Electrical Resistivity Measurements on Reservoir Rock Samples by the Two-Electrode and Four-Electrode Methods

Experimental evidence is presented showing that reproducible formationresistivity factor measurements and resistivity index determinations onreservoir core samples may be made utilizing either the two- or four-electrodemethods. Equipment is described which permits the application of eithertechnique to the core specimen without loss of time and with a minimum amountof effort. Since electrical resistivity measurements on reservoir rocks areimportant not only in electric log interpretations, but also in the study offundamental rock parameters, it was considered desirable to compare theapplicability of both techniques of resistivity measurement. Introduction The techniques employed in the measurement of electrical resistivity onreservoir rocks, as practiced in the petroleum industry, may be reduced to twofundamental methods: one utilizing two electrodes only, usually referred to asthe two-electrode method; the other using two additional electrodes andcommonly known as the four-electrode method. In the former, the core samplewhose resistivity is to be measured is mounted between two electrodes, usuallymetal discs, which serve to pass a current of known magnitude through thesample. The potential drop across the sample, measured between these same twoelectrodes, and the core dimensions furnish the necessary data to compute theresistivity of the sample. In the four-electrode technique the end plates, again usually of metal, still serve as current electrodes. Additional probes orelectrodes dispersed along the sample are utilized to measure the potentialdrop. Both methods are used in practice, and it is the purpose of this paper tocompare the two techniques as applied to the determination of formationresistivity factors and the resistivity index exponent n, in Archie's empiricalequation. To evaluate the two techniques it is desirable to point out theexperimental precautions which must be taken to insure reliable results. It isevident from the electrode arrangements of the two- and the four-electrodemethods that, for a given sample, the volume of sample included by thefour-electrode method is less than the volume included by the two-electrodemethod. One of the prime difficulties associated with the two-electrode method isthe possible occurrence of an abnormally high resistance between the electrodesand the brine-saturated sample, which results in erroneous resistivity valuesfor the core sample itself. T.P. 3397