Electricity: measuring and testing – Of subsurface core sample
Reexamination Certificate
1999-07-22
2001-05-08
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Of subsurface core sample
C324S698000, C250S255000, C073S152090, C073S038000
Reexamination Certificate
active
06229312
ABSTRACT:
FIELD OF THE INVENTION
The object of the invention is a method and a device for measuring the resistivity index curve of a solid sample independently of the capillary pressure curve.
Measurement of the resistivity index of small cores is necessary to obtain a precise estimation of the water saturation from log data.
BACKGROUND OF THE INVENTION
Various more or less fast and accurate methods have been proposed to measure the resistivity index of rocks. Several combinations of these techniques have been proposed. A known method for measuring the resistivity index essentially consists in combining a multicore air-water “semipermeable membrane” desaturation technique with a two-electrode resistivity measuring technique and in calculating the average saturation by weight difference. It appears that this method is very imprecise and depends too much on the handling quality; it is very slow and does not take account of a possible wettability effect.
Different solutions using the same principle have been found to improve the precision thereof with, in return, greater implementation complexity and a correlative cost increase. It is for example possible to
carry out separate measurements on individual core samples in order to better control the capillary equilibrium and thus to obtain uniform saturation profiles,
implement a technique using four electrodes so as to avoid overestimation of the electric resistance,
implement a continuous injection technique in order to accelerate experiments when only curve Ir is necessary, as described for example by de Waal et al., 1991,
optimize the duration of the desaturation process by using a micropore membrane and by reducing the length of the core samples as described by
Longeron D. et al., “Water-Oil Capillarity Pressure and Wettability Measurements Using Micropore Membrane Technique” SPE 30006, 1995, or
Fleury M. et al., “Combined Resistivity and Capillarity Pressure Measurements using Micropore Membrane Technique” Proceeding of the International Symposium of the Society of Core Analysts, Montpellier 1996.
However, when the measurement of Ir is linked with the determination of the capillary pressure curves, it is very difficult to reduce the duration of the experiments.
Other desaturation techniques such as the centrifugal method can be selected and implemented in the same way as the aforementioned “semipermeable membrane” method with multiple cores. Implementation of the centrifugal method also proved to be imprecise because of the accumulation of two important problems linked with the saturation profile and the contact resistance. On account of the known Archie relation connecting Ir and Sw (Ir=Sw
−n
), the measurements are very sensitive to saturation. It is then also possible to determine the saturation profile (during a fluid injection for example) and to reduce the duration of the experiment by using a device for measuring the local saturation in situ and multiple electrodes as described by
Jing et al., “Resistivity Index from non Equilibrium Measurements using Detailed in situ Saturation Monitoring”, SPE 26798 Offshore European Conference 1993.
The method according to the invention allows continuous measurement of the resistivity index of a porous solid sample, combining rapidity, precision and low cost, while avoiding the drawbacks of the prior methods and notably the obligation to perform in-situ saturation monitoring, which is long and expensive.
SUMMARY OF THE INVENTION
The method according to the invention allows to rapidly and continuously obtain the variations of the resistivity index (Ir) of a porous solid sample, initially saturated by a first fluid, by means of a device comprising an elongate containment cell, means for exerting a radial pressure on the sample, electrodes pressed against the peripheral wall of the sample, allowing application of an electric current and detection of the potential differences appearing between distinct points in response to the application of the electric current, the electrodes being connected to a device for measuring the complex impedance of the sample, a first semipermeable filter permeable to the first fluid and placed substantially in contact with a first end of the sample, and pressure means for injection under pressure of a second fluid through a second end of the sample. The method is characterized in that
it uses electrodes whose longitudinal extension is relatively great in relation to the length of the sample but shorter than this length, so selected as to involve the largest possible part of the volume of the sample in the impedance measurements while avoiding short circuits through the ends of the sample, likely to distort measurements, and
at least one injection pressure stage is applied for the second fluid and precise continuous measurement of the complex electric impedance variations of the sample is carried out at several frequencies during a phase of displacement of the saturating fluid (drainage phase or imbibition phase), measurement being achieved without waiting for a capillary pressure equilibrium to be reached in the sample in response to each pressure stage.
Electrodes whose length ranges between ¼ and ¾ of the length of the sample and is preferably of the order of ½ of its length can be used for example.
The device according to the invention comprises an elongate containment cell, means for exerting a radial pressure on the sample, electrodes pressed against the peripheral wall of the sample, allowing application of an electric current and detection of the potential differences appearing between distinct points in response to the application of an electric current, the electrodes being connected to a device for measuring the impedance of the sample. a first semipermeable filter permeable to the first fluid and placed substantially in contact with a first end of the sample, and injection means (
14
) for injection under pressure of a second fluid through a second end of the sample.
The device is characterized in that the electrodes have a relatively great longitudinal extension in relation to the length of the sample (between ¼ and ¾ of the length of the sample and preferably of the order of ½ of its length) but shorter than this length, so as to involve the largest possible part of the volume of the sample in the impedance measurement while avoiding short circuits through the ends of the sample.
The method according to the invention, in relation to the prior methods, is particularly advantageous since it allows
to draw a very precise drainage continuous resistivity index curve in a short time (about 2 days for a typical 100 mD sandstone whereas the typical time required with the continuous injection technique is often of the order of two weeks),
the method is not linked with a capillary pressure equilibrium,
the effect of non -uniform saturation profiles during measurement is negligible. This is due to the combination of three factors: (i) the radial resistivity measuring technique, (ii) the presence of semipermeable filters on the outlet side, (iii) all the volume of the core is analysed by means of electric measurements (this is verified when the diameter of the core is greater than its length).
REFERENCES:
patent: 5164672 (1992-11-01), Gilliland et al.
patent: 5679885 (1997-10-01), Lenormand et al.
patent: 4208953 (1992-09-01), None
patent: 2724460 (1996-03-01), None
Deflandre Francoise
Fleury Marc
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
Jolly Anthony
Williams Hezron
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