Abstract

A reserve estimate is strongly influenced by the value of the saturation exponent used. There has been a long standing problem in the petroleum industry as to what value should be used for a given wetting condition. This problem arises due to remarkable divergence in conclusions derived from laboratory works. A non-laboratory investigation is therefore needed to study the effect of wettability on the saturation exponent.

The present study has been directed toward fractal modeling through the use of thin-sectioned core samples in determining the electrical properties. The advantage of this type of work over laboratory measurements is that it is mainly independent of the influence of rock-fluid equilibrium problems. A general equation of electrical resistivity has been developed in this study. The effects of electrical tortuosity, clay content, and rock wettability are incorporated in the equation. The present work employed twenty thin-sections of limestone and sandstones.

The results show that the saturation exponents obtained range from 1.6 to 5.0. The exponent consistently increases as the wetting condition is shifted from strongly water-wet toward oil-wet. It is close to 2.0 for clean sandstones at strongly water-wet, thus supports the empirical formula of Archie. The results also demonstrate that a higher clay content leads to a lower exponent. An increased fractal dimension of pores results in a slightly higher exponent for both strongly water-wet and mixed-wet systems, but it reduces the exponent for oil-wet systems. The impact of this study is that given knowledge of in-situ reservoir wettability, one can use the saturation exponent correctly when evaluating reserve estimates.