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Abstract

A new equation that directly relates shaly sand conductivity to the membrane potential is presented. 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 Smits and Waxman (1968) is used to test the model. The Ep measured at different salinities are compared with model predictions using a single membrane potential measurement. Excellent agreement is obtained. versus curves are then generated using the model equations and compared with measured data. Again the agreement is excellent. Experimental results are presented for membrane potential measurements made in our lab on 50 tight gas sand samples. It is shown that the measurements can be correlated with Q although the Q measured by grinding up the samples was always higher than that estimated from Ep measurements. The Ep 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 possible to determine the water saturation in shaly sands from one membrane potential measurement at a known salinity together with a set of 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.