A mechanistic model for the complex conductivity of clay materials. II. Comparison to experimental data

SUMMARY Clays are very abundant minerals in the Earth's crust and express a high conductivity response that can be observed at the scale of the geological formation by electrical and electromagnetic methods. However, these minerals have a complex microstructure that renders difficult the quantitative petrophysical interpretation of the electrical field measurements. In this study, we developed a new approach to interpret spectral induced polarization (SIP) signals measured on clay materials in terms of microstructural and electrical double layer (EDL) properties, including surface conductivity, using the physical model recently developed by Leroy et al. With a restrained set of optimized physical–chemical parameters, that is, the fraction of the counter-charge and cation effective ion mobility in the Stern layer the number of stacked sheets per montmorillonite particle, and clay aggregate effective shape and size distribution, our model well reproduces the measured laboratory SIP spectra in the mHz to kHz frequency range on kaolinite, illite and montmorillonite muds at different NaCl concentrations. Our results suggest that most of the EDL counter-charge controlling SIP spectra are located in the Stern layer on the basal surfaces. First Archie's law explains the in-phase conductivity measurements provided that the contribution of the diffuse layer to water conductivity is properly included. The ratio of imaginary surface conductivity to real surface conductivity is highly dependent on frequency and slightly dependent on salinity. Finally, the measured quadrature conductivity is proportional but not necessarily equal to the imaginary surface conductivity. Our study is a step forward to better understand the complex electrical conductivity of clays.