EP1255911B1

EP1255911B1 - A device for and method of separating gas and liquid in a wellstream

Info

Publication number: EP1255911B1
Application number: EP01910250A
Authority: EP
Inventors: Geir Inge Olsen; Gunder Homstevdt; Bjornar Svingen
Original assignee: Aker Kvaerner Subsea AS
Current assignee: Aker Solutions AS
Priority date: 2000-02-18
Filing date: 2001-02-19
Publication date: 2007-04-11
Anticipated expiration: 2021-02-19
Also published as: EP1255911A1; WO2001061149A1; AU2001237826A1; NO20000816D0

Description

The present invention regards a device for and method of separating gas and liquid in a wellstream in accordance with the preamble of the independent claims.
In production of gas from gas wells, the volume of produced water will increase as the production proceeds. If this water is not removed continuously, a gradual reduction in production will result, and eventually the well will stop producing gas. As long as the gas rate is high compared with the water rate, the water will be carried out of the well in the form of dispersed water droplets in the gas. As the water/gas ratio increase however, the water droplets will start to accumulate in the well, leading to a reduced bottom hole pressure. It is thus important to maintain the water level in the well at an acceptable level in order to achieve the maximum production life for the well. By performing the separation downhole, the separated water may be injected into a suitable area of the formation. If the conditions of the formation are favourable, such water injection may contribute towards maintaining the pressure in the production zone. Reducing the volume of water being passed up to the surface will also reduce the costs associated with cleaning the produced water in order to be able to discharge it to the surrounding environment or reinject it from the surface.
From international patent application WO 94/25729 there is known a one stage, downhole gas/liquid separator for separation of gas and oil, in which the oil is passed up to the surface while the gas is reinjected into the well. The separator unit consists of a tubular body with an outer diameter that is smaller than the diameter of the borehole, in which tubular body is arranged one or more helical plates. The gas/liquid mixture is introduced at the lower end of the separator unit and will as a result of the rotation induced by the helical plates be subjected to a centrifugal force, which causes the heavier fraction, the liquid, to be flung out towards the walls of the tubular body, while the lighter fraction, the gas, moves towards the centre of the tubular body. A gas outlet pipe is provided at the centre of the separator unit, which leads the gas into the annulus between the casing and the separator unit. The liquid carried out of the separator unit and further into the riser.
From GB 2 297 573 there is known a similar separator for separation of a wellstream, as well as a method of separating wellstreams. This is a three stage process in which gas and liquid is first separated by means of cyclone separation, followed by cyclone separation of condensate/oil from water at a high pressure, in addition to separation of undesirable gases such as H₂S, CO₂ and H₂O by means of membranes. The separated water and the undesirable gases may be reinjected into the well.
From US patent 5 794 697 there is known a device for and a method of increasing the oil production from wells containing a mixture of oil and gas, in which the gas is injected back into the well in order to maintain the well pressure. The device disclosed therein consists of a separator having one or more helical plates similar to the plates described in the above WO publication. The separated gas is passed through a compressor and into the well. The liquid and the remaining gas are first passed through a turbine, which drives the gas compressor, and then out of the well.
From US patent 5 693 225 there is known a liquid/liquid separator system to be positioned downhole in an oil producing well. The separator system consists of one or more separators such as filters, membranes, electrostatic devices, separators with rotating fins, static or moving centrifugal separators or any combination of such separation devices. The water that is separated out at each stage is led to a common liquid sump for possible injection into a suitable area of the borehole.
From GB patent 2 333 540 there is known a separator for a submersible pumping system for separation of well fluids in a borehole. The heaviest fractions from each stage of separation are led to a common liquid sump, and the system has a centrally positioned shaft for operating the pumps.
From WO 99/10070 there is known a separator for separation of well fluids and in particular separation of water from a gas producing well. The actual separator is not placed in an oil or gas producing well but in a so-called "dummy well" that has been drilled into the seabed. The separator consists of a continuous screw with a varying pitch, creating one or more stages of separation. This separator has a central channel that is used for collecting the separated gas.
From US patent 5 988 275 there is known a downhole separator device for separating a three phase wellstream. The separated water and gas fractions are injected into separate zones of the well, while the oil fraction is sent up to the surface.
From international patent application WO 97/46323 there is known a cyclone separator for separating gas, liquid and fluids having different densities. The separator comprises a first chamber with a vortex finder and a core body. A guide vane is arranged coaxially with and adjacent to the internal surface of the vortex finder, in order to increase the rotational velocity components of the gas and entrained fluids that pass through the vortex finder by converting the axial velocity component to a rotational velocity component. The unit also comprises an arrangement for preventing carry-over of gas into the liquid outlet.
From European patent application EP 0 711 903 A2 there is known a device for separation of a wellhead fluid mixture. After a first separation in a first separation centrifugal unit the resulting mixture is conveyed to a second centrifugal unit through a substantially open interstage region which is used to distribute the wet gas more evenly before entering the second centrifugal unit. The mixture enters the second centrifugal unit through radial openings in the unit, where a set of static, tangential vanes imparts a tangential spin on the flow in the second centrifugal unit.
A limitation of the previously known solutions for downhole separation of liquid and gas is the fact that the liquid/gas mixture is subjected to a relatively low centrifugal force. This results in a corresponding ineffective separation. Another limitation is the ability to handle slug flow. A further limitation of several of the known solutions is the fact that control cables and gas pipes are arranged between the separation device and the well casing. This reduces the effective diameter of the separation device by the order of 10-15 %, with a follow-on reduction in separation efficiency in the order of 20 - 30%.
An object of the present invention is therefore to provide a separation device of the type mentioned at the beginning, in which the liquid/gas mixture is subjected to a higher centrifugal force than that which is known from prior art, in order to achieve more efficient separation.
Another object of the present invention is to achieve an improved separation effect under slug flow conditions.
Yet another object of the invention is to achieve more efficient separation by using an optimum part of the cross section of the borehole for separation, while at the same time using the central, low separation efficiency section of the cross section to run cables, shafts, pipes etc.
A further object of the present invention is to provide a separation device of the type mentioned at the beginning, which can handle large variations in the incoming gas/liquid flow.
These and other objects and advantages are achieved by a device for separating gas and liquid, which device includes at least two separator units arranged in a common housing, the separated liquid fraction from each of the separator units being sent to a common liquid sump, characterised by the device in combination comprising:

a first gas separator unit for separating a well stream or a pre-treated wellstream, which first gas separator unit comprises an axially arranged feed pipe having a diameter smaller than the diameter of the housing, radially arranged static vanes that imparts a tangential spin to the gas flow, and a wall enclosing the vanes,
a second gas separator unit comprising an axially arranged pipe having a smaller diameter than that of the wall enclosing the vanes of the first gas separator unit, which pipe is provided with apertures or slots for draining off separated liquid and such that the tangential spin of the gas flow is increased further by the gas flowing out through a pipe having a smaller diameter than that of the housing enclosing the vanes of the first gas separator unit.

The device preferably comprises a further gas/liquid separator unit located upstream of the first gas separator unit, which gas/liquid separator unit comprises an enclosing wall with radial inlets for introduction of a wellstream to be treated and static vanes arranged in close proximity to the inlets in order to impart a tangential spin to the wellstream.
The device preferably comprises a pump designed to inject the separated liquid into an injection zone in the well or pump the liquid to the surface at a rate controlled on the basis of the liquid level in the liquid sump.
Preferably, the device is provided with one or several centrally placed pipes for leading through cables, shafts, pipes or similar for supplying energy to the pump or a turbine, electric motor or mechanical device that drives the pump.
Preferably, the device is provided with a centrally placed pipe designed to carry the gas flow from the separation device and out of the well.
Preferably, the device also comprises means of supplying driving power to the pump located centrally in the separator housing, which driving power is supplied in the form of a rotating shaft or reciprocating rod from the surface, or a drive unit positioned above the separation device in the well, or as electric or hydraulic energy to a motor positioned below the separation device.
In accordance with a particular embodiment, the device is placed in an essentially horizontal or steeply inclined section of the well, whereby the inlets are arranged solely in the upper half of the cross section of the well, the gas/liquid separator unit of the separator has a closed bottom, and the liquid from each of the separator units is pumped out by a pump, preferably an ejector pump.
In accordance with the latter embodiment, the device preferably comprises an outlet cyclone downstream of the pump in order to prevent foaming at the outlet.
The invention also regards a method of separating a gas/liquid stream from a gas producing well, which separation preferably takes place downhole, and which method includes the steps of:

a) leading a wellstream or pre-treated wellstream containing gas and liquid into a first separator unit and imparting to this stream a spin having a centrifugal acceleration in the range 50 - 500 G, preferably approximately 250 G;
b) leading the separated liquid from the separator unit to a liquid sump;
c) leading the remaining gas to a second separator unit and imparting to this stream a spin having a centrifugal acceleration in the range 300 - 1500 G, preferably approximately 800 G;
d) leading the separated liquid from the second separator unit to the liquid sump; and
e) leading the separated gas out of the separation device via a pipe located centrally in the separation device.

If the wellstream contains a relatively large liquid fraction, the well stream is passed into a gas/liquid separator unit prior to the above step a), and the a spin is imparted to the well stream, giving it a centrifugal acceleration in the range 1 - 100 G, preferably approximately 25 G, and the separated liquid then flows to the liquid sump.
The separated liquid is either carried out of the well or injected into a suitable area of the well.
While previously known systems for downhole gas dehydration make use of axially arranged, tilted vanes (screws) for generating the centrifugal force for separation, the present invention is based on such a centrifugal force initially being generated by means of radially arranged, static vanes, while the increase of spin in the axial direction is achieved by a reduction of the cross-section of flow. The effect of the second gas separator unit is enhanced considerably by drops of a certain size (determined by the gravitational field (the G field)), which still follow the gas after the first gas separator unit, not being allowed to flow out through the outlets due to the strong centrifugal field being generated. Such drops will remain in the outer enclosing chamber until such time as they are collected and drained off.
It is appreciated that variations of the suggested embodiment may be appropriate where there is not a great need for liquid removal and/or the incoming liquid fraction is small.
Such variations of the preferred embodiment may be as follows:

1. Advantageously, in the case of a low liquid/gas ratio in the incoming fluid and a great need for liquid removal, only the 2. and 3. separator units are used.
2. Advantageously, in the case of a high liquid/gas ratio in the incoming fluid and a moderate need for liquid removal, only the 1. and 2. separator units are used.
3. Advantageously, in the case of a low liquid/gas ratio in the incoming fluid and a moderate need for liquid removal, only the 2. separator unit is used.
4. Advantageously, in the case of a moderate to high liquid/gas ratio in the incoming fluid and little need for liquid removal, only the 1. separator unit is used.

It is also recognised that the separation principle described herein for downhole application may also be applicable for surface and seabed based separation systems, in that case located in a pipe as the enclosing chamber as a replacement for the well casing as otherwise given as such chamber in the present description.
The present solution shows far better performance under conditions of slug flow than existing solutions. The biggest slugs are extracted in the first separator unit, with the downstream separator units removing those liquid drops that will pass through the first separator unit under such conditions.
Arranging control cables, gas and possibly liquid pipes and similar centrally in the separation device allows the effective diameter of the separation device according to the invention to be maintained while achieving a significant improvement in separation efficiency in comparison with previously known solutions.
In the following, the invention will be explained in greater detail with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic diagram of an embodiment of the device in accordance with the present invention;
Fig. 2 shows a system arrangement of an embodiment of the device in accordance with the invention;
Fig. 3 schematically shows an embodiment of the device in accordance with the invention, for liquid injection into the well;
Fig. 4 shows an embodiment of the device in accordance with the invention, for transport of liquid to the surface;
Fig. 5 shows an embodiment of the device in accordance with the invention, for use in horizontal or steeply inclined wells with the pump placed below the separation device;
Fig. 6 shows an embodiment of the device in accordance with the invention, for use in horizontal or steeply inclined wells with the pump placed above the separation device; and
Figs. 7 a) and b) show sections through static vanes for the gas/liquid separator unit and the first gas separator unit respectively, in accordance with the invention.

In the following, the invention will be explained in greater detail through various examples of embodiments. In this description, the terms gas/liquid separator unit and gas separator unit have been used. The gas/liquid separator unit denotes a separator for separation of a well stream with a relatively high liquid content (> 5 % liquid) while the term gas separator unit denotes a separator for separation of a wellstream or pre-treated wellstream with a relatively low liquid content (< 5 % liquid). In this context, the term "liquid" signifies condensate: water or other hydrocarbon-containing liquids or mixtures of these.
Figure 1 shows a schematic diagram of the separation device in accordance with the present invention. The device in accordance with the embodiment shown consists of two gas separator units 2, 3 in which a gas stream 8 from a production zone, which stream contains a relatively small fraction of water, is led to a first gas separator unit 2. The liquid from the first gas separator unit 2 is transported to a pump sump 5 in the form of a stream 9 . The remaining gas fraction from the first gas separator unit 2 is carried further to a second gas separator unit 3 in the form of stream 10, the separated liquid from which separator unit is led to pump sump 5 in the form of a stream 11 and the gas fraction 12 from which separator unit is sent to the surface. The liquid fraction in the pump sump 5 is either injected into a suitable area of the well or pumped to the surface by pump 6. The two gas separator units 2, 3 consist of separators that are based on the centrifugal principle. In those cases where the well stream contains a large fraction of liquid, a liquid/gas separator 1 is provided upstream of the two gas separator units 2,3. The liquid fraction from this separator is also carried to a common liquid sump in the form of a stream 7, whereas the gas fraction is passed to the first gas separator unit 2 in the form of a stream 8. This liquid/gas separator is known from international patent application WO 97/46323, which is incorporated herein as a reference.
Figure 2 shows an arrangement of the separation device of Figure 1 in more detail, which device is located in a borehole defined by a well casing 20. The separation device is positioned near a production zone 18. Expansion packings 16 and 17 are provided above and below the separation device in order to isolate the separation device from the rest of the well. All the separator units are located in one separator housing 14, and will be explained in more detail later. Liquid/gas 4 from a production zone is introduced into the separation device/separator housing 14, and the gas fraction 12 is passed to the surface via a centrally located pipe 15 while the total liquid fraction 22 flows to a pump 6 that, in the embodiment shown, pumps the liquid fraction to an injection zone 19 in the borehole.
The pump 6 will either lift the liquid from the well or transport it to an injection zone 19. The pump will also maintain the level in the well below that of the inlet perforations to the production zone. The pump 6 may be driven by an electric or hydraulic motor 21 or mechanically by means of a shaft with its motor positioned over the separation device or on the surface. It is also possible to use a reciprocating pump, the driving rod of which passes through the centre of the separation device and up to the surface (piston rod pump).
Figure 3 shows an embodiment of the separation device in accordance with the present invention, with liquid injection in the well.
The gas/liquid mixture 4 from the production zone 18 enters the first separator unit 1 of the separation device through radial inlets 33 in the separator housing through static vanes 25, typically arranged as per Fig. 7a. The vanes impart a moderate tangential rotation to the gas/liquid mixture 4, which creates a centrifugal acceleration in the order of 1 - 100 G, preferably approximately 25 G. Much of the liquid will separate out and collect on the enclosing cylindrical surface that is formed by the centrally located pipe 23. The initial liquid separation will therefore take place in the gas/liquid separator unit 1 of the separation device, in addition to that which is separated out in the well itself due to gravity. The separated liquid will flow down into the liquid sump 5 while gas with a reduced liquid content, in the form of a stream 8 is forced through an axially arranged pipe 23 and into the first gas separator unit 2. The gas stream 8 enters the first gas separator unit 2 axially, and the movement of the gas stream 8 is accelerated by means of static vanes 27, typically arranged as per Fig. 7b. The centrifugal acceleration due to the tangential velocity is in the range 50 - 500 G, preferably approximately 250 G at the outlet from the vanes 27. Most of the liquid will separate and collect on the wall 28 that enclosed the vanes. The separated liquid flows down to the liquid sump 5 through drainage pipe 29.
The gas stream is forced into the second gas separator unit 3 via an axially arranged pipe 35 with a smaller diameter than that of the first gas separator unit 2, causing the rotational velocity of the gas mixture to increase further. The ratio of diameters between the outer wall 35 of the second gas separator unit 3 and the actual separator housing 14 is in the range 0.1 - 0.75, preferably approximately 0.5. As such, the centrifugal acceleration will here increase to the order of 300 - 1500 G, preferably approximately 800 G at the outer wall 35 of the separator unit 3. This will result in most of the free liquid drops separating through drainage orifices 30 in the outer wall 35 of the separator unit 3, so as to create a dry gas stream 12. The separated liquid is conducted down to the liquid sump 5 by means of a drainage pipe 31.
The gas stream 12 is sent out of the well for further treatment, while the liquid fraction in the liquid sump 5 is injected into a suitable area 19 of the well (Fig. 2) by the pump 6.
Figure 4 shows an embodiment of the device in accordance with the present invention, in which the liquid fraction is also transported out of the well. The separation device itself corresponds to that which is shown in Figure 3, apart from the fact that the liquid fraction is carried out of the well in the form of a stream 13, via a centrally located pipe 38.
The operation of the pump 6 is the same for the two embodiments shown in Figures 3 and 4. The pump 6 may be run by a hydraulic or electric motor 21 placed in the borehole. The power supply to the motor 21 is placed centrally in the borehole so as to minimise the reduction of the effective diameter of the separation device. If the motor 6 is driven hydraulically by means of pressurised water, the water exiting from the motor 21 may be sent directly to the liquid sump 5 and either be injected into the well along with the separated water (Fig. 3) or be pumped to the surface with the separated water (Fig. 4).
If the pump is electrically driven, electric power may be conducted to the motor 21 through a centrally located pipe 32.
As an alternative (not shown), the pump may be driven mechanically by a drive shaft or a driving rod being led from a motor positioned over the separation device downhole or from a motor on the surface, down to the pump 6. This drive shaft or driving rod is placed centrally in the borehole, in the same manner as the power supply described above.
Fig. 5 shows an embodiment in which the separation device is placed in a horizontal or steeply inclined section of the well. The separation device itself corresponds to that which is shown in Figure 3, apart from the fact that the introduction of the gas/liquid mixture and the draining of the separated liquid are different.
The separation device is rotationally oriented in such a manner in the well that the inlets 33 are positioned in the upper half of the cross section of the well. Besides, the liquid/gas separator unit of the separation device has a closed bottom and a drainage outlet 26 corresponding to 29 and 31 for the first and second gas separator units respectively. Due to the low drainage height above the liquid sump 5 in such horizontal or steeply inclined wells, it will in some cases be necessary to pump the liquid out of the separator chambers by using a small auxiliary pump. This is illustrated by an ejector pump 39 run by a small side stream from the injection water stream. A small hydraulic pump run by this side stream may also be used.
Due to the different pressures in the different separator units the liquid drainage pipes must be led up to the auxiliary pump separately.
In order to prevent foaming at the liquid outlet, the outlet is designed as a small outlet cyclone (40) that separates liquid and gas, and where the liquid outlet is submerged in the liquid.
Fig. 6 shows a variant of the alternative shown in Fig.5. Here, the injection water pump is located above the actual separation device while the injection water is sent back through central pipe 32, to an injection zone below the separator. In this case, the auxiliary pump, typically an ejector pump, is run by a small side stream from this pressurised injection water, corresponding to Fig.5.
Figs. 7 A and B show sections through typical embodiments of the static vanes 25 and 27 for the gas/liquid separator unit 1 and the first gas separator unit respectively.
For all embodiments, the injection pump 6 must be controlled so that the liquid level in the liquid sump 5 is maintained between a highest level corresponding to the lowest inlet 33 to the separation device and a lowest level corresponding to the pump inlet. This is typically done by a level gauge 36 controlling the pumping of the injection water.
Even though the embodiments shown in Figures 2 - 4 show a separation device arranged vertically and the embodiments shown in Figures 5 and 6 show a separation device arranged horizontally, it is also conceivable that the separation device may be arranged at any angle between the horizontal and the vertical orientation.
In those cases where the incoming liquid/gas ratio is low and the need for liquid removal is high, it would also be possible to use the first and second gas separator units 2, 3 only.
In those cases where the incoming liquid/gas ratio is high and the need for liquid removal is moderate, it would also be possible to use the liquid/gas separator unit 1 and the first gas separator unit 2 only.
In those cases where the incoming liquid/gas ratio is low and the need for liquid removal is moderate, it would also be possible to use the first gas separator unit 2 only.
In some cases (not shown) it may be of interest to place the gas dehydration stages (first and second gas separator units 2, 3) high up in the well while placing the liquid/gas separator unit 1 near the production zone. The partially dried gas will then be transferred between the liquid/gas separator unit 1 and the gas separator units 2 and 3 through a dedicated pipe. Such a configuration would be able to produce even drier gas at the outlet of the well, as liquid particles that separate out due to the pressure drop through the production tubing will be removed by the gas separator units 2, 3.

Claims

A device for separating gas and liquid, which device is located downhole in a well that produces from a gas reservoir, and which device comprises at least two separator units arranged in a common housing (14), the separated liquid fraction from each separator unit being passed to a common liquid sump (5), the device in combination comprises:
a first gas separator unit (2) for separation of a well stream (4) or a pre-treated wellstream (8), which first gas separator unit (2) comprises an axially arranged feed pipe (23) with a diameter that is smaller than that of the housing (14), radially arranged static vanes (27) that impart a tangential spin to the gas stream (8), and a wall (28) enclosing the vanes (27), and

a second gas separator unit (3) comprising an axially arranged pipe (35), which pipe (35) is provided with apertures or slots (30) for drainage of separated liquid,
characterised in that
the axially arranged pipe (35) is of a diameter that is smaller than that of the wall (28) enclosing the vanes (27) of the first gas separator unit (2), and has an axial opening to receive the axial flow of the gas stream (8) from the first gas separator unit (2), which reduction in diameter causes the tangential velocity of the gas stream (8) passing from the first separator unit (2) to the second separator unit (3) to increase further..
A device in accordance with Claim 1 for establishing a pre-treated wellstream (8) suitable for gas dehydration,
characterised in that it further comprises a gas/liquid separator unit (1) provided upstream of the first gas separator unit (2), which gas/liquid separator unit (1) includes radial inlets (33) for introduction of a wellstream to be treated, radially arranged static vanes (25) in close proximity to the inlets (33) for imparting a tangential spin to the wellstream, an enclosing wall (14) for collection of liquid, and a centrally positioned outlet pipe (23) for drawing off gas for further dehydration.
A device in accordance with one or more of the preceding claims, which device further comprises a pump (6) designed to inject the separated liquid into an injection zone (19) in the well or pump liquid to the surface,
characterised in that the pumping rate is controlled on the basis of the liquid level in the liquid sump (5).
A device in accordance with one or more of the preceding claims,
characterised in that the device is provided with one or more centrally positioned pipe(s) (32, 38) for lead-through of cables, shafts, pipes or similar for supply of energy to the pump (6) or a turbine, electric motor or mechanical device driving the pump (6).
A device in accordance with one or more of the preceding claims, which device is disposed in the essentially horizontal or steeply inclined section of the well
characterised in that the inlets (33) are provided in the upper half of the cross section of the well only, that the gas/liquid separator unit (1) of the separator has a closed bottom, and that the liquid from each of the separator units (1, 2, 3) is pumped out by a pump (3 9), preferably an ejector pump.
A device in accordance with Claim 5,
characterised in that it further comprises an outlet cyclone (40) downstream of the pump (39) in order to prevent foaming at the outlet.
A method of separating a gas/liquid stream from a well that produces from a gas reservoir by means of a device according to any of claims 1-6, which separation preferably takes place downhole,
characterised in that the method includes the steps of:
a) leading a wellstream or pre-treated wellstream containing gas and liquid into a first separator unit (2) and imparting to this stream a spin having a centrifugal acceleration in the range 50-500 G, preferably approximately 250 G;

b) leading the separated liquid from the separator unit (2) to a liquid sump (5);

c) leading the remaining gas to a second separator unit (3) and imparting to this stream a spin having a centrifugal acceleration in the range 300-1500 G, preferably approximately 800 G;

d) leading the separated liquid from the second separator unit (3) to the liquid sump (5),

e) collecting liquid drops in an outer, enclosing chamber between the first gas separator unit (2) and the second gas separator unit (3) and conducting these to a liquid sump (5), and

f) leading the separated gas out of the separation device via a gas pipe (15) located centrally in the separation device.
A method in accordance with Claim 7 for establishing a pre-treated wellstream (8) suitable for gas dehydration,
characterised in that the wellstream (4) is introduced into a gas/liquid separator unit (1) and a spin with a centrifugal acceleration in the range 1 - 100 G, preferably approximately 25 G, is imparted to the wellstream, and that the separated liquid is carried to the liquid sump (5).
A method in accordance with one or more of the claims 7 - 8 for carrying the separated liquid out of the well or for injection into a suitable area of the well
characterised in that the rate of such liquid removal is based on level measurements (34) in the liquid sump (5).