The Yuan-Diederix equation relates the conductivity of a brine-saturated rock to the brine conductivity and membrane and Nernst potentials for the sample for a given salinity contrast. The equation has only one parameter, namely, the shaly sand formation resistivity factor, F*. Because the equation is essentially local in salinity, data from Waxman-Smits Group 2 samples for which conductivity and membrane potential data have been simultaneously measured allow the determination of F* as a function of salinity.
For situations where the clay conductivity is smaller than the brine conductivity, the Waxman-Smits assumption of constant F* is valid. However, where the clay conductivity exceeds the brine conductivity, F* may no longer be constant, implying that the Waxman-Smits assumption of constant F* is not always valid. An implication of salinity-dependent F* is that membrane potential measurements may be necessary for cases where the clay conductivity exceeds the brine conductivity. This can be critical for shaly sand evaluation.
New simultaneous conductivity and membrane potential data are reported for a set of very shaly sandstones which confirm the salinity dependence of F* when the clay conductivity exceeds the brine conductivity. The effect is greatest for montmori1lonite-bearing samples. Procedures for applying membrane potential measurements are described.