A new equation is derived that directly relates shaly sand conductivity to the membrane potential. The relationship between the resistivity index and the water saturation is also expressed in terms of a single membrane potential measurement. The data presented by Smits (1968) and Waxman and Smits (1968) are used to test the model. The membrane potentials measured at different salinities are compared with model predictions using a single membrane potential measurement. Excellent agreement is obtained. The curves of conductivity of the reservoir rock versus the conductivity of the saturating fluid are then generated using the model equations and compared with measured curves. Again the agreement is excellent.

Experimental results are presented for membrane potential measurements made in our laboratory on 30 tight gas sand samples. It is shown that the measurements can be correlated with the cation exchange capacity, although the cation exchange capacity measured by grinding up the samples was always higher than that estimated from membrane potential measurements. The membrane potential measurements are combined with the equations for partial saturation developed in this paper to obtain water saturations in a tight gas sand well. Through this example it is shown that it is possible to determine the water saturation in shaly sands from one membrane potential measurement at a known salinity together with a set of resistivity logs. The results clearly demonstrate the applicability of the method as a practical procedure for shaly sand formation evaluation.