A simple physical model describing shaly sand conductivity was previously described by Waxman and Smits. The electrical conductivity of a water-saturated shaly sand was expressed as a function of a geometry factor, the brine conductivity, an effective concentration of clay exchange cations, and their equivalent ionic conductance. Experimental data support this relation. The model was extended to cases where the sands were oil-bearing. An expression was obtained relating the resistivity index to water saturation, brine conductivity, and the clay parameters mentioned above.
The assumptions involved in the model for oil-bearing sands have now been confirmed by additional laboratory conductivity measurements. We conclude that the effective concentration of clay exchange cations is increased by a quantity proportional to the decrease in water saturation. Twelve different rock samples from seven fields were utilized in these studies, incorporating a wide range of brine conductivities, oil saturations, and cation exchange capacities.
Further, the temperature coefficients of electrical conductivity were measured for a set of shaly sands equilibrated with salt solutions covering a wide range of concentrations. When compared at the same electrolyte concentration, these temperature coefficients increased systematically with increasing clay content of the sands.
