GB2322942A - Device for taking electrical resistivity measurements on a solid sample - Google Patents

Abstract

A device for taking electrical resistivity measurements on a solid sample (S) inside a rigid receptacle 2 by connecting to the surface of the sample electrodes E arranged between the sample and an elongate containment sheath 1. To obtain an excellent coupling between electrodes and solid, means 7,8 for mechanically clamping the sheath whereby an axial compression with concomitant radial deformation is exerted thereon, or alternatively a heat-shrinkable sheath which is shrunk once the electrodes have been positioned may be used. The electrodes and the electric conductors C linking them to an electric measuring apparatus 6 can be incorporated in a flexible, multi-layered circuit arranged between the sample and the sheath.

Description

DEVICE FOR TAKING ELECTRICAL RESISTIVITY XHASUREMENTS ON A SOLID SAMPLE The present invention relates to a device for taking electrical resistivity measurements on a solid sample and a method of implementing the device.

The device of the invention finds applications in numerous fields and in particular in the measurement of the electrical resistivity of geological samples as a means of determining various parameters in the presence of fluids.

The resistivity of a solid sample can be measured by means of electrodes, which are placed in contact with the surface at selected points, between which an electric current is passed. The measurement of the difference in potential occurring between the positions of the electrodes gives a direct measurement of the resistivity. To ensure that the measurements are meaningful, the contact between the electrodes and the surface must be as tight as possible.

A known method, for example, consists in placing the sample to be tested in an elastic sheath. The electrodes are placed between the sample and the sheath and connected through the sheath by means of electric conductors to an electrical conductivity measuring system. The sheath is placed in a containment receptacle. A pressurised fluid is allowed to pass into the sheath, the effect of which is to press the sheath and hence the electrodes firmly against the sample.

A method such as this based on using a pressurised fluid to press a sheath against a sample is used, for example, as one of the petro-physical tools described in patents FR-A-2 708 742 and 2 724 460 filed by the applicant or in patent US-A-5 105 154, for example. The method is easy to apply in all instances where the receptacle containing the sample is static. It would be more difficult to conceive of using this method if resistivity measurements had to be taken during a process whereby the sample is placed in a relatively small and light mobile apparatus and subjected to rapid displacements.

This situation arises with certain types of measurements when studying the movements of fluids in a sample of a porous material in a relatively light cell or bucket subjected to the action of centrifugal force, due to mass and space constraints.

Such a centrifugation system adapted to handle a sample of a porous material containing a fluid (such as brine, for example) is described, for example, in patents FR-A-2.666.147 (US 5,253,529), FR-A-2 699.282 (US 5,463,894) filed by the applicant. This centrifugation system comprises a motor driving several arms in rotation. The sample, in the form of a cylindrical bar possibly sheathed around its periphery, is placed in a bucket containing another fluid such as oil, for example, and fixed to the end of an arm so that the centrifugal force drives the denser fluid out radially from the sample.

The device of the invention allows electrical resistivity measurements to be taken on a solid sample placed in a relatively narrow recipient (and in particular a sample that will be subjected to centrifugation) having electrodes arranged between the sample and a containment sheath and an electrical measuring device connected to the electrodes.

It is characterised in that it has mechanical clamping means so that the electrodes can be applied tightly against the sample by virtue of a mechanical action on the sheath.

In one embodiment, the sheath is made from a deformable material and the device has a rigid recipient or bucket, the section of which is just sufficient to contain the sample in its sheath, and mechanical means to apply a compression force directed along the direction of elongation of the sheath and produce a centripetal expansion which will couple the electrodes with the sample, this force being produced by tightening a nut, for example.

In a second embodiment, the sheath is of the heatshrinkable type and the mechanical clamping means include a device for temporarily heating the sheath once it has been placed around the sample.

In a preferred embodiment, the device has a flexible sheet of insulating material placed around the sample and the electrodes are connected to the measuring apparatus by connecting means with conductor tracks running through the thickness of the sheet. This flexible sheet is used in conjunction with a heatshrinkable sheath, for example, the purpose of which is to press it against the sample.

By preference, the electrodes and possibly the conductor tracks are made from a stainless conductive material: noble metal, stainless steel, etc., incorporated in the sheet.

The device of the invention is particularly well suited to producing a substantial lateral coupling force on a sample in applications where centrifugation is used, for example, in which case this sample is placed in a relatively light mobile apparatus in which the available space around it is restricted.

The invention also relates to a method for implementing the device as defined above, where the device is incorporated in a recipient linked to means which will apply a centrifugal force thereto and variations in the resistivity of the sample due to displacements of fluids in the sample under the effect of the centrifugal force applied are measured.

Other features and advantages of the 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 appended drawings, of which - Fig. 1 depicts a first mode of coupling electrodes with a sample in a containment receptacle by compressing a sheath; - Fig. 2 illustrates a second mode of coupling electrodes and connecting conductor tracks with a sample in a containment receptacle using a heat shrinkable sheath; - Fig. 3 shows a detail of how the electrodes and the connecting conductor tracks are laid out; - Fig. 4 depicts another embodiment of the sheath arranged around the sample; - Fig. 5 illustrates a centrifugation system for a receptacle containing a sample to be tested; and - Fig. 6 is a schematic illustration of the apparatus used to measure the electrical resistivity of a sample; - Fig. 7 illustrates an example of how the electrical resistivity measurements of a sample are distributed as a function of the water saturation thereof, obtained using the device of the invention.

The device of the invention (Figs. 1 to 4) is used in the examples described below to take electrical resistivity measurements on a sample S in the form of an elongate bar, cut in a rock fragment, for example, in which fluids are displaced. The sample S is laterally confined by a sealed sheath 1 made from a dielectric material and placed in a receptacle. The inner cavity of the receptacle 2 is delimited at a first end by a wall 3 having an opening 4 which is able to communicate with a hydraulic system 5 allowing fluids to be displaced through the sample S.

In order to measure the resistivity of the sample, several electrodes E are arranged in contact with the lateral wall thereof. They communicate with an electric measuring device 6, set up to circulate electric currents selectively and measure the differences in potential between the electrodes.

In the embodiment illustrated in Fig. 1, the sheath is made from a deformable material, a plastics or elastomer material, and the internal section of the receptacle is substantially the same as that of the sheath 1. At its end opposite the wall 3, the receptacle 3 is delimited by a screw-down ring 7 having an annular rim 8, the section of which is the same as that of the sheath surrounding the sample and the size of which is selected so that it will compress the sheath 1 longitudinally when the cap is completely screwed down. The resulting centripetal expansion has the effect of applying the electrodes E with force against the lateral wall of the sample S.

In order to improve the compression force applied to each of the electrodes E still further, the device may have radial screws 9 in orifices provided through the wall of the receptacle behind each of the electrodes which can be adjusted as required.

In the embodiment illustrated in Fig. 2, the containment sheath 1 around the sample is made from a heat-shrinkable material. The electrodes E are inserted between the external wall of the sample and the sheath 1 and the latter is exposed to sufficient heat to cause the sheath to shrink so that it applies the electrodes E with force against the lateral wall of the sample.

In a preferred embodiment, the electrodes E are incorporated in an insulating plastic sheet 10 (Fig. 3) enclosing the sample S. They are connected to external circuits C providing a link to the measuring apparatus 4 by means of electric conductors 11 running through the thickness of the sheet. This latter may be a multilayered sheet (Fig. 3), in which case the electric conductors will be photo-etched onto an inner face of one of them (10a) using a known technique. By preference, these electrodes E and these electric conductors 11 are made from a stainless material such as stainless steel or a noble metal so as to improve their reliability. The chip forming the electrode E may be inserted by means of electrolysis in an orifice arranged in the layer 10a in contact with the rock. The plastic sheet 10 incorporating the electrodes and connecting tracks 11 is arranged around the sample S and is then tautened around it by adding and shrinking a heat-shrinkable sheath 1, for example.

In order to provide a better seal, a sheath 1 can be used which has (Fig. 4) a first layer made from elastomer arranged around the sample S provided with its electrodes in conjunction with an outer heatshrinkable sheath lb.

The two modes of applying electrodes against a sample described above can be used, for example, as a means of taking resistivity measurements incorporated in a system for displacing fluids by centrifugation as described in patent FR-A-2.699.282 (US 5,463,894) mentioned above.

Placed in a bucket containing oil is a sample saturated with a denser liquid (such as brine, for example, and simultaneous measurements are taken on the volumes of fluid displaced under the effect of the centrifugal force and the resistivity of the sample using the electrodes pressed against the sample (Figs.

1-3). The purpose of these combined measurements is to determine the variation in resistivity as a function of saturation. The outer sheath ensures that the lateral surface of the sample is sealed and fluid can be displaced in the direction of elongation thereof only.

The centrifugation system consists of (Fig. 5) a circular tank 12, an electric motor 13 resting on a base 14 integral with the wall of the tank 12, which drives a hub 15 fitted with two arms 16 of the same length arranged opposite one another in rotation. Two buckets 2 substantially of the same mass, for reasons of balance, are pivotally mounted on each of the respective ends of the two arms 16 so that they are spontaneously aligned with the direction of the centrifugal force applied. Each bucket has a tapered terminal portion 2a in which the fluids extracted from the sample by centrifugal force accumulate. A graduated transparent window 17 is arranged in this terminal portion. Using a stroboscope (not depicted), the window is illuminated so that the position of the water/oil meniscus can be detected in order to deduce the volume of fluid expelled.

The electric conductors associated with the various electrodes form a cable 18 which runs along the arm as far as a rotating connector 19 of a known type.

The peripheral part 20 of this rotating connector is held in position by a bracket 21 secured to the wall of the tank 12 and linked by a collector cable 22 to an electric measuring device 6 driven by a control system 23.

Three pairs of electrodes (el, e'1), (e2, e'2) and (e3, e'3), for example, are placed in contact with the sample (Fig. 6), the electrodes of each pair being arranged symmetrically relative to the axis of the bar.

The electrodes el and e3 are inter-connected and linked via a resistor R0 to a first terminal of an alternating generator 24. The electrodes e'l and e'3 are also inter-connected and linked via a switch I1 to a second terminal of the generator 24. The pairs of electrodes (el, e3), (e'1, e'3) as well as the electrodes e2 and e'2 are connected respectively to the different terminals of a switch C. The different voltages Vo-V3 occurring between them and the earth (first terminal of the generator 24) are measured by a voltmeter. The intensity passing through the sample is expressed by I = vo/Ro and the electrical resistance R of the sample is calculated by the equation: R = (V2-Vl)/I where R=R0.(v2-v1)/Th In practice, in accordance with the conventional rules of the art, a known four-conductor connection is used to connect the three pairs of electrodes to the measuring apparatus 6.

The resistivity measurement (in Q.m) of a reservoir rock sample taken from a geological formation as a function of saturation by wetting/non-wetting fluids will provide information about the interactions between these fluids and the rock minerals (distribution of fluids in relation to mineralogical nature and superficial composition). These interactions will determine behaviour in terms of wettability, relative permeability values, etc., a knowledge of these values being crucial in terms of understanding and modelling hydrocarbon recovery from petroleum deposits.

The constraints resulting from centrifugation are not the same at all points of the sample. Any distribution of saturations will depend on the geometry and rotation speed of the centrifuge as well as the different densities of the two fluids. Measuring the quantity of fluid produced by the sample will allow the mean saturation to be calculated. The value of capillary pressure at the inlet of the sample can be calculated. It will then be possible to calculate the capillary pressures at the cross-section of the sample at which the electrodes are positioned in order to derive saturation at this point. Given that the resistivity measurement R and saturation S are known, the law of variation R(S) can then be applied.

The resistivity of rocks is one of the parameters which can be obtained by taking well measurements in situ and adjustment by comparison with laboratory measurements is therefore valuable. The relationship between water saturation and the resistivity of the rock is conventionally expressed by Archie's law: IR = R/R100=SW-n where IR is the resistivity index, the ratio of the resistance R of the rock measured at saturation Sw to the resistance of the rock entirely saturated with water. The exponent n is usually close to 2 in the case of water wet samples.

It would not be a departure from the scope of the invention if the device of the invention were to be used in combination with a system for measuring the travel time of acoustic impulses through the bar at different points along each bucket, as described in the above-mentioned patents filed by the applicant.

Claims (11)

1. A device for taking electrical resistivity measurements on a solid sample (S) placed in a relatively narrow, rigid receptacle (2) having electrodes (E) arranged between the sample and a containment sheath (1) and an electric measuring device (6) connected to these electric electrodes (6), characterised in that it has mechanical clamping means to apply the electrodes tightly against the sample by a mechanical action on the sheath.

2. A device as claimed in claim 1, characterised in that the sheath is made from a deformable material, the cross-section of the rigid receptacle (2) being just sufficient to contain the sample in its sheath, the device having mechanical means (7, 8) for applying a compression force directed along the direction of elongation of the sheath and producing a centripetal expansion to couple the electrodes with the sample.

3. A device as claimed in claim 1, characterised in that the sheath (1) is of the heat-shrinkable type and the mechanical clamping means also incorporate a device for temporarily heating the sheath once it has been placed around the sample.

4. A device as claimed in claim 1, characterised in that the sheath (1) has a first layer (la) made from a flexible material and a second layer (lb) of the heat-shrinkable type, the mechanical clamping means having a device for temporarily heating the sheath once it has been placed around the sample.

5. A device as claimed in any one of the preceding claims, characterised in that it comprises a sheet (10) of an insulating material placed around the sample and the electrodes (E) are connected to the electric measuring apparatus (6) by connecting means consisting of tracks (9) running through the thickness of the sheet (10).

6. A device as claimed in claim 5, characterised in that the electrodes and the conductor tracks are made from a conductive material such as a noble metal or stainless steel and incorporated in the sheet (10).

7. A device as claimed in one of claims 4 or 5, characterised in that the sheet (10) is placed inside a heat-shrinkable sheath (10).

8. A device as claimed in one of the preceding claims, characterised in that the receptacle (2) is linked to a centrifugation system.

9. Application of the device as claimed in one of the preceding claims to the study of reservoir rocks in which fluids are displaced.

10. A method for implementing the device as claimed in one of claims 1 to 7, characterised in that the device is incorporated in a receptacle linked to means for applying a centrifugal force thereto and the variations in the resistivity of the sample resulting from the displacements of fluids in the sample under the effect of the centrifugal force applied are measured.

11. A device for taking electrical resistivity measurements on a solid sample substantially as hereinbefore described with reference to the accompanying drawings.