Induced polarization of clay-rich materials — Part 1: The effect of desiccation

Abstract

A petrophysical model describing spectral induced polarization (IP) has been developed for clay rocks accounting for the Maxwell-Wagner polarization. It is also used to connect the complex conductivity to the relative permeability of the material. This model is applied to the Callovo-Oxfordian clay rock of the Paris Basin (France) where the Meuse/Haute-Marne Underground Research Laboratory is located. Laboratory experiments are performed using eight clay-rock cores to study the effect of desiccation on their spectral-IP response. The measurements are performed along the foliation plane. Complex conductivity spectra are measured over a frequency range of 1 mHz to 45 kHz. These spectra are fitted with a double Cole-Cole model to extract the evolution of the Cole-Cole parameters with the saturation during the desiccation process. The low-frequency Cole-Cole model corresponds to IP phenomena, whereas the high-frequency Cole-Cole model corresponds to the Maxwell-Wagner contribution. We obtain the value of the first and second Archie’s exponents and we check the relationship between the surface conductivity and the cation exchange capacity of the clay rocks. We are also able to connect the relative permeability curve to the second (saturation) Archie’s exponent. The monitoring of the complex conductivity can be used to predict how the permeability of the clay-rock formation changes with the water content.