Feras Abdulsamad

Desiccation influences the complex conductivity of porous media with disseminated metallic particles. We expand the mechanistic model developed in the previous papers of this series to include the effect of saturation upon the complex conductivity of mixtures of mineral grains, pyrite, and pore water. During desiccation, the salt is assumed to be segregated in the liquid pore water; therefore, the conductivity of the pore water increases when saturation decreases. We have performed 14 experiments corresponding to 91 complex conductivity spectra. In these experiments, the saturation of the water phase is changed over time by desiccation. The resulting spectra are fitted by a double Cole-Cole model used as the fitting model. We also developed a mechanistic model in which the chargeability and the relaxation time of the low-frequency polarization are expected to change with saturation in a predictable way. We first characterized the properties of the background material made by an illitic clay material. We determine how the Cole-Cole parameters depend on saturation. The Cole-Cole exponent is essentially independent on saturation. When the chargeability of the mixture is dominated by the presence of pyrite, it becomes independent of the saturation but a small effect on the chargeability is observed at low pyrite contents. The instantaneous conductivity of the background decreases with the saturation in a predictable way. The relaxation time depends on the inverse of the instantaneous conductivity and therefore on saturation. This dependence is well-explained through numerical simulations made with the finite-element method. Finally, we analyze the complex conductivity spectra of two clay-rock core samples from the Callovo-Oxfordian formation in the Paris Basin (France). The spectra are shown as a function of their desiccation and explained thanks to the newly developed model.

We investigate the influence of temperature and salinity upon the spectral induced polarization of 10 samples including rocks with their mineralization (galena, chalcopyrite) plus sand mixed with semi-conductors such as magnetite grains, graphite, and pyrite cubes of two different sizes. Measurements are made in a temperature-controlled bath with a high-precision impedance meter and using NaCl solutions. We cover the temperature range 5-50°C and the frequency range 10^-2 Hz to 45 kHz. For one large pyrite cube, we also investigated 6 salinities from 0.1 to 10 S m^-1 (at 25°C, NaCl) and three salinities for graphite. The spectra are fitted with a Cole Cole complex parametric conductivity model for which we provide a physical meaning to the four Cole Cole parameters. As expected, the Cole Cole exponent and the chargeability are both independent of the temperature and salinity. The instantaneous and steady state (DC, Direct Current) conductivities depend on both salinity and temperature. This temperature dependence can be fitted with an Arrhenius’ law (combining the Stokes-Einstein and Vogel-Fulcher-Tammann equations) with an activation energy in the range 15±1 kJ Mol^-1. This activation energy is the same than for the bulk pore water conductivity demonstrating the control by the background electrolyte of these quantities, as expected. The instantaneous and DC conductivities depend on the salinity in a predictable way. The Cole Cole relaxation time decreases with the temperature and decreases with the salinity. This behavior can be modeled with an Arrhenius law with an apparent activation energy of 7±3 kJ mol^-1. A finite element model is used to analyze further the mechanisms of polarization and can reproduce the temperature and salinity dependencies observed in the laboratory.