GB2321527A - Maintaining sample cell membrane gas tightness - Google Patents

Abstract

A method and device for improving the duration over which semi-permeable membranes remain gas-tight when used in a receptacle such as a test cell, for example, communicating with a hydraulic system allowing pressurised multi-phase fluids to be displaced through the sample. The hydraulic system comprises hydraulic circuits 11, 17, 13, 15, burettes 12, 14 for separating the fluids, pumps OP, WP and switching means (V) allowing fluids to be displaced in the sample (drainage and imbibition) as well as means for establishing a circulation of wetting liquids in contact with the membranes M1, M2 in order to keep them saturated with liquid, thereby preventing gas from passing through them for a long period during phases when gas is being injected into the samples. Application to three-phase measurements on geological samples, for example.

Description

METHOD AND DEVICE FOR IMPROVING THE DURATION OVER WHICH MEMBRANES INSERTED IN A FLUID DISPLACEMENT SYSTEM REMAIN TIGHT The present invention relates to a method of improving the period over which semi-permeable membranes used in a fluid displacement process remain gas-tight and a device for implementing the method. The method is particularly well suited to membranes capable of confining a gas injected into a container such as a test cell, for example, communicating with a displacement system for multiphase fluids.

The method of the invention finds applications specifically in measuring operations used as a means of determining various physical parameters of porous samples placed in a cell in the presence of two- or three-phase fluids.

A device is disclosed in patent FR-A-2.731.073 which allows geological samples to be tested, for example, so that various parameters such as the capillary pressure of rocks in drainage or imbibition phases can be determined, their wettability, their relative permeability, their resistivity index, etc. The sample to be tested is placed inside a rigid body in a containment receptacle laterally delimited by a flexible sheath and having two end-pieces at its two opposite ends. Passages through these end-pieces place the sample in communication via semi-permeable membranes with fluid circulating means comprising a pump delivering a first fluid (such as water) , an injection column for a second fluid such as oil, two burettes to receive the fluid displaced out of the sample and a delivery device connected to valves delivering gas which can be injected into the containment receptacle in order to take measurements with three-phase fluids. The distribution circuits and control valves can be used for both two-phase and othree-phase measurements.

During certain measurement operating phases in the presence of three-phase fluids where gas is injected in specifically determined stages over relatively long periods, sometimes as much as several days, the semi-permeable membranes initially saturated with liquid become rapidly desaturated and the gas is able to penetrate and build up in the burettes. The tests then have to be halted whilst the membranes are restored or replaced.

The method of the invention avoids the disadvantages outlined above. It improves the period over which semipermeable membranes remain gas-tight in a receptacle communicating with a multi-phase fluid displacement system and provides at least one opening fitted with a membrane that is wettable by a certain liquid.

It is characterised in that the membrane is kept permanently saturated with liquid by contact with a circulation of this liquid so as to retain its searing capability.

In one embodiment in which the container is a test cell for a sample of porous material in the presence of multi-phase fluids, for example, having at least one opening communicating with a liquid circulation system and fitted with a membrane wettable by a liquid and an orifice communicating with gas injection means, a displacement system is used whereby each membrane is kept permanently saturated with wetting liquid so as to keep it Aessentially gas-tight.

Once such a circulation of liquid has been established, gas can be injected into the cell without it being able to escape. Gas injections can then be operated at constant pressure (injection stages) for relatively long periods, as much as several days, for example. Running operations of a longer duration is thereby simplified.

The device of the invention is able to improve the duration over which semi-permeable membranes will remain gas-tight, these being used in a test cell for a sample of porous material in the presence of multi-phase fluids, which is delimited by at least one rigid tubular sleeve with two end-pieces at its two opposite ends, having an elastic sheath inside the cell where the sample is placed, the two end-pieces being provided with a terminal section designed to locate in the interior of the sheath at the opposite ends thereof and passages communicate with a hydraulic system in order to produce a displacement of liquids through the sample under pressure, pressure means apply the sheath radially against the sample, at least one semi-permeable membrane wettable by a liquid is inserted between at least one of the end-pieces and one of the ends of the sample and means provide selective communication between the interior of the sheath and a source of pressurised gas.

The device is characterised in that it has means within the hydraulic system for establishing a circulation of wetting liquid in contact with each membrane ln order to keep the latter permanently saturated during the phases when gas is being injected into the sample. These circulating means consist, for example, of two circuits connected respectively to the two opposite ends of the sample through the two end-pieces, each incorporating a fine tube for separating the phases, a pump for circulating the liquid and switching means so that the liquids can be displaced selectively inside the sample or a liquid can be circulated in contact with each membrane in order to keep them saturated.

The device has pressure and volume measurement means, for example, and a management device comprising means for acquiring measurement signals supplied by the measuring means as well as a control unit for coordinating the circulation of different phases through the sample or in contact with the membranes.

In one embodiment, the device has means for selectively applying over-pressure to one of the two liquids during phases when gas is being injected at a pressure below a critical pressure (Pc) for spontaneous re-imbibition of the sample by the other liquid.

Other features and advantages of the method and device of the invention will become clear from the following description of an embodiment, which is not restrictive in any respect, and with reference to the attached drawing, which provides a schematic illustration of the layout of the device.

The device consists of (Fig. 1) a rigid, elongate containment receptacle 1, which is cylindrical for example and which may be closed at its two opposite ends by two removable caps 2, 3. Fixing and sealing means (not illustrated) allow these to be secured to the ends of the receptacle. The sample to be tested S is placed inside a deformable sheath 4 and the two are placed inside the receptacle 1. The two caps 2, 3 are designed to locate in the terminal portions of the sheath 4 when they are secured to the vessel 1. The annular space 5 around the sheath 4 communicates with a source of pressurised gas 6 by means of a passage 7. It may be a pressurised gas cylinder, a piston or a hydraulic pump. This fluid is applied in order to place the sample to be tested under a given pressure.

Inserted between the sample S to be tested and the two end-pieces 2, 3 are semi-permeable membranes M1, M2 of variable thickness. These membranes are pressed against the porous sample S in order to ensure good capillary continuity. They are selected as a function of the fluids to be displaced inside the sample. The membranes M1 and M2 are wettable by oil and water respectively, for example.

Passages 8a, 8b at one side and 9a, 9b at the other side arranged parallel with the axis of the vessel are pierced through the two caps 2, 3. Connected to the passage 8a in cap 2, for example, is a line 10 which connects with the output of a pump OP, preferably of the double-acting type, via=a valve Vl and with an oil tank OI viaa solenoid valve V2.

The other passage 8b is connected to a line 11 which connects via a solenoid valve V3 and a valve V4 to the median part of a first glass tube or burette 12, the internal diameter of which is calibrated, containing a first fluid such as oil. A bubble trap BT1 is inserted on the line 11.

Connected to the passage 9a is a line 13 communicating via a solenoid valve V5 and a valve V6 with the median part of a second glass tube or burette 14, whose internal diameter is also calibrated, containing a second fluid such as water. A second bubble trap BT2 is interconnected into the line 13. The output of a pump WP for the second fluid, by preference a pump of the double-acting type, supplies a line 15 linked via a valve V7 to the passage 9b in the cap 3. An inlet of the pump WP is connected via a line 16 fitted with a valve V8 to the base of the burette 14. A first pressure detector PD1 is used to measure the pressure in the burette 14. The base of the first burette 12 is connected to a line 17 linked via a valve V9 to the output of the pump OP. A tank 18 containing a pressurised gas such as nitrogen is connected by means of a line 19 to the top of the first burette 12 via a control valve V10 and by means of a line 20 to an inlet of a differential pressure detector PD2. A second inlet of the detector PD2 is connected to the base of the burette 12 and the line 17. The pressure in the gas tank 18 is treasured by a pressure detector PD3.

Inserts (not illustrated) are arranged through the sheath 4, allowing lines 21 to be connected to a second gas tank 22 via valves V12.

A line 23 provides communication between the interior of the body 1 of the cell and a source of compressed air 24, so that a radial confining pressure can be applied to the sample S inside its sheath 4 during operation. The confining pressure applied to the sample is measured by a pressure detector PD4.

Opto-electronic type detectors (not illustrated) allow variations in the volumes of fluid inside the burettes 12, 14 to be measured. The measurement signals M issued by the various detectors as well as all the pressure detectors PD1 to PD4 are collected by an acquisition device 25 connected to a control micro-computer 26.

Measurement cycles Similarly to the device described in patent FR-A2.731.073 mentioned above, the device of the invention can be used to perform the following operations: - oil-water two-phase drainage - water-oil two-phase imbibition - relative permeabilities - gas-oil-water (mobile or irreducible) three-phase drainage - water-oil-gas three-phase imbibition - three-phase relative permeabilities.

Two-phase measurements Drainage During an initial period, the sample S placed in the cell is saturated with a wetting fluid such as water, for example. The two-phase drainage of the sample is produced by displacing the wetting phase with the non-wetting phase, in this case oil, in progressive pressure stages. The purpose of this operation is to plot a curve showing the differential pressure between the wetting and non-wetting fluid as a function of saturation with non-wetting fluid.

The oil from the oil tank OI is injected via the passage 8a. It displaces the water contained in the sample, which is evacuated through the cap 3 across the membranes M2 up to a certain volume imposed by the capillary pressure.

The effluents are recovered in the burette 14 and the change in water height is measured.

At instants selected by the operator, the acquisition unit 25 piloted by the micro-computer 26 records the various physical measurements: pressures, volumes, etc.. When production of water stops, the porous sample may be scanned if required to check saturation of the two phases. The progressive pressure stages are halted when the watersaturated membrane M2 allows the oil phase to pass through.

Imbibition The imbibition operations can be performed by decreasing the pressure in regular stages. The steps are similar to those described in respect of drainage.

Three-phase operations Drainage To produce a capillary pressure curve for drainage in the presence of a mobile or a non-mobile water, the following steps are performed: - saturation of the porous medium with water, - plotting of the capillary pressure curve for water-oil drainage (Fig. 3) until a pressure is reached which produces the desired irreducible water saturation SWi, - possible lowering of the pressure in the oil in order to produce a re-imbibition of water until a given water saturation Sw is reached, - three-phase drainage by injecting gas in progressive pressure stages in order to displace the oil.

The layout of the circulating device behind the membranes makes it more difficult if not impossible for the injected gas to escape from the cell through the membranes M1 and M2. The passages 8a, 8b, 9a, 9b through the two caps 2, 3 are connected in the one case to a closed circuit (10, 11, 17) incorporating the pump OP and in the other to a closed circuit (13, 15, 16) incorporating the pump WP. This means that when the gas is being injected via the opening of valves V12 on the lines 21, the fluids can circulate respectively behind each of the membranes M1 and-M2. Wetted in this way and thus permanently saturated, the membranes continue to be not very permeable to the gas.

The layout of the device when adapted to apply a back-pressure on the fluid in the oil burette 12 also allows better control of the pressure in the oil circuit, which is necessary in certain three-phase operations, particularly on porous samples wettable by water. The water is driven by the oil until irreducible saturation Swi is reached. If the pressure applied to the oil is lowered to a level below the critical pressure Pc for re-imbibition, the sample reimbibes spontaneously. If it is desirable to lower the gas pressure to a stage lower than that of critical pressure Pc, the pressure of the oil circulating in the oil circuit (11, 17) must be maintained at an adequate level. To this end, the valve V11 is opened in order to place the burette 12 in communication with the gas tank 18, thereby applying a pressure to the oil in the oil circuit.

Claims (9)

1. A method of improving the duration over which semi-permeable membranes remain gas-tight in a receptacle communicating with a multi-phase fluid displacement system and having at least one opening fitted with a membrane wettable by a certain liquid, characterised in that the membrane is kept permanently saturated with liquid by means of contact with a circulation of this liquid in order to maintain its tightness capacity.

2. A method as claimed in claim 1, in which the receptacle is a test cell for a sample of a porous material in the presence of multi-phase fluids having openings communicating with means for injecting liquids, at least one of the openings being fitted with a membrane wettable by a liquid, and an orifice communicating with gas injection means, characterised in that before injecting gas into the sample a circulation of liquid is established in contact with each membrane in order to keep it saturated with this liquid.

3. A device for improving the duration over which semi-permeable membranes remain gas-tight when used in a test cell for a sample of a porous material in the presence of multi-phase fluids, which is delimited by at least one rigid, tubular sleeve (1) and having two end-pieces (2, 3) at its two opposite ends, an elastic sheath (4) inside the cell where the sample (S) is placed, the two end-pieces being provided with a terminal portion designed to locate inside the sheath at the opposite ends thereof, and passages (8, 9) communicating with a hydraulic system in order to produce a displacement of liquids through the sample under pressure, pressure means (24) to press the sheath radially against the sample (S), at least one semi-permeable membrane (M1, M2) wettable by a liquid inserted between at least one of the end-pieces (2, 3) and one of the ends of the sample (S) and means to provide selective communication between the interior of the sheath and a source of pressurised gas, characterised in that it has means within the hydraulic system for establishing a circulation of wetting liquid in contact with each membrane in order to keep the latter permanently saturated during phases when gas is being injected into the sample.

4. A device as claimed in claim 3, characterised in that the hydraulic system has two circuits (13, 15), (11, 17) connected respectively to the two opposite ends of the sample (S) through the two end-pieces (2, 3) , each incorporating a fine tube for separating the phases (12, 14), a pump for circulating liquid (OP, WP) and switching means (V) for selectively displacing the liquids inside the sample or establishing a circulation of liquid in contact with each membrane in order to keep these saturated.

5. A device as claimed in claim 3 or 4, characterised in that the membranes (M1, M2), which are respectively wettable by the two liquids, are inserted between the sample and each of the two end-pieces, each end-piece (2, 3) having two passages (8a, 8b) , (9a, 9b) connected to one of the circuits allowing a circulation of liquid to be established in contact with each of the membranes.

6. A device as claimed in one of claims 3 or 4, characterised in that it has pressure and volume measuring means and a management device incorporating means (25) for acquiring measurement signals supplied by the measuring means as well as a control unit (26) for coordinating the circulation of different phases through the sample or in contact with the membranes.

7.--A device as claimed in claim 3, charasterised in that it has means for selectively applying an over-pressure to one of the two liquids during phases when gas is being injected at a pressure below a critical pressure (Pc) for spontaneous imbibition of the sample by the other liquid.

8. A method of improving the duration over which semi-permeable membranes remain gas-tight in a receptacle communicating with a multi-phase fluid displacement system substantially as hereinbefore described with reference to the accompanying drawing.

9. A device for improving the duration over which semi-permeable membranes remain gas-tight when used in a test cell for a sample of a porous material in the presence of multi-phase fluids substantially as hereinbefore described with reference to the accompanying drawing.