High-salinity polarization of sandstones

Induced polarization measurements on numerous samples from multiple databases indicate a polarization maximum at a pore fluid conductivity between 320 and 4600 mS/m. Hitherto, limited measurements above this salinity suggest a decrease in the imaginary part of the complex conductivity (sigma ''), a behavior that has up to now not been adequately considered in the mathematical formulations of recently proposed double-layer polarization models. We measured variations in the complex conductivity spectra over a wide range of pore fluid conductivity (sigma(w) between 25 and 15,000 mS/m) for four Eocene sandstone samples. All samples exhibited a strong decrease of the imaginary conductivity for fluid conductivity exceeding 3000 mS/m. A Stern-layer polarization model, previously presented to fit the dependence of sigma '' on sigma(w) assuming a high-salinity asymptote, is extended with the addition of an exponential term to describe the polarization decrease at high fluid salinity. This mathematical model with four free parameters was applied to measurements on additional sandstone samples of available databases, and the dependence of the four model parameters on petrophysical properties was determined. The parameter of the high salinity exponential term varied over a small range, and it was not significantly related to the available petrophysical quantities. This observation suggested that the polarization decrease resulted from a reduction in the mobility of the counterions in the electric double-layer at high ionic concentrations. Our results have implications for the application of induced polarization in well logging of briny reservoirs.