary tool, and the retrieval tool string 100 further comprises a secondary tool connecting the wireline fishing tool 1 to the wireline. The secondary tool comprises a driving unit for conveying the retrieval tool string 100 down the well, an anchor unit for anchoring the retrieval tool string 100 at a location in the well and/or a stroking tool, also called a pulling tool, for providing an axial pulling force on at least the wireline fishing tool 1.

The retrieval method for retrieving a fish in a well thus comprises connecting a retrieval tool string 100 to a wireline, submerging the retrieval tool string down 100 the well, reaching a fish at a first location in the well, engaging the fish profile engagement device 9 to the profile of the fish, pulling the fish for releasing the fish from the well tubular 101, moving the retrieval tool string 100 and the fish closer to the top of the well to a second location, anchoring the second tool part 8 in the well tubular at the second location, disconnecting the first tool part 7 from the second tool part 8, moving the first tool part 7 closer to the top and out of the well, leaving the second tool part 8 at the second location, entering the well, connecting the first tool part 7 to the second tool part 8, releasing the second tool part 8 from the second location, and pulling the retrieval tool string 100 and the fish out of the well.

A stroking tool or a pulling tool is a tool providing an axial force along the axial extension of the tool string. The stroking tool comprises an electric motor for driving a pump. The pump pumps fluid into a piston housing to move a piston acting therein. The piston is arranged on the stroker shaft. The pump may pump fluid out of the piston housing on one side and simultaneously suck fluid in on the other side of the piston.

By “fluid” or “well fluid” is meant any kind of fluid that may be present in oil or gas wells downhole, such as natural gas, oil, oil mud, crude oil, water, etc. By “gas” is meant any kind of gas composition present in a well, completion or open hole, and by “oil” is meant any kind of oil composition, such as crude oil, an oil-containing fluid, etc. Gas, oil and water fluids may thus all comprise other elements or substances than gas, oil and/or water, respectively.

By “casing”, “well tubular” or “well tubular metal structure” is meant any kind of pipe, tubing, tubular, liner, string, etc., used downhole in relation to oil or natural gas production.

In the event that the tool is not submergible all the way into the casing, a driving unit such as a downhole tractor can be used to push the tool all the way into position in the well. The downhole tractor may have projectable arms having wheels, wherein the wheels contact the inner surface of the casing for propelling the tractor and the tool forward in the casing. A downhole tractor is any kind of driving tool capable of pushing or pulling tools in a well downhole, such as a Well Tractor®.

Although the invention has been described above in connection with preferred embodiments of the invention, it will be evident to a person skilled in the art that several modifications are conceivable without departing from the invention as defined by the following claims.