M.R. Taherian, T.M. Habashy, R.J. Schroeder, D.R. Mariani, and M-Y. Chen
Schlumberger
ABSTRACT
We have devised and implemented a method for generating spontaneous potential (SP) signals in the laboratory. By placing synthetic, ion-selective membranes between aqueous, saline solutions of different concentrations, we created source distributions that approximated those encountered by borehole logging tools. Configurations of cation-selective and anion-selective membranes simulated cases of piston-like invasion, sawtooth-shaped invasion and no invasion of a central sand zone flanked by two shale layers. We recorded 1-dimensional scans and 2-dimensional potential maps by scanning a double junction reference electrode in the saline baths wetting the membranes. A forward model we developed to analyze these data represents the membranes by double layers whose strengths are determined by the measured transference numbers of the membranes and the Nernst equation. We have also devised an inversion scheme to estimate the transference numbers of the membranes, assuming that their geometric distribution is known. Results from both the forward and inverse models are in good agreement with the experimental data.