Quantification Of Surface Conductivity In Clean Sandstones Available to Purchase

Many investigators have noted that the apparent formation factor of porous materials seems to diminish as the interstitial waters become more resistive. This effect has been attributed both to the presence of "conductive solids" (usually clays) and to the existence of a concentrated layer of ions attracted to the surfaces of the rock matrix. This study was designed to determine (1) if surface conductivity in clean sandstones was an important consideration in the evaluation of fresh water aquifers and (2) if so, what parameters were necessary to evaluate the effect. Nine cores, two of which had small percentages of clay were chosen with average grain sizes ranging from .09 to .29 mm. Resistivities were measured over a wide concentration range using solutions of NaCl, Na2SO4, NaHCO3, and CaCl2. The results were found to correlate with a simple equation that is easily applied to routine log analysis. Several examples of actual log analyses are presented and compared to measured water resistivities with an average deviation of less than 7 1/2% between the log derived values and the measured values. If surface effects were neglected in the same cases the calculated Rw could have been as much as 60% too low.