Certain Wettability Effects on Electrical Resistivity in Porous Media
Authors: Mungan N. , Moore E.J. 
Published: March 2010

A laboratory study was made on the relation of wettability and wetting equilibrium to electrical resistivity, particularly under dynamic conditions. Teflon cores and synthetic fluids as well as reservoir cores and fluids wereused. Resistivities were measured by a four-electrode system. Under static conditions and with the wetting equilibrium prevailing, exponents as high as 9 were calculated from Archie's Equation when the interstitial conductive liquid was the non-wetting phase. When it was the wetting phase, saturation exponents ranged from 2 to 3 for the same core. This means that in laboratory measurements of Archie's saturation exponents on reservoir cores, the corewettability must represent the wettability of the reservoir. Otherwise, thecalculated connate water saturations will be in error. Under dynamic conditions, wetting equilibrium prevails only for very low displacement rates. At high flow rates the rate has an important bearing on the measured resistivity, indicating non-equilibrium fluid distribution in some experiments, the porous medium was saturated with the non-wetting liquid first. It was shown that, in these tests, the wetting equilibrium is either notobtained at all or it is obtained extremely slowly. These laboratory resultsare explained in terms of fluid distribution in porous media. A qualitative discussion is given on the implications of this work in electrical loginterpretation and the recovery of crude oil by chemical waterfloods.

INTRODUCTION

HYDROCARBON-BEARING FORMATIONS are generally identified through themeasurement and interpretation of their electrical properties by well logging. The empirical equation used in the estimation of the hydrocarbon saturations from resistivity data is: {See equation in full paper},

In this equation, S, is connate water saturation, R. is the resistivity ofthe 100 per cent water-saturated formation, Rt is the resistivity of the formation containing both water and hydrocarbon, and n is the saturation exponent. The true resistivity, R, of the un-invade zone and R. are obtained from the logs; n is found from laboratory measurements on reservoir cores. Implicit in the above approach are the assumptions that:the saturation-resistivity relationship is unique,laboratory-derived n valuesapply to the reservoirn is constant for a given porous medium andall of the water contributes to the flow of the electric current.

Numerous studies on the electrical resistivities of cores partly saturated with sodium chloride solution have shown that the above assumptions are notnecessarily invalid. For the same saturations, the resistivity of cores hasbeen found to vary significantly with wetting conditions, clay content, textureand salinity. To find the effect of wettability on resistivity, some studieshave been performed on natural or synthetic cores in which the wettability waschanged through a chemical reaction, i.e. hot solvent extraction, oxidation or coating the surfaces with a chemical. There are some complicating factors insuch an approach; namely, the resulting core wettability is not uniform, and cannot be described or measured, and exchangeable ions from natural mineral scan change the resistivity of the electrolyte filling the pores. Also, the porosity of the core, particularly the effective porosity, can change.