Multi-stage pump assembly having a pressure controlled valve for controlling recirculation of fluid from the pump stage outlet to the pump stage inlet

Abstract

The invention provides a pump comprising a pump inlet, a pump outlet, at least two threaded rotors and a pressure controlled valve, the pressure controlled valve being capable of controlling re-circulation of fluid from the pump outlet to the pump inlet. The pressure controlled valve can be a control valve. The invention also provides a multiple stage pump assembly comprising at least two pumps arranged in series, wherein at least one of the pumps is the aforementioned pump.

Claims

1. A multiple stage pump assembly for pumping a fluid, the multi stage pump assembly having an intake end and a discharge end, the multiple stage pump assembly comprising: a plurality of pumps arranged in series, wherein the plurality of pumps includes a first pump at the intake end, a second pump at the discharge end, and a third pump positioned between the first pump and the second pump, wherein the second pump and the third pump each comprise a pump inlet, a pump outlet, at least two threaded rotors, and a pressure controlled valve; a first conduit connecting the pump outlet of the second pump to the pump inlet of the second pump and configured to re-circulate the fluid from the pump outlet of the second pump to the pump inlet of the second pump, wherein the pressure controlled valve of the second pump is configured to control re-circulation of the fluid through the first conduit from the pump outlet of the second pump to the pump inlet of the second pump; and a second conduit connecting the pump outlet of the third pump to the pump inlet of the third pump and configured to re-circulate the fluid from the pump outlet of the third pump to the pump inlet of the third pump, wherein the pressure controlled valve of the third pump is configured to control re-circulation of the fluid through the second conduit from the pump outlet of the third pump to the pump inlet of the third pump.

2. The assembly of claim 1, wherein the pressure controlled valve of the second pump is configured to control a flow rate of the fluid through the first conduit in proportion to the gas to liquid ratio of the fluid being pumped by the second pump; wherein the pressure controlled valve of the third pump is configured to control a flow rate of the fluid through the second conduit in proportion to the gas to liquid ratio of the fluid being pumped by the third pump.

3. The assembly of claim 1, wherein each pressure controlled valve is a control valve.

4. The assembly of claim 1, wherein the pressure controlled valve of the second pump is located wholly or partly within the first conduit or adjacent one or other end of the first conduit; wherein the pressure controlled valve of the third pump is located wholly or partly within the second conduit or adjacent one or other end of the second conduit.

5. The assembly of claim 1, wherein each pump outlet comprises a recess configured to selectively flow liquid rather than gas from an enclosure in which the corresponding threaded rotors are located to the corresponding conduit.

6. The assembly of claim 1, wherein the pressure controlled valve of the second pump is configured to respond to an absolute pressure difference between the pump outlet of the second pump and the pump inlet of the second pump such that the pressure controlled valve of the second pump permits fluid to flow through the first conduit when the absolute pressure difference between the pump outlet of the second pump and the pump inlet of the second pump reaches a threshold level.

7. The assembly of claim 1, wherein the pressure controlled valve of the second pump is configured to respond to a ratio between the pressure at the pump outlet of the second pump and the pressure at the pump inlet of the second pump, such that the pressure controlled valve permits fluid to flow through the first conduit when the ratio between the pressure at the pump outlet of the second pump and the pressure at the pump inlet of the second pump reaches a threshold ratio.

8. The assembly of claim 7, wherein the pressure controlled valve of the second pump comprises a piston having an inlet face and an outlet face, wherein a surface area of the inlet face is greater than a surface area of the outlet face and a ratio between the surface area of the inlet face to the surface area of the outlet face prescribes the threshold ratio between the pressure at the pump outlet of the second pump and the pressure at the pump inlet of the second pump.

9. The assembly of claim 1, wherein the pressure controlled valve of the second pump is configured to respond to a ratio between the pressure difference between the pump outlet of the second pump and the pump inlet of the second pump (dPstage) and the pressure difference between first and second pressures (dPassembly) which, in use, are communicated to the pressure controlled valve of the second pump, such that the pressure controlled valve of the second pump permits fluid to flow through the first conduit when the ratio between dPstage and dPassembly reaches a threshold ratio.

10. The assembly of claim 9, wherein the pressure controlled valve of the second pump comprises: a piston having a pair of end faces, a first chamber, and a second chamber, wherein the first chamber is in fluid communication with an intake of the multi stage pump assembly disposed at the intake end of the multiple stage pump assembly and the second chamber is in fluid communication with a discharge of the multi stage pump assembly disposed at the discharge end of the multiple stage pump assembly; wherein a pressure in the first chamber opposes an inlet pressure at the inlet of the second pump and a pressure in the second chamber opposes an outlet pressure at the outlet of the second pump; wherein a ratio of a surface area of the end faces to a cross-sectional area of the chambers prescribes a threshold ratio at which the pressure controlled valve will permit fluid flow through the first conduit.

11. The assembly of claim 1, wherein each pump has a swept volume, and wherein the swept volume of each pump is the same.

12. The assembly of claim 1, wherein a swept volume of the first pump is greater than a swept volume of the third pump, and wherein the swept volume of the third pump is greater than a swept volume of the second pump.

13. A method of pumping a fluid from a first location to a second location comprising: providing a multiple stage pump assembly having an intake end and an discharge end opposite the intake end, wherein the multiple stage pump assembly includes a plurality of pumps in series, wherein the plurality of pumps include a first pump disposed at the intake end, a second pump is disposed at the discharge end, and a third pump positioned between the first pump and the second pump, wherein each pump includes a pump inlet and a pump outlet; positioning the intake end of the multiple stage pump assembly at or near the first location; activating the multiple stage pump assembly to pump the fluid from the first location to the second location; recirculating fluid from the outlet of the second pump to the inlet of the second pump while pumping the fluid from the first location to the second location; and recirculating fluid from the outlet of the third pump to the inlet of the third pump while pumping the fluid from the first location to the second location.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

(2) FIG. 1 is a schematic of a multiple stage twin screw pump assembly;

(3) FIG. 2 is a view from above of one of the pumps forming the multiple stage pump assembly of FIG. 1;

(4) FIG. 3 is a schematic of a sleeve valve;

(5) FIG. 4 is a chart showing flow rate against pressure difference for a typical sleeve valve as shown in FIG. 3;

(6) FIG. 5 is a schematic of another valve which may be used in the pumps shown in FIG. 1;

(7) FIG. 6 is a schematic of yet another valve which may be used in the pumps shown in FIG. 1;

(8) FIG. 7 is a schematic of a second embodiment of the invention;

(9) FIG. 8 is a schematic of another embodiment of the invention;

(10) FIG. 9 is a schematic of another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) A multiple stage pump assembly 1 in accordance with the second aspect of the invention can be seen in FIG. 1. The multiple stage pump assembly 1 is suitable for pumping a multiphase fluid in the direction marked by arrows A. It can be understood that this multiple stage pump assembly could be used to lift fluids from a well.

(12) The multiple stage pump assembly 1 comprises four pumps 2,3 in series. The first pump 2 in the series (first since it is at the intake end 4 of the multiple stage pump assembly) is a conventional rotary screw pump as known in the art. The second, third and fourth pumps 3 are in accordance with the first aspect of the invention. The fourth pump is referred to as the last one in the series as it is at the discharge end 5 of the multiple stage pump assembly 1. Thus, the first pump 2, the second pump 3, the third pump 3, and the fourth pump 3 may be used to describe the serial order of the pumps 2, 3 moving from the intake end of pump assembly 1 to the discharge end 5 of the pump assembly 1. Alternatively, the pump assembly 1 may be described as including a first pump 2 at the intake end 4 of pump assembly 1, a second pump 3 at the discharge end 5 of the pump assembly 1 (the last pump 3 in the series), and one or more third pumps 3 positioned between the first pump 2 at the intake end 4 and the second pump 3 at the discharge end 5.

(13) Each pump 2,3 has two threaded rotors 6 located in a rotor chamber 15 for driving fluid from an inlet 7 to an outlet 8 of that particular pump. Although two rotors are depicted in FIG. 1 (i.e. a twin-screw arrangement), other numbers of rotors could be used instead, such as three (triple screw arrangement) or more. Also, although FIG. 1 (and later figures) depicts a single pair of rotors driving fluid in one direction, it is possible for each pump to comprise opposing pairs of rotors such that the fluid drawn into the inlet of each pump is split into two streams, each stream being driven through one of the pairs of rotors and then re-combined before the outlet of the pump, as described in U.S. Pat. No. 6,413,065.

(14) It is known in multiple stage rotary screw pump assemblies to include one or more additional units, such as units associated with each of the pump stages (e.g. between the pump stages). For example, these units may include gear modules, spacer units, sealing units or plenum chambers and the like. In this example, a single spacer unit 9 is depicted between each pump which transfers the drive from one pump to the next, and a gear module 10 is located at the discharge end 5 of the multiple stage pump assembly. Although not shown in detail, the spacer units 9 and the gear module 10 naturally have conduits 16 there-through to allow the passage of fluid from one pump to the next. However, it may be unnecessary to provide any units between the pump stages depending on the nature of the rotary screw pump. The precise design of the rotary screw pump and whether any associated units are required will be apparent to the person skilled in the art and is not the subject of this invention.

(15) Each pump in accordance with the invention comprises a conduit 11 in fluid communication with the pump inlet 7 and the pump outlet 8. Specifically, one end 12 of the conduit 11 is open to the pump inlet 7 and the other end 13 of the conduit 11 is open to the pump outlet 8. As depicted in FIG. 1 and in the top view shown in FIG. 2, channels 17 in the end faces of the pump connect the pump inlet 7 and the pump outlet 8 to the conduit 11.

(16) A pressure controlled valve 14 is positioned in the conduit 11, although the valve 14 could actually be located at or adjacent either end 12,13 of the conduit 11. Ideally, and as shown, the entry to the valve 14 is arranged below the pump outlet 8 when the multiple stage pump assembly 1 is arranged vertically, as in use.

(17) The valve 14 is a sleeve valve as shown in FIG. 3. The sleeve valve comprises an outer sleeve 18 and an inner sleeve 19 positioned co-axially within the outer sleeve 18. The outer sleeve 18 is formed with a rectangular (ignoring the effect of the curvature of the sleeve) aperture 20 there-through. The inner sleeve 19 is also formed with an aperture 21 which has a curved edge 22. As is known in the art, a spring (not shown) biases the valve to the closed position in the absence of a sufficient pressure difference across the spring.

(18) As pressure across the valve 14 increases, the inner sleeve 19 moves further into the outer sleeve 18, and the apertures 20,21 overlap to a greater extent. A greater fluid volume can flow through the valve with increased overlap of the sleeves. The volumetric flow rate (V) compared with pressure difference (dP) across the valve is depicted in FIG. 4.

(19) In use, before the multiple stage pump assembly is installed in a well, the overall pressure increase to be obtained by the multiple stage pump assembly is divided by the number of pumps in the series to obtain the threshold pressure of the pressure controlled valves 14. The threshold pressure of the valves is then set to this value. Alternatively, the threshold pressure is set slightly above the value calculated. For example, if the required pressure increase for this multiple stage pump assembly comprising four pump stages is 2000 psi (13.8 MPa), then the threshold pressure for each pressure controlled valve 14 can be set to 550 psi (3.79 MPa) (i.e. slightly above 2000/4). The pump can then be installed in the well.

(20) In situations where the fluid in the well is all liquid, the pump operates as a conventional twin screw multiple stage pump assembly. Specifically, the liquid is pressurised equally at each stage and so the pressure difference across each pump stage is about 500 psi (3.45 MPa). The valves do not, therefore, open.

(21) However, where the fluid comprises gas, the last pump in the series begins to perform more work than the other pumps and the pressure difference across that pump increases. If the pressure difference across the last pump is greater than the threshold pressure of the pressure controlled valve 14, then the valve 14 will open and fluid, primarily liquid, will be re-circulated from the outlet 8 of the pump through the conduit 11 and to the inlet 7 of the last pump.

(22) By re-circulating liquid back to the inlet of the last pump, the pressure difference across the third pump is increased. Since the flow rate of the third pump is unchanged, it can be seen from equation 1 above that this means that the third pump assembly is caused to work harder (increased power). Additionally, the increase in pressure difference across the third pump causes the valve of the third valve to open, permitting liquid to be re-circulated back to the inlet of the third pump.

(23) In turn, the valve of the second pump is caused to open and re-circulate liquid to the inlet of the second pump.

(24) Consequently, each of the third, second and first pumps are forced to work harder and contribute more effectively to the pressure boost obtained by the multiple stage pump assembly.

(25) It will be understood that the pressure difference across the first pump 2 will also increase. However, since, in this embodiment, the first pump 2 is a conventional twin screw pump, the pump will simply be forced to work harder.

(26) In practice, the valves 14 of each of the last, third and second pumps 3 open quickly, one after another, to varying degrees to allow liquid to re-circulate across or around the pumps establishing an equilibrium pressure distribution. Thus, the conduit 11 of the pump 3 at the discharge end 5 may also be referred to as the first conduit, the conduit 11 of the adjacent upstream pump 3 may also be referred to as the second conduit 11, and so on. If the gas to liquid ratio increases over time, the required volume differences between the pumps 3 will increase causing the valves 14 to open further, permitting a greater volume of liquid to be re-circulated (see FIG. 2).

(27) It can be seen, therefore, that the pump assembly of the invention automatically regulates the opening of the valves to evenly distribute the work done by each pump in the assembly. Further, the pump assembly automatically and continuously responds to variations in the fluid composition being pumped.

(28) In another embodiment, the first pump in the series can also be in accordance with the first aspect of the invention. In this case, liquid can be re-circulated from the outlet to the inlet of the first pump, thereby controlling the pressure difference across, and therefore work done by, the first pump. Whilst this may ensure longevity of the first pump, it will control the maximum power which the multiple stage pump assembly can achieve.

(29) FIG. 5 illustrates another valve which may be used in the invention. The valve 14 of FIG. 5 comprises a piston 23 having an inlet face 24 and an outlet face 25. The inlet face 24 is the face which is exposed, in use, to the pump inlet pressure and the outlet face 25 is the face which is exposed, in use, to the pump outlet pressure. The surface area of the inlet face 24 is greater than the surface area of the outlet face 25. A passage 26 extends through the piston to permit fluid flow through the valve. The exit 27 of the passage 26 can be shaped to permit a varying fluid flow rate, similar to the aperture 21 in FIG. 2.

(30) It will be appreciated that the piston 23 in FIG. 5 acts as an actuator to control the opening of the passage 26. Since the passage extends through the piston, the actuator is integral with that part of the valve which provides a fluid flow path (the valve element). However, it is possible for the actuator to be remote from that part of the valve which provides the fluid flow path whilst still actuating and controlling it.

(31) The pressure controlled valve 14 responds to the ratio between the pressure at the pump outlet (which is acting on the outlet face 25 of the piston) and the pressure at the pump inlet (which is acting on the inlet face 24 of the piston). When the ratio between the pressure at the pump outlet and the pressure at the pump inlet reaches a threshold, the valve permits fluid to flow there-through. The threshold corresponds to the ratio between the surface area of the inlet face 24 to the surface area of the outlet face 25.

(32) The ratio between the surface area of the inlet face 24 to the surface area of the outlet face 25 decreases from the first pump to the last pump in the series, so that approximately the same pressure can be added by each pump stage. For example, if it desired that each pump stage should increase the fluid pressure by about 500 psi (3.45 MPa) and the bottom hole pressure is thought to be about 750 psi (5.17 MPa), the ratio between the surface area of the inlet face 24 to the surface area of the outlet face 25 for the first pump stage is about 1.67; for the second pump stage the ratio is about 1.4; for the third pump stage the ratio is about 1.29; and for the last pump stage the ratio is about 1.22.

(33) Using valves of this type, the overall pumping pressure that can be obtained by the multiple stage pump assembly is not limited in the way mentioned above when each pump includes a valve of the type depicted in FIG. 2.

(34) Yet another example of a valve 14 which can be used in the present invention is depicted in FIG. 6. This valve comprises a piston 28 having end faces 29, a shaft 30 and first and second chambers 31,32. The first chamber 31 is in fluid communication with the intake 4 of the multiple stage pump assembly 1 and the second chamber 32 is in communication with the discharge 5 of the multiple stage pump assembly 1. Ports 34 through the valve side wall allow the chambers 31,32 to be put in fluid communication with the intake 4 and discharge 5 of the multiple stage pump assembly 1.

(35) It can be understood from the figure that the chambers 31, 32 are annular shaped around the shaft 33 of the piston 28. It will be further understood that the pressure in chamber 31 which corresponds to the intake pressure of the multiple stage pump assembly opposes the inlet pressure of the pump stage. Similarly, the pressure in chamber 32 which corresponds to the discharge pressure of the multiple stage pump assembly opposes the outlet pressure of the pump stage.

(36) As with the valve shown in FIG. 5, the valve may alternatively be structured such that the piston is remote from the fluid flow path.

(37) The ratio of the surface area of the end faces 29 of the piston to the cross-sectional area of the chambers 31,32 prescribes a threshold ratio. When the ratio of the pressure difference between the outlet and the inlet of the pump stage (dP.sub.stage) and the pressure difference between the discharge and intake of the overall multiple stage pump assembly (dP.sub.assembly) reaches the threshold ratio, the valve will permit fluid flow there-through.

(38) To set the ratio for a multiple stage pump assembly comprising n pumps, the ratio of the surface area of the end faces 29 to the cross-sectional area of the chambers 31,32 is n:1. Accordingly, in a multiple stage pump assembly such as that shown in FIG. 1 which has 4 pump stages, the surface area of the end faces 29 of the piston 28 should be about four times the cross-sectional area of the chambers 31,32.

(39) For a valve with a given piston end face 29 surface area, the ratio between the end face 29 surface area and the chamber 31,32 cross-sectional area can be varied by varying the diameter of the piston's shaft 30.

(40) With such an arrangement, it is possible to distribute the work across all the pumps without knowing what the bottom hole pressure is. Although the chambers 31, 32 have been described as annular, and this is advantageous chambers of other shapes may also be used. The function of the chambers 31, 32 is to enable the valve of FIG. 6 to be actuated based on the ratio of the pressure difference across an individual pump to the pressure difference of the multi-pump assembly as a whole. For example, in a multiple stage pump assembly comprising a plurality of individual pumps arranged in series, the inlet of an individual pump may be coupled to the outlet of that individual pump by a fluid bypass arranged to enable recirculation of fluid from the outlet of that individual pump to its inlet. The fluid bypass typically comprises a control valve, configured to control the recirculation based on the pressure drop across the individual pump, e.g. the pressure difference between the outlet of that individual pump and its inlet. The control valve may also be controlled based on the pressure between the inlet of the multiple stage pump and the outlet of the multiple stage pump. This enables, for example the control valve to control recirculation through the fluid bypass of an individual pump based on the ratio of the pressure drop across the individual pump to the pressure drop across the multiple stage pump assembly. This may be achieved as described above by providing fluid couplings into the control valve from the outlet/inlet of the multiple stage pump assembly or by other means for example by electronic control of the control valves.

(41) FIG. 7 shows another example of a multiple stage twin screw pump assembly similar to that shown in FIG. 1, and so like numerals refer to like parts. The multiple stage twin screw pump assembly shown in FIG. 7 is made up of four pumps. Each pump is a conventional twin screw pump 2. The second, third and fourth pumps each further comprise an inlet 40 and an outlet 41 adaptor which are connected to each other via a conduit 42, such as a pipe. It can be seen that the conduits 42 are external to the conventional twin screw pumps 2. A pressure controlled valve 14 is positioned in each conduit 42, though it could also be positioned at the inlet or outlet to the conduit 42.

(42) Accordingly, it can be seen that a conventional twin screw pump can be used to make a pump in accordance with the present invention.

(43) The inlet/outlet adaptors 40,41 are units which can be connected to the inlet/outlet 7,8 of the conventional twin screw pump and which have a chamber for containing the fluid. Fluid is discharged from the outlet of a conventional pump 2 into the adjacent outlet adaptor 41 so that it can be passed on to the next pump assembly in the series. According to the invention, some of the fluid can be re-circulated to the inlet adaptor 40 when the pressures across the conventional twin screw pump 2 cause the valve to open. The valve can be any of the valves described above. The conduits 42 are connected to the chambers inside the respective outlet adaptors 41 near the bottom so that the chambers can act as small separation tanks, thereby enabling liquid to be preferentially re-circulated to the inlet adaptors 40.

(44) In this way, a multiple stage pump assembly can be constructed using conventional rotary screw pumps.

(45) FIG. 8 shows another embodiment of the invention in which conventional rotary screw pumps are used to form pumps and a multiple stage pump assembly in accordance with the invention. Again, like reference numerals refer to like parts.

(46) In this embodiment, rather than providing the conventional pumps with inlet and outlet adaptors adjacent the inlets and outlets of the second, third and fourth conventional pumps, only outlet adaptors 45 are provided. An outlet adaptor 45 is coupled to the outlet 8 of each of the conventional twin screw pumps 2 so that fluid is delivered from the pump to a chamber inside the outlet adaptor.

(47) Each outlet adaptor 45 is also connected to the outlet adaptor 45 of the adjacent pump assemblies via a conduit 46. As can be seen from the figure, the conduit 46 is a single conduit with a connection point 47 for each outlet adaptor 45. Pressure controlled valves 14 are positioned in the conduit 46 to separate each connection point.

(48) Where the fluid being pumped is 100% liquid, the valves 14 remain closed.

(49) However, as in the first example described above with respect to FIG. 1, if gas is present in the fluid, the pressure difference across the last pump will increase, causing the valve 14 located between the outlet adaptors 45 of the fourth and third pump assemblies to open. Fluid will flow from the outlet adaptor 45 of the last pump assembly and into the conduit 46. Since the outlet adaptor 45 of the third pump assembly is in fluid communication with the inlet of the fourth pump assembly, the pressure in that outlet adaptor is lower than the pressure of the fluid being re-circulated in the conduit 46 and so the fluid will flow into the outlet adaptor of the third pump assembly.

(50) In turn, the pressure difference across the third pump in the series increases and the corresponding valve opens to re-circulate liquid, and so on for the second and first pumps. In practice, the valves open and reach equilibrium almost instantly.

(51) It can be understood that this arrangement of outlet adaptors 45, valves 14 and the conduit 46 can be used with conventional twin screw pumps to form pumps and a multiple stage pump assembly in accordance with the invention.

(52) In an alternative arrangement, the conduit 46 may not be a single conduit. There may instead be separate conduits connecting adjacent outlet adaptors 45. In that case, the outlet adaptors connected to the outlets of the second and third pump assemblies each have two conduits connected thereto; one which feeds pressurised fluid into the outlet adaptor and one which takes fluid away for re-circulation.

(53) FIG. 9 shows a multiple stage twin screw pump assembly which is tapered. The pump assembly comprises four pumps of decreasing swept volume from the intake end 4 to the discharge end 5. Thus, the swept volume of the pump 3 at the intake end 4 is greater than the swept volume of the adjacent pump 3, the swept volume of the pump 3 immediately adjacent the pump 3 at the intake end 4 is greater than the swept volume of the pump 3 immediately adjacent the pump 3 at the discharge end 5, and the swept volume of the pump 3 immediately adjacent the pump 3 at the discharge end 5 is greater than the swept volume of the pump 3 at the discharge end 5. The decreasing swept volume may be achieved as is well known in the art. For example, the pitch of the threads on the rotors may decrease from the intake end to the discharge end.

(54) Each pump is constructed in accordance with the first aspect of the invention, in that it has a conduit 11 and pressure controlled valve 14 to selectively allow re-circulation of fluid from the outlet to the inlet of the respective pump. Accordingly, these pumps are similar to those described above with respect to FIG. 1, except that they form a tapered pump assembly. Accordingly, like reference numerals refer to like parts.

(55) In use, it is well known in the industry that a tapered pump can be designed specifically for a particular gas to liquid ratio. Accordingly, the swept volume of each of the four pumps is selected as is known to the skilled reader so that the multiple stage pump can handle a predefined gas to liquid ratio. If, in use, the gas to liquid ratio of the fluid encountered increases above the predefined ratio, the pump operates in the same way as described above with reference to FIG. 1. Specifically, the valves open and re-circulate fluid to the respective pump inlets.

(56) If the gas to liquid ratio decreases below the predefined ratio, then the first pump delivers too much fluid to the second pump, the second pump delivers too much fluid to the third assembly and so on. The pressure differences across the pumps therefore increase and so the valves open and re-circulate liquid from the respective outlets to the respective inlets. However, in contrast to the discussion above, in this situation, the valve of the first pump reacts first, followed by the valves of the subsequent pumps. Again, though, successive opening of the valves is, in practice, relatively quick.

(57) An example of where this embodiment can be useful is where a well has been killed by injecting heavy kill fluid (primarily liquid) into a well. It may be known that the well typically produces a fluid with a particular gas to liquid ratio. A tapered multiple stage pump assembly in accordance with the invention can be tailored for that gas to liquid ratio. Although the pump is optimised for the normal composition of the well fluid, it is still able to pump the heavy kill fluid out of the well when it is desired to put the well back into operation, since fluid can be re-circulated as described above. Specifically, for the period when the kill fluid is to be pumped out, the gas to liquid ratio is lower than the ratio for which the pump is tailored. Too much fluid is delivered to the subsequent pumps. Liquid would be re-circulated initially from the outlet to the inlet of the first pump and then of subsequent pumps in the series.

(58) It can be seen that a tapered pump as described above can efficiently pump a wide variety of gas to liquid ratios.

(59) It is to be understood that features described above with reference to one of the embodiments may be used in conjunction with other embodiments. Also, variations will be apparent to the skilled reader, for example the tapered pump shown in FIG. 9 may comprise a conventional twin screw pump in the first or last stage instead of the pump of the invention, so as to handle more or less gas respectively than the gas to liquid ratio for which the tapered pump is designed. Also, any of the described valves can be used in any of the embodiments of pump assembly.