Transport in Porous Media 29: 85–98, 1997. 85 © 1997 Kluwer Academic Publishers. Printed in the Netherlands. A Percolation Study of Wettability Effect on the Electrical Properties of Reservior Rocks DENGEN ZHOU ⋆ , SEPEHR ARBABI and ERLING. H. STENBY ⋆⋆ Petroleum Engineering Department, Stanford University, Stanford, CA 94305-2220, U.S.A. (Received: 4 November 1996; in final form: 9 June 1997) Abstract. Measurements of the electrical resistivity of oil reservoirs are commonly used to estimate other properties of reservoirs, such as porosity and hydrocarbon reserves. However, the interpretation of the measurements is based on empirical correlations, because the underlying mechanisms that control the electrical properties of oil bearing rocks have not been well understood. In this paper, we employ percolation concepts to investigate the effect of wettability on the electrical conductivity of a reservoir formation. A three-dimensional simple cubic network is used to represent an ideal reservoir formation, for which the effect of the wettability can be isolated from the others. The phase distribution in the network is analyzed for different flow processes, and the conductivity is then estimated using a power law approximation of the percolation quantities. The proposed conceptual model predicts the generic behavior of reservoir resistivities of different wettabilities. It demonstrates that the resistivity index depends on saturation history and wettability. For strongly oil-wet systems, significant hysteresis is expected, while there is little hys- teresis for strongly water-wet systems, and some hysteresis is also expected for intermediate wet systems. One of the interesting results from this study is that for intermediate wet systems, Archie’s saturation exponent is between 1.9 and 3.0. Key words: percolation, network modeling, wettability, electrical conductivity, resistivity index, saturation hysteresis. 1. Introduction The amount of hydrocarbon reserves is one of the most important parameters in the decision-making process in developing a reservoir. Accurate estimation of the reservoir’s reserves can reduce the uncertainty of financial forecasting. The estima- tion of hydrocarbon reserves is commonly obtained from electrical logging. The interpretation of the logging data is based on Archie’s law (Archie, 1942), which is an empirical correlation of the electrical resistivity index and its brine saturation. Archie’s law indicates that the resistivity index is only a function of the conducting phase saturation for a given formation, as expressed by the following equations: F f = R s R b = a φ m , (1) ⋆ To whom correspondence should be addressed. ⋆⋆ Chemical Engineering Department, Technical University of Denmark, DK-2800 Lyngby, Denmark.