APPARATUS AND METHOD FOR PROVIDING A FLUID SAMPLE IN A WELL

Abstract

Apparatus and a method are provided for bringing a well-fluid sample in a well through a sampling device which is in fluid communication with the apparatus. The apparatus includes: a dump chamber for receiving the well-fluid sample; a first hydraulic piston arranged in a first cylinder housing; a second hydraulic piston arranged in a second cylinder housing; a piston rod for mechanically connecting the first piston to the second piston to form a piston arrangement, the dump chamber being defined by the first piston and a portion of the first cylinder housing. The apparatus further includes at least one holding device arranged to switch between an active position, in which the piston arrangement is held fixed against movement, and an inactive position, in which the piston arrangement is allowed a movement relative to the cylinder housings.

Claims

1. An apparatus for bringing a well-fluid sample in a well through a sampling device which is in fluid communication with the apparatus, the apparatus comprising: a dump chamber for receiving the well-fluid sample; a first hydraulic piston arranged in a first cylinder housing; a second hydraulic piston arranged in a second cylinder housing; a piston rod for mechanically connecting the first piston to the second piston to form a piston arrangement, characterized in that the dump chamber is defined by the first piston and a portion of the first cylinder housing, and that the apparatus further comprises at least one holding device which is arranged to switch between an active position, in which the piston arrangement is held fixed against movement, and an inactive position, in which the piston arrangement is allowed a movement relative to the cylinder housings, said movement of the piston arrangement being brought about by means of a well fluid which, in a position of application, is in fluid communication with the second hydraulic piston so that the movement brings about an increase in a volume of the dump chamber when the holding device is in its inactive position.

2. The apparatus according to claim 1, wherein the area of a piston face of the first piston is larger than the area of the piston face of the second piston.

3. The apparatus according to claim 1, wherein the first cylinder housing is provided with an end plug, wherein the end plug, a piston face of the first piston and a portion of the first cylinder housing define a first cylinder-housing chamber which is arranged to accommodate a pressurized fluid, and wherein the holding device is a valve arranged for selectively bleeding off the pressurized fluid.

4. The apparatus according to claim 1, wherein the second piston is in continuous fluid communication with the well fluid when the apparatus is in its position of application.

5. The apparatus according to claim 1, wherein the area of a piston face of the first piston is smaller than the area of the piston face of the second piston.

6. The apparatus according to claim 5, wherein the second cylinder housing is provided with an end plug, wherein the end plug, the piston face of the second piston and a portion of the second cylinder housing define a second cylinder-housing chamber which is provided with the holding device in the form of a valve which, in its active position, shuts off fluid communication of well fluid into the second cylinder-housing chamber and, in its inactive position, allows fluid communication of well fluid into the second cylinder-housing chamber.

7. The apparatus according to claim 1, wherein the piston rod has a cross-sectional area which is smaller than the area of a piston face of the first piston and the area of a piston face of the second piston, and wherein a chamber wall with an opening complementarily adapted to the piston rod provides a partition between the dump chamber and a partition-wall chamber defined by a portion of the piston rod, the second cylinder housing and the second piston.

8. The apparatus according to claim 7, wherein at least the first cylinder housing is provided with an end plug, wherein the end plug, the piston face of the first piston and a portion of the first cylinder housing define a first cylinder-housing chamber which is arranged to accommodate a pressurized fluid, and wherein the holding device is a valve arranged for selectively bleeding off the pressurized fluid, the valve, in its active position, shutting off fluid communication of the pressurized fluid out of the first cylinder-housing chamber and, in its inactive position, allowing fluid communication of the pressurized fluid out of the first cylinder-housing chamber, and wherein a pressure in the partition-wall chamber is lower than atmospheric pressure at least when a volume in the dump chamber is at a minimum.

9. The apparatus according to claim 8, wherein a second cylinder-housing chamber defined by the second piston face and the second cylinder housing is arranged to receive the well fluid.

10. The apparatus according to claim 9, wherein the second cylinder housing is provided with an end plug which is provided with a holding device in the form of a valve to control the inflow of well fluid into the second cylinder-housing chamber, the valve, in its active position, shutting off fluid communication of well fluid into the second cylinder-housing chamber and, in its inactive position, allowing fluid communication of well fluid into the second cylinder-housing chamber.

11. A system for collecting a well sample from a well, comprising: a sampling device comprising a measuring device provided with at least one sensor; and an apparatus configured to bring a well-fluid sample in a well through the sampling device, wherein the sampling device is in fluid communication with the apparatus, and wherein the apparatus comprises: a dump chamber for receiving the well-fluid sample; a first hydraulic piston arranged in a first cylinder housing; a second hydraulic piston arranged in a second cylinder housing; a piston rod for mechanically connecting the first piston to the second piston to form a piston arrangement, characterized in that the dump chamber is defined by the first piston and a portion of the first cylinder housing, and that the apparatus further comprises at least one holding device which is arranged to switch between an active position, in which the piston arrangement is held fixed against movement, and an inactive position, in which the piston arrangement is allowed a movement relative to the cylinder housings, said movement of the piston arrangement being brought about by means of a well fluid which, in a position of application, is in fluid communication with the second hydraulic piston so that the movement brings about an increase in a volume of the dump chamber when the holding device is in its inactive position.

12. The system according to claim 11, wherein the sensor includes a chromatograph selected from the group of a gas chromatograph, a liquid chromatograph and an ion chromatograph, or a combination thereof.

13. The system according to claim 11, wherein the sampling device comprises a tracer sample chamber which, by means of valves, is arranged to isolate a fluid sample which is drawn from the well by means of the dump chamber, and wherein the sampling device, the valves and a fluid line upstream of the tracer sample chamber is provided with an internal surface which consists of a material which is substantially non-reactive in relation to substances carried from the well into the tracer sample chamber, so that trace elements that are carried into the tracer sample chamber can be brought out of the tracer sample chamber when the well-fluid sample is analysed.

14. The system according to claim 11, wherein the measuring device is placed upstream of the tracer sample chamber, and wherein the tracer sample chamber is arranged to be isolated from the measuring device and the dump chamber by means of valves.

15. The system according to claim 14, wherein the tracer sample chamber includes a portion of a fluid line extending between an outlet portion of the sampling device and the dump chamber.

16. The apparatus according to claim 1, wherein the holding device is a releasable mechanical holding device which, in its active position, is configured to lock the piston arrangement in an initial position in which the volume of the dump chamber is at a minimum.

17. A method of bringing a well-fluid sample through a sampling device (3) in a well, comprising: providing an apparatus configured to bring the well-fluid sample in the well through the sampling device, wherein the sampling device is in fluid communication with the apparatus, and wherein the apparatus comprises: a dump chamber for receiving the well-fluid sample; a first hydraulic piston arranged in a first cylinder housing; a second hydraulic piston arranged in a second cylinder housing; a piston rod for mechanically connecting the first piston to the second piston to form a piston arrangement, characterized in that the dump chamber is defined by the first piston and a portion of the first cylinder housing, and that the apparatus further comprises at least one holding device which is arranged to switch between an active position, in which the piston arrangement is held fixed against movement, and an inactive position, in which the piston arrangement is allowed a movement relative to the cylinder housings, said movement of the piston arrangement being brought about by means of a well fluid which, in a position of application, is in fluid communication with the second hydraulic piston so that the movement brings about an increase in a volume of the dump chamber when the holding device is in its inactive position; arranging the dump chamber with a smallest possible volume and then activating the holding device of the apparatus; bringing the apparatus together with the sampling device to a desired location in the well; and initiating movement of the piston arrangement by deactivating the at least one holding device so that a well fluid sets the piston arrangement in motion in order thereby to increase the volume of the dump chamber and suck the well-fluid sample through the sampling device.

18. The method according to claim 17, wherein the method further comprises stopping the movement of the piston arrangement by reactivating the holding device comprising a valve, so that well fluid is no longer sucked via the sampling device and into the dump chamber.

19. The method according to any one of claim 17, wherein the movement of the piston arrangement is facilitated by means of a fluid in a partition-wall chamber, the fluid having a pressure which is lower than atmospheric pressure at least when the volume of the dump chamber is at a minimum.

20. The method according to claim 17, wherein the dump chamber is arranged with the smallest possible volume before the apparatus is run into the well.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In what follows, an example of a preferred embodiment is described, which is visualized in the accompanying drawings, in which:

[0050] FIG. 1A shows an apparatus according to the present invention just after the piston arrangement of the apparatus has been set in motion by the well pressure and a dump chamber in the apparatus is close to a smallest volume;

[0051] FIG. 1B shows the apparatus of FIG. 1A, but the piston arrangement has been driven by the well pressure so that the volume of the dump chamber has been further increased;

[0052] FIG. 1C shows the apparatus of FIGS. 1A and 1B, the piston arrangement being at its end position and the sucking-in of a well-fluid sample into the dump chamber having ceased;

[0053] FIGS. 2A-2C show an alternative embodiment of the apparatus shown in FIGS. 1A-1C;

[0054] FIGS. 3A-3C show an alternative embodiment of the apparatus shown in FIGS. 1A-1C and FIGS. 2A-2C; and

[0055] FIG. 3D shows the apparatus of FIGS. 3A-3C before it has been run into the well and while the piston arrangement is in the process of being placed the position which is shown in FIG. 3A.

DETAILED DESCRIPTION OF THE INVENTION

[0056] In what follows, positional specifications such as “over”, “under”, “upper”, “lower”, “right” and “left” relate to what is shown in the figures.

[0057] Like or corresponding elements may be indicated by the same reference numeral in the different figures. For the sake of clarity, the reference numerals of some of the elements may have been left out in some of the figures.

[0058] In the figures, the reference numeral 1 indicates an apparatus according to the present invention.

[0059] The apparatus 1 includes a first hydraulic piston 11 movably arranged in a first cylinder housing 13 and a second hydraulic piston 15 movably arranged in a second cylinder housing 17. The first hydraulic piston 11 is mechanically connected to the second hydraulic piston 15 by means of a piston rod 19. The piston rod 19 is thus a connecting means between the first hydraulic piston 11 and the second hydraulic piston 15.

[0060] The apparatus 1 is in fluid communication with a sampling device 3 which is provided with an inlet portion 5 and an outlet portion 7. The inlet portion 5 is in fluid communication with a well fluid, and the outlet portion 7 is in fluid communication, via a fluid line 8, with a dump chamber 9 in the apparatus 1. The fluid line 8 is connected to the apparatus 1 by means of a coupling device 8′ which is attached, for example by means of a threaded connection, to a cut-out in the second cylinder housing 17. The cut-out is in fluid communication with the dump chamber 9 as indicated in the figures.

[0061] The sampling device 3 may be a measuring device 3 comprising at least one sensor. In an alternative embodiment, the sampling device 3 may be a sample chamber, for example a tracer sample chamber. In yet another alternative embodiment, the sampling device 3 may include both a measuring device and a tracer sample chamber. In a particular embodiment, the sampling device 3 includes the measuring device with the at least one sensor, whereas the tracer sample chamber is arranged in, for example defined by, a portion of the fluid line 8.

[0062] In the embodiment in which the sampling device 3 is a tracer sample chamber, this is provided with valves (not shown) arranged for example at the inlet portion 5 and the outlet portion 7. In the embodiment in which the tracer sample chamber is arranged in a portion of the fluid line 8, the fluid line 8 is provided with valves (not shown) arranged at the outlet portion 7 and at the coupling device 8′, for example.

[0063] The description that follows will be directed, to a great degree, towards an embodiment in which the sampling device 3 is a measuring device comprising at least one sensor. However, it will be understood that the apparatus 1 according to the invention could just as well be used for a sampling device including a sample container such as said tracer sample chamber, or a combination of the measuring device and the sample container. The point of the invention is that the well-fluid sample is sucked into and through the sampling device 3 by means of the apparatus 1 according to the present invention.

[0064] The dump chamber 9 is defined by the first piston 11, the first cylinder housing 13 and the portion of the piston rod 19 that is in the first cylinder housing 13 at any time.

[0065] To provide a fluid seal between the pistons 11, 15 and the respective cylinder housings 13, 17, each of the pistons 11, 15 is provided with a seal 11′ and 15′, respectively.

[0066] In the embodiment shown in FIGS. 1A-1C, the area of a piston face 12 of the first piston 11 is larger than the area of a piston face 16 of the second piston 15. In said figures, the piston rod 19 has a cross-sectional area which is equal to the cross-sectional area (and the piston face 16) of the second piston 15.

[0067] In an upper end portion, the first cylinder housing 13 is provided with an end plug 21 attached to it by means of a threaded connection. The end plug 21 is provided with seals 21′ to be able to fonts a fluid-tight connection between the first cylinder housing 13 and the end plug 21. The end plug 21, piston face 12 of the first piston 11 and the portion of the cylinder housing 13 which at all times is defined by said piston face 12 and an end face 21″ of the end plug 21, define a first hydraulic chamber 23.

[0068] The first hydraulic chamber 23 is arranged to accommodate a pressurized fluid such as a hydraulic liquid. The hydraulic liquid is carried into the chamber 23 via the end plug 21.

[0069] The end plug 21 is further provided with a holding device in the form of a valve 25 which is arranged to be controllable both ways between an open position and a closed position. The control of the valve 25 can be performed by means of means that will be well known to a person skilled in the art.

[0070] The valve 25 is in fluid communication with the first hydraulic chamber 23 via a fluid channel 27 and in fluid communication with a receiving unit, not shown, for hydraulic liquid from the hydraulic chamber 23 via an outlet line 29. For the embodiment shown, the receiving unit may be, for example, a container, not shown, having a pressure, when filled with the fluid from the hydraulic chamber 23, lower than an ambient pressure in the well. Alternatively, the receiving unit may for example be somewhere higher up in the well or outside the well so that the fluid may selectively be bled off from the hydraulic chamber 23.

[0071] The purpose of the valve 25 is thus to allow the hydraulic liquid to be bled off from the hydraulic chamber 23.

[0072] In a lower end portion, the second cylinder housing 17 is provided with an opening 31 for communication of well fluid between the well and the second cylinder housing 17.

[0073] A second hydraulic chamber 33 is thus defined by the piston face 16 of the second piston 15, the portion of the cylinder housing 17 which at all times is defined by said piston face 16, and the opening 31.

[0074] In the embodiment shown in FIGS. 1A-1C, the opening 31 is a bore through a lower end portion of the second cylinder housing 17. As soon as the apparatus 1 is run into the well, the well fluid will be carried towards the piston face 16 of the second piston 15. In consequence of the pressure from the well fluid, the piston 15 will exert an upward force as is shown in the figures. To prevent the piston arrangement 15, 19, 11 from being moved in an uncontrolled manner, the pressure of the fluid in the first hydraulic chamber 23 must be of such a magnitude that a downward force exerted by the first piston 11 is larger than the opposite force in consequence of the pressure of the well fluid in the second hydraulic chamber 33. Thus, the necessary pressure that will have to be supplied to the first hydraulic chamber 23 depends on the maximum well pressure to which the apparatus 1 is exposed to in the relevant situation of application.

[0075] Normally, fluid at the desired pressure will be supplied to the hydraulic chamber 23 of the apparatus 1 before the apparatus 1 is run into the well. But it is conceivable that the hydraulic chamber 23 is pressurized as the apparatus 1 is being run into the well. However, this requires fluid communication with the surface or a pump/valve device from a possible container which is arranged to be selectively put into fluid communication with the chamber 23. Especially the latter alternative with a pump/valve device will complicate the apparatus 1 and is thus a less attractive solution.

[0076] FIG. 1A shows the apparatus 1 in its position of application just after the valve 25 in the end plug 21 has been brought from the closed position to the open position. The valve 25 is of a kind known per se, which is arranged to be remotely operated, for example from a vessel connected to the well. When the valve 25 is opened, pressurized fluid from the first hydraulic chamber 23 will be bled off via the fluid channel 27 and the outlet line 29 and further to the receiving unit not shown. The fluid flow out of the first hydraulic chamber 23 is indicated by an arrow F.sub.b.

[0077] The pressure in the well fluid flowing in through the opening 31 of the second cylinder housing 17 and into the second hydraulic chamber 33 causes the piston arrangement 15, 19, 11 to move upwards in the cylinder housings 17, 13. The inflow of well fluid through the opening is illustrated by arrows F.sub.w.

[0078] In consequence of the movement of the piston arrangement 15, 19, 11, the volume of the dump chamber 9 increases. In consequence of this volume increase, well fluid will be sucked/drawn or, depending on the volume-increase rate of the dump chamber 9 in relation to the entering rate of the well-fluid flow F.sub.i, flow into and through the sampling device 3 and into the dump chamber 9. The flow of a well-fluid sample into the sampling device 3 is illustrated by the arrow F.sub.i.

[0079] In an embodiment in which the sampling device 3 is a measuring device comprising at least one sensor, the flow of “exhaust fluid”, i.e. fluid that has passed the measuring device 3, is illustrated by an arrow F.sub.e. The exhaust-fluid flow F.sub.e may be identical to F.sub.i. But if the measuring device 3 is of the gas-chromatograph type, for example, F.sub.e will be a mixture of the well-fluid flow F.sub.i and a carrier gas which is supplied from a carrier-gas container, not shown, which is in fluid communication with the measuring device 3, as will be known to a person skilled in the art.

FIG. 1B shows the apparatus of FIG. 1A after the piston arrangement 15, 19, 11 has moved about half the stroke length in the cylinder housings 17, 13.

[0080] The piston arrangement 15, 19, 11 will be moved upwards as long as the bleed-off of the fluid pressure in the first hydraulic chamber 23 is maintained, i.e. as long as the valve 25 is open and as long as the downward force from the first piston 11 is smaller than the opposite force from the second piston 15.

[0081] When the desired measurement has been achieved, the valve 25 is closed. By opening the valve 25 again after a certain time, either with the apparatus 1 in the same position in the well or after the apparatus 1 has been moved to another position, for example higher or lower in the well, another measurement can be carried out. Such “stepped” measurements may be repeated one or more times until the dump chamber 9 has reached its maximum volume as shown in FIG. 1C where the fluid flows F.sub.b, F.sub.w, F.sub.i and F.sub.e have ceased.

[0082] A person skilled in the art will know that in those cases in which the sampling device 3 comprises a measuring device with at least one sensor, it may be very important that the fluid flow F.sub.i into and through the at least one sensor of the measuring device 3 takes place at a controlled, stable flow rate.

[0083] Such a controlled, stable flow rate is achieved by controlling the degree of opening of the valve 25. It will therefore be understood that the term “open valve” means that the position of the valve is not necessarily in its fully open position, but in a position different from fully closed, so that the desired flow rate of the fluid flow F.sub.i into and through the measuring device 3 is achieved.

[0084] FIGS. 2A-2C show an alternative embodiment of the apparatus shown in FIG. 1A-1C.

[0085] In FIGS. 2A-2C, the area of the piston face 12 of the first piston 11 is smaller than the area of the piston face 16 of the second piston 15. In said figures, the piston rod 19 has a cross-sectional area which is smaller than the cross-sectional areas (and the piston faces 12, 16) of both the first piston 11 and the second piston 15.

[0086] A chamber wall 35 provides a partition between the dump chamber 9 and a partition-wall chamber 37 which is further defined by a portion of the piston rod 19, the second cylinder housing 17 and the second piston 15.

[0087] The chamber wall 35 is provided with an opening 39 which is complementarily adapted to the piston rod 19 and arranged coaxially with it. In the periphery of the opening 39, annular grooves are arranged for receiving a set of seals 41 (two shown in the figures) in order to provide, together with the piston rod 19, fluid-sealing between the partition-wall chamber 37 and the dump chamber 9.

[0088] A substantial difference between the apparatus 1 shown in FIGS. 1A-1C and the apparatus 1 shown in FIGS. 2A-2C is that the latter does not need a closable first hydraulic chamber 23 which, in the embodiment shown in FIGS. 1A-1C, initially contains a fluid at a pressure sufficient to counteract the movement of the piston arrangement 15, 19, 11. The first hydraulic chamber 23 may thus, but need not, be open towards the well as shown.

[0089] To be able to provide the desired inflow F.sub.i of a well-fluid sample into the sampling device 3, an end portion of the second cylinder housing 17 is provided with an end plug 22 which bears many similarities to the end plug 21 shown in FIGS. 1A-1C. But in FIG. 2A and FIG. 2B the fluid flow F.sub.w is from the well, via the valve 26 and the fluid channel 28 and to the second hydraulic chamber 33.

[0090] By controlling the degree of opening of the valve 26 in the end plug 22 shown in FIGS. 2A-2C, the upward movement of the piston arrangement 15, 19, 11 in the cylinder housings 17, 13 can be controlled in the same way as explained above in connection with FIG. 1A-1C.

[0091] A person skilled in the art will understand that the apparatus 1 according to the exemplary embodiment shown in FIGS. 2A-2C is run into the well with the piston arrangement 15, 19, 11 in the position shown in FIG. 2A. This means that the partition-wall chamber 37 is filled with a fluid, such as air or a liquid or a mixture thereof.

[0092] A draining device in the form of a valve 43 is arranged in a fluid-communication channel in the second cylinder housing 17, near the chamber wall 35.

[0093] The purpose of the valve 43 is to allow the fluid present in the partition-wall chamber 37 to be bled off as the volume of said chamber 37 is being reduced in consequence of the fluid flow F.sub.w into the second hydraulic chamber 33 and consequent upward movement of the piston 15. In the embodiment shown, the fluid is bled off towards the well.

[0094] The valve 43 thus enables a full movement of the piston arrangement 15, 19, 11 from the position which is shown in FIG. 2A to the position which is shown in FIG. 2C.

[0095] In its simplest form, the valve 43 is a one-way valve, but it is also conceivable for it to be a controllable valve. However, a controllable valve may complicate the apparatus 1 and is therefore not the primarily preferred solution.

[0096] FIGS. 3A-3C show yet another alternative embodiment of the apparatus 1 shown in FIGS. 1A-1C and FIGS. 2A-2C.

[0097] In FIGS. 3A-3C, the area of the piston face 12 of the first piston 11 is equal to the area of the piston face 16 of the second piston 15. In said figures, the piston rod 19 has a cross-sectional area which is smaller than the cross-sectional area (and the piston faces 12, 16) of the first piston 11 and thus also that of the second piston 15.

[0098] In FIGS. 3A-3C, the apparatus 1 is provided with an end plug 21 and a valve 25 arranged in association with the first hydraulic chamber 23, in a manner corresponding to that shown in FIGS. 1A-1C. Further, the apparatus 1 is provided with a second end plug 22 and a valve 26 arranged in association with the second hydraulic chamber 33, in a manner corresponding to that shown in FIGS. 2A-2C. However, it will be understood that the latter end plug 22 and valve 26 are, in terms of operation, of the embodiment shown in FIGS. 3A-3C. In addition, it will be understood that the pistons 11, 15 do not need to be of the same size. For example, the piston arrangement 15, 19, 11 with associated cylinder housings 17, 13 shown in FIGS. 2A-2C could be used also for the arrangement shown in FIGS. 3A-3C.

[0099] In FIG. 3A, the piston arrangement 15, 19, 11 has been driven to a position in the apparatus 1 such that the dump chamber 9 has a smallest volume. This preparation or “charging/tensioning” of the apparatus 1 has been performed by supplying the first hydraulic chamber 23 with a pressurized fluid preferably before the apparatus 1 has been run into the well, so that the pressurized fluid supplied has driven the piston arrangement 15, 19, 11 from the position the apparatus has in FIG. 3C, possibly from a position between those shown in FIGS. 3C and 3A, for example from the position as shown in FIG. 3B to the position that is shown in FIG. 3A.

[0100] By such a displacement, the volume in the partition-wall chamber 37 mentioned above will increase. The partition-wall chamber 37 is fluid-tight in consequence of seals 41 in the chamber wall 35 and the seal 15′ in the second piston 15. Thus, by said displacement, an underpressure will form in the partition-wall chamber 37. The underpressure will act against an internal piston face 16′ of the second piston 15 and thus exert an upward force which acts against the force from the first piston 12 in consequence of the pressurized fluid in the first hydraulic chamber 23.

[0101] During the above-mentioned “charging” of the apparatus 1, fluid must be evacuated from the dump chamber 9. In the embodiment shown, in which an end plug 22 is arranged in association with the second hydraulic chamber 33 as well, the valve 26 must be in an open position so that fluid is also allowed to evacuate from the second hydraulic chamber 33.

[0102] FIG. 3d shows the fluid flows through the apparatus 1 during said “charging”.

[0103] The valve 43 shown in FIGS. 2A-2C may be superfluous in the embodiment shown in FIGS. 3A-3C provided the “charging” is carried out from the position that is shown in FIG. 3C in which the second piston abuts against the chamber wall 35.

[0104] An advantage of arranging an end plug 22 with an associated valve 26 in association with the second hydraulic chamber 33 as well, is that the well fluid F.sub.w may selectively be prevented from entering the chamber 33, for example while the apparatus 1 is being run into the well. Thus, the fluid in the first hydraulic chamber 23 may be supplied at a pressure which is sufficient to overcome the underpressure in the partition-wall chamber 37.

[0105] The valve 26 may be used as a means of at least contributing to the control of the movement of the piston arrangement 15, 19, 11. By adjusting the valve 26 controlling the inflow of well fluid into the second hydraulic chamber 33 while, at the same time, the valve 25, which controls the bleed-off of the pressurized fluid in the first hydraulic chamber 23, is at least partially open, the fluid pressure in the second hydraulic chamber 33 may be controlled into being positive, contributing to an upward movement of the piston arrangement 15, 19, 11, that is, or negative, reducing or braking an upward movement of the piston arrangement 15, 19, 11, that is.

[0106] It is also conceivable that the valve 25 associated with the first hydraulic chamber 23 may be fully or partially closed, and that the rate of motion of the piston arrangement 15, 19, 11 and thereby the rate of volume change in the dump chamber 9 is controlled by means of the valve 26 controlling the inflow of well fluid into the second hydraulic chamber 33. A pressure increase will then occur in the first hydraulic chamber 23.

[0107] From the above it will be understood that the movement of the piston arrangement 15, 19, 11 is affected by the well fluid in the second hydraulic chamber 33.

[0108] It should be noted that all the above-mentioned embodiments illustrate the invention, but do not limit it, and persons skilled in the art may construct many alternative embodiments without departing from the scope of the independent claims. In the claims, reference numbers in brackets are not to be regarded as restrictive. The use of the verb “to comprise” and its different forms does not exclude the presence of elements or steps that are not mentioned in the claims. The indefinite article “a” or “an” before an element does not exclude the presence of several such elements.