EP1160932A1 - Waterproof electrical connection device for electrodes by shielded cables and petrophysical measuring system - Google Patents

Abstract

A tubular protector sleeve (M) is fastened to wall (18) of an enclosure. Plugs (24,28) are connected to electrodes (E,15) with a base plate that fits into inner cavity of the sleeve. The plug is electrically connected to core (21) of shielded cable inside a metal tube (23). Seals (19,25) are provided to insulate the inside of the enclosure from outside and inner cavity of the tubular sleeve. An Independent claim is also included for physical quantity measurement system.

Description

The present invention relates to a device for establishing precisely in laboratory the curve of the resistivity index of a solid sample independent of the capillary pressure curve, suitable for high measurements frequency.

Measuring the resistivity index of small carrots is necessary for obtain an accurate estimate of water saturation from data from logs obtained for example by the measurement technique during drilling (MWD).

State of the art

By patents FR2 781 573 and FR 2 762 681 (US 5,979,223) of the applicant, in particular, there are known methods and devices for continuously measuring the resistivity index curve of a solid sample initially saturated with a first wetting fluid, such as a geological sample, regardless of the capillary pressure curve. The porous solid sample is contained in a sheath sealed which is placed in an elongated containment cell between two end pieces. Channels through the two end pieces communicate with an injection system allowing a second non-wetting fluid to be injected into the sample at a first end of the cell and drain the first fluid out of the cell to the opposite end, through a semi-permeable membrane, permeable to first fluid. The sample is contained in a sheath and subjected to pressure radial by injecting pressurized oil into the annular space between the body of the cell and sheath. A membrane wettable only by the second fluid is interposed between the sample and the first end of the cell to perform reimbibition operations.

Electrodes interposed between the sample and its sheath allow applying an electric current and detecting potential differences appearing between distinct points in response to the application of current electric. The electrodes being connected to an impedance measuring device sample complex. The longitudinal extension of the electrodes is relatively important compared to the length of the sample so as to involve the most as much of the sample volume as possible in impedance measurements while avoiding short-circuits at the ends of the sample which could distort measures.

One or more injection pressure levels are applied, the continuous variations in the resistivity index as a function of the variation in saturation average without waiting for the capillary balances to be established.

The annular space between the sheath and the outer wall of the cell being under high pressure, the electrical conductors connecting the electrodes to the device measure cross the outside wall of the cell by tight crossings (glass bead connectors for example).

Studies have shown that the resistivity index of porous rocks varies noticeably with frequency. As the log probes measure the electrical resistance of the terrain crossed at often very high frequencies, it is necessary to properly relate the measurements obtained by the well tools and the resistivity index measurements obtained in the laboratory using cells, that these can operate precisely in the same frequency range.

The results obtained with the preceding cells are satisfactory when the frequency range of the applied electric currents remains within the limit of a few KHz or a few tens of KHz. They lose a lot of their meaning when impedance measurements are made at much higher frequencies included for example in the range 1MHz - 10MHz. At such frequencies, of course use shielded cables of constant impedance. The link continues electrodes to the measuring device by shielded cables is difficult to achieve because because of sealing problems. If you use a conventional connector glass bead type for example, this results in a break in the continuity of the shielding. This discontinuity, which would have no significant effect at low frequency, is the high frequency source, stray reflections and significant attenuation signals.

THE DEVICE ACCORDING TO THE INVENTION

The device according to the invention makes it possible to make the connection by shielded cable at least one electrode to a measuring device, located on either side of a wall separating a pressurized enclosure from the outside environment. It includes at least a rigid protective sleeve of insulating material which passes through the wall waterproof and extends to the immediate vicinity of the electrode, in which pass the shielded cable, this rigid sleeve containing a tube of conductive material in electrical contact with the cable shield, and being rigidly associated and sealingly to a connection means electrically connecting the core of the shielded cable to the electrode.

The electrical connection means comprises for example a plug connected to the electrode, with a base which is rigidly and tightly fixed in a sleeve cavity, and electrically connected to the core of the shielded cable.

To take into account a certain possible latitude of movement of the electrode, the plug is engaged in a recess in the electrode and adapted to maintain electrical contact with the electrode as it moves.

According to one embodiment, the device comprises an electrical connector associated with the rigid sleeve on the outside of the wall for the connection of a shielded wire connected to the measuring device, a shielded cable element inside the rigid sleeve the core of which is connected by means of connection, and the shielding is electrically connected to the conductive tube which extends towards the inside of the rigid sleeve up to the connection zone of the core by means of connection.

The wall is for example the wall of the body of a cell intended for measurement variations in the resistivity index of a porous solid sample coated with a sheath and subjected to radial pressure by injection of a pressurized liquid into the body of the cell, these variations being consecutive to displacement operations forced a first fluid out of the sample by injection of a second fluid, one of the two fluids being electrically conductive and the other not, by means of electrodes arranged between the sample and the sheath and each provided with a extension passing through the sheath, each rigid sleeve passing through the wall of the body cell and extending substantially to the sheath.

The measurement system according to the invention comprises a confinement cell elongated for a sample in a coating sheath, means for injecting a pressurized liquid in the cell body to exert radial pressure on the sample, electrodes placed between the sample and the coating sheath, allowing the application of an electric current and the detection of differences in potential appearing between distinct points in the sample in response to the application of electric current. The electrodes are each provided with a extension passing through the sheath and being connected to a device for measuring the impedance of the sample, outside the cell body, a first semi-permeable filter, permeable to the first fluid and disposed substantially in contact with a first end of the sample, and injection means for injection under pressure of a second fluid through a second end of the sample. The system includes connection devices to connect the different electrodes to the meter by shielded cables and each device includes at least one rigid protective sleeve of insulating material which passes through the wall in a sealed manner and extends to the immediate vicinity of the electrode, in which the shielded cable is passed, this rigid sleeve containing a tube in conductive material in electrical contact with the cable shield and being associated rigidly and sealingly to a connection means electrically connecting the core shielded cable to the electrode.

Preferably, the electrodes have a relatively longitudinal extension important compared to the length of the sample (between ¼ and ¾ of the length of the sample and preferably of the order of half) but less than this length, so as to involve as much of the sample volume as possible in impedance measurements while avoiding short circuits at the ends of the sample.

The connection device as just defined, is advantageous in that that it allows the tight crossing of a wall by a shielded wire without there being discontinuity of the core and the shielding of the cable likely to affect the signals which are transmitted there, this in a frequency range extending up to several tens of MHz.

• d'établir une courbe très précise d'indice de résistivité continue en drainage et en peu de temps (environ 2 jours pour un grès typique de 100 mD alors que la durée typique nécessaire en utilisant la technique d'injection continue est souvent de l'ordre d'une quinzaine de jours) ;
• l'incidence de profils de saturation non uniformes au cours des mesures, s'avère négligeable. Ceci est dû à la combinaison de trois facteurs : (i) la technique de mesure de résistivité radiale, (ii) la présence de filtres semi-perméables côté sortie, (iii) la totalité du volume de la carotte est analysée au moyen de mesures électriques (ceci est vérifié lorsque le diamètre de la carotte est supérieur à sa longueur) ;
• de fournir des mesures d'indice de résistivité précis dans une gamme de fréquence très étendue prolongée jusqu'à des fréquences de l'ordre de plusieurs dizaines de MHz.

The measurement system with its or its various connection devices as just defined is particularly advantageous in that it allows:

• to establish a very precise curve of index of continuous resistivity in drainage and in a short time (approximately 2 days for a typical sandstone of 100 mD whereas the typical duration necessary using the technique of continuous injection is often of the fortnightly order);
• the incidence of non-uniform saturation profiles during the measurements is negligible. This is due to the combination of three factors: (i) the technique of measuring radial resistivity, (ii) the presence of semi-permeable filters on the outlet side, (iii) the entire volume of the core is analyzed by means of measurements electric (this is checked when the diameter of the carrot is greater than its length);
• to provide precise resistivity index measurements in a very wide frequency range extended up to frequencies of the order of several tens of MHz.

Presentation of the figures

• la Fig.1 montre schématiquement en coupe longitudinale une cellule de mesure permettant la mesure de la résistivité d'un échantillon poreux ;
• la Fig.2 montre en coupe transversale la disposition des électrodes autour d'un échantillon permettant d'injecter un courant électrique et mesurer la différence de potentiel engendrée par le passage du courant au travers de l'échantillon ;
• la Fig.3 montre schématiquement en coupe longitudinale un dispositif de connexion permettant la connexion électrique étanche d'un câble blindé reliant une électrode à un appareil de mesure extérieur ;
• la fig. 4A représente les variations comparées du module d'impédance Z d'un circuit électrique de test placé à l'extérieur de la cellule (en trait plein) et à l'intérieur de la cellule et raccordée à l'impédancemètre par l'intermédiaire du dispositif de connexion décrit (en pointillés),
• la Fig.4B représente dans les mêmes conditions les diagrammes d'Argand : partie réelle de Z en abscisse et partie imaginaire de Z en ordonnée) qui leur correspondent ;
• la Fig.5A, 5B représentent respectivement les variations comparées du module d'impédance normalisée en fonction de la fréquence pour deux échantillon de grès de Fontainebleau mouillables respectivement à l'eau (ww) et à l'huile (ow) et pour des saturations Sw respectivement de 1 et de 0,38 ;
• les Fig.6A, 6B) représentent dans les mêmes conditions les diagrammes d'Argand qui leur correspondent respectivement ; et
• les Fig.7A, 7B montrent les effets dispersifs de la fréquence sur les courbes représentatives de la variation de l'index de résistivité en fonction de la saturation en saumure, pour deux échantillons de grès de Fontainebleau mouillable l'un à l'eau (ww) (Fig.7A), l'autre à l'huile (ow) (Fig.7B) et la réduction rapide de la pente des courbes au-dessus de 500 KHz.

Other characteristics and advantages will appear on reading the following description of a non-limiting example of embodiment, with reference to the appended drawings where:

• Fig.1 shows schematically in longitudinal section a measuring cell for measuring the resistivity of a porous sample;
• Fig.2 shows in cross section the arrangement of the electrodes around a sample for injecting an electric current and measuring the potential difference generated by the passage of current through the sample;
• Fig.3 shows schematically in longitudinal section a connection device for the sealed electrical connection of a shielded cable connecting an electrode to an external measuring device;
• fig. 4A represents the compared variations of the impedance module Z of an electrical test circuit placed outside the cell (in solid lines) and inside the cell and connected to the impedance meter via the connection device described (dotted),
• Fig.4B represents under the same conditions the Argand diagrams: real part of Z on the abscissa and imaginary part of Z on the ordinate) which correspond to them;
• Fig. 5A, 5B respectively represent the compared variations of the normalized impedance module as a function of the frequency for two samples of Fontainebleau sandstones wettable respectively with water (ww) and oil (ow) and for saturations Sw respectively 1 and 0.38;
• FIGS. 6A, 6B) represent, under the same conditions, the Argand diagrams which correspond to them respectively; and
• Figs. 7A, 7B show the dispersive effects of frequency on the curves representative of the variation of the resistivity index as a function of brine saturation, for two samples of Fontainebleau sandstone, wettable one with water ( ww) (Fig.7A), the other with oil (ow) (Fig.7B) and the rapid reduction of the slope of the curves above 500 KHz.

DETAILED DESCRIPTION

The connection device will be described without this constituting a any limitation, to an experimental system for measuring variations the resistivity index of a porous solid sample, consecutive to forced displacements of a first wetting fluid conducting electricity such as brine for example, by injecting a second non-conductive fluid such as only oil for example (drainage phase) or the second fluid by the first (imbibition phase) such as that described in the aforementioned patents of the applicant.

It includes (Fig. 1) a core containment cell which includes a hollow body 1 with cylindrical symmetry closed at its two opposite ends by two end caps 2, 3. The sample S is placed inside a cylindrical piece in elastomer 4 whose longitudinal section is U-shaped, constituting a sheath for sample S. The set of sample S and its sheath 4, is installed in an interior cavity of the body 1 and is delimited axially on either side by the two tips 2, 3. On the side of the tip 2, the sample S is in contact with a semi-permeable filter 5, wettable by the first fluid such as a filter ceramic. On the side of the opposite end piece 3, the sample S is in contact with a membrane 6 wettable by the second fluid. The interior faces of both end caps 2, 3 are provided with a network of grooves 7 (Fig. 2). Fastening means not shown allow the two end pieces to be rigidly fixed to each other.

Channels 8 pass through the nozzle 3 and communicate the network of grooves 7 on its end face, with a first source 9 delivering the second fluid under pressure. Likewise, channels 10 pass through the end piece 2 and communicate the network of grooves 7 corresponding with a second pressure source 11 of the first fluid drained from the sample due to the injection of the second fluid. An element 12 is installed on the circuit 10 to measure the volume of fluid displaced out of sample S. Preferably, a low cost capacitive sensor is used with an accuracy of 0.05 cc and a resolution of 0.01 cc, similar to that used in the device described in patent application FR 2,772,477 of applicant.

The device comprises for example two pairs of electrodes E1, E2 which are molded inside the sheath 4, so as to press tightly against the wall device of the sample, allowing the application of an electric current. At by means of another pair of electrodes E'1, E'2, likewise molded, the potential difference V created in response to the application of electric current.

This separate assignment of the pairs of electrodes, each, to the application of a current, and the other, to measure potential differences, avoids resistance due to contact. The electrodes are for example of square shape and made in Monel. The angular extension of a pair of electrodes around the sample is less than 90 °. Their length must be less than the length of the sample so as to avoid electrical end short circuits outside the sample, directly through the fluids, which would distort the measurements. However their length must be sufficiently large compared to the length of the sample so that the current lines embrace most of the of its volume with a relatively regular distribution. This length may vary in significant proportions depending on the size of the sample diameter. In the experiments that were performed, we found that the length of the electrodes could advantageously be between ¼ and ¾ of the length of the sample and preferably be of the order of half this length.

The annular space 13 between the body 1 and the sheath 4 communicates with pressure means 14 allowing the injection of a pressurized liquid which exerts a radial confining pressure on the sample S. The radial pressure of confinement around the sample is for example of the order of a few Mpa, sufficient to ensure good electrical contact of the electrodes. So in under normal conditions, the contact resistance is generally of the same order of magnitude as the resistance of the sample to be measured with a low water saturation.

The assembly is placed in a thermostatically controlled enclosure (not shown).

All the electrodes E are provided with a hollow through extension 15 the thickness of the sheath 4, and are connected to an RLC 16 impedance meter coupled with a set 17 of measurement acquisition, by the connection device which will be described below.

The connection device comprises for each electrode E (Fig. 3) a tubular sleeve M of insulating material which engages in a bore N in the outer end wall 18 of the body 1 and rigidly fixed to it. Seals sealing 19 are arranged in grooves of the tubular sleeve M. A BNC type electrical connector 20 well known to those skilled in the art for example, is fixed against the outer wall of the tubular sleeve M. One of its terminals is connected to the core 21 of a portion of screened cable, the other to the braid 22 of this same cable. The braid 22 is in close electrical contact with a stainless steel tube 23 disposed in a cylindrical cavity of the tubular sleeve M. The base 24 of a plug is engaged in another cavity at the opposite end of this same tubular sleeve M and fixed to it by a threaded ring 26. The fixing is sealed by gaskets 25. This base 24 is provided with a first extension 27 to which is welded the core 21 of the shielded cable and at its opposite end is integral with the plug 28 intended to come to engage in a housing of the extension 15 of each electrode E. To strengthen the electrical contact with the extension 15 of the electrode E, the plug 28 is provided with a leaf spring 29. A certain amount of travel is allowed plug 28 in its slot of electrode E to take account of the displacements of the sheath 4 in elastomer when it is pressed against the sample S by injection of liquid into the annular space 13.

The stainless steel tube 23 extends towards the inside of the tubular sleeve M of so as to cover and electrically insulate the area where the cable core is welded to the extension 27. As it is connected to the shield 22, the core 21 is electrically insulated to its junction with plug 24.

The wire 21 has a certain looseness between the connector 20 and the end piece 24 inside the tubular sleeve M for mounting requirements.

OPERATION

The sample S saturated with the first fluid is placed in the enclosure and applies a radial confinement pressure by connection with the means of pressure 14.

A second fluid such as oil is then injected through the channels 8 at a first pressure and the variations in the complex impedance of the sample are continuously measured for several frequencies between 0.1 Hz and a few tens of MHz. , which are recorded by the acquisition assembly 16, 17. The data are analyzed using a generalized resistivity index or impedance index depending on the saturation and the frequency f, defined as follows: Ir ( Sw ) = | Z ( Sw ) | | Z ( Sw = 1) = g ( Sw , f ) or | Z | = ( Re ( Z ) 2 + Im ( Z ) 2 ) ½

We check that the frequency acts strongly on the resistivity index curves Ir above 500 KHz (Fig.7A). For the water-wettable sample, the data can be adjusted by application of well-known Archie law and the exponent of saturation drops from 2 to 1 KHz to 1.5 to 2 MHz. For the wettable sample at oil, the influence of the frequency is different (Fig. 7B) and the curve is strongly nonlinear in a log-log scale. At high frequency, the deviation from the curve at 1KHz depends on saturation. At 2 MHz, the difference forms towards Sw = 0.7 and the curve gradually flattens at low saturation. If we measured only one point of the curve with low saturation, we would find a saturation exponent of 2.

The curves of Figs. 5, 6, the dispersion curves can be extracted from data recorded for two saturation values. For a water saturation of 100%, there is little difference at high frequency (Fig.5A). The frequency of cut (i.e. the top of the semicircle in the Argand diagram) (Fig. 6A), is in the order of 5 MHz in both cases. At low frequency (between 0.1 and 1 kHz) the deviations are due to different surface roughness for the two samples. For a lower saturation (Sw = 0.38), there is a decrease cutoff frequency (around 500KHz). The phenomena attributed to the roughness of surface are shifted to lower frequencies (0.1 Hz).

Tests

To test the quality of the connection device, an electrical circuit consisting of a resistance of some 1 KΩ and a capacitor with a capacity of the order is placed inside the cell and connected to the electrodes E. of 200 pF, typically reproducing the electrical behavior of a sample S. The complex impedance Z of this circuit has been measured, for all frequencies up to 20 MHz when this circuit is placed outside the cell and at l inside the cell and connected by the connection device described above. It can be seen in FIGS. 4A, 4B that the results obtained are completely identical and that the cell and the connection device do not in any way affect the quality of the measurements.

Claims (10)

Device for making the connection by shielded cable, through the wall (18) of a pressure vessel, of at least one electrode (E, 15) in the vessel and of a measuring device outside the enclosure, characterized in that it comprises at least one tubular protective sleeve (M) made of insulating material which is fixed to the wall and crosses it right through and in a sealed manner, this tubular sleeve (M) comprising an interior cavity through which the shielded cable (C) is passed, a tube of conductive material (23) attached in the tubular sleeve (M) and in electrical contact with the shielding (22) of the cable, an electrical connection means (24, 28 ) comprising a plug (24, 27) connected to the electrode (E, 15) with a base (24) which is engaged in the internal cavity of the tubular sleeve (M) and electrically connected to the core (21) of the cable shielded inside the conductive tube (23), and sealing means (19, 25) associated with u tubular sleeve (M) and by means of electrical connection, to isolate the interior of the enclosure from the exterior and the interior cavity of the tubular sleeve (M). Device according to claim 1, characterized in that the plug (24, 28) is engaged in a recess of the electrode (15, E) and adapted to maintain electrical contact with the electrode when the latter undergoes displacement. Device according to one of the preceding claims, characterized in that it comprises an electrical connector (20) associated with the tubular sleeve (M) outside the wall (18) for the connection of a shielded wire connected to the measuring device, a screened cable element (21, 22) inside the tubular sleeve whose core (21) is connected to the connection means (24, 28), and the screen (22) is electrically connected to the conductive tube (23) which extends inwards from the tubular sleeve to the connection zone of the core (21) by means of connection (24, 28). Device according to one of the preceding claims, characterized in that the wall (18) is that of the body of a cell (1) intended for measuring variations in the resistivity index of a porous solid sample (S) coated with a sheath (4) and subjected to a radial pressure by injection of a pressurized liquid into the body of the cell, these variations being consecutive to operations of forced displacement of a first fluid out of the sample by injection of a second fluid, one of the two fluids being electrically conductive and the other not, by means of electrodes (E) arranged between the sample and the sheath (4) and each provided with a extension (15) passing through the sheath, each tubular sleeve (16) passing through the wall of the cell body (1) and extending substantially to the sheath (4). System for measuring physical quantities of a porous solid sample (S) placed in a coating sheath (4), comprising an elongated confinement cell for the sample in its sheath, means (14) for injecting a liquid under pressure in the body (1) of the cell making it possible to exert a radial pressure on the sample by injection of a liquid under pressure, and means intended for measuring the variations in the resistivity index of the 'sample (S), following operations of forced displacement of a first fluid out of the sample by injection of a second fluid, one of the two fluids being electrically conductive and the other not, comprising electrodes (E) arranged between the sample and the sheath (4) and each provided with an extension (15) passing through the sheath, and a measuring device (16, 17) external to the cell connected to the electrodes, characterized in that it comprises at least one tubular sleeve d protection (M) made of insulating material which is fixed to the wall and crosses it right through and in a sealed manner, this tubular sleeve (M) comprising an interior cavity in which the shielded cable (C) is passed, a tube of conductive material (23) attached to the tubular sleeve (M) and in electrical contact with the shield (22) of the cable, an electrical connection means (24, 28) comprising a plug (24, 27) connected to the electrode (E, 15) with a base (24) which is engaged in the internal cavity of the tubular sleeve (M) and electrically connected to the core (21) of the shielded cable inside the conductive tube (23), and sealing means (19, 25) associated with the tubular sleeve (M) and by means of electrical connection, for isolating the interior of the enclosure from the exterior and the interior cavity of the tubular sleeve (M). Measuring system according to claim 5, characterized in that the electrodes (E) allow the application of an electric current and the detection of potential differences appearing between distinct points of the sample (S) in response to the application of the electric current, and the cell comprises a first semi-permeable filter (5), permeable to the first fluid and disposed substantially in contact with a first end of the sample, and injection means (8, 9) for the injection under pressure of a second fluid through a second end of the sample. System according to claim 6, characterized in that the electrical connection means comprises a plug (24, 28) connected to the electrode (15, E) with a base (24) which is rigidly and tightly fixed in a cavity of the tubular sleeve (M), and electrically connected to the core (21) of the shielded cable. System according to claim 7, characterized in that the plug (28) is engaged in a recess of the electrode and adapted to maintain electrical contact with the electrode when the latter undergoes displacement. System according to one of Claims 5 to 8, characterized in that it comprises an electrical connector (20) associated with the tubular sleeve (M) outside the wall (18) for the connection of a connected armored wire to the measuring device, a screened cable element (21, 22) inside the tubular sleeve whose core (21) is connected to the connection means (24, 28), and the screen (22) is electrically connected to the conductive tube (23) which extends towards the inside of the tubular sleeve up to the connection zone of the core (21) by means of connection (24, 28). System according to one of Claims 5 to 9, characterized in that it comprises electrodes having a relatively large longitudinal extension relative to the length of the sample but less than this length, so as to involve the greatest possible part of the sample volume in impedance measurements while avoiding short circuits at the ends of the sample.

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