On charge accumulation in heterogeneous porous rocks under the influence of an external electric field

Abstract

Electric polarization is described as the sum of charge accumulations (free charge density) and orientation of polar molecules such as those of bound and free water molecules (bound charge polarization). Charge accumulation in porous materials cannot be described with Ohm’s law alone. Nonequilibrium thermodynamics or the upscaling of the local Nernst-Planck equation imply that the drift of ions in porous media is controlled by the gradient of their electrochemical potentials and not solely by the electric field. In porous media, electrochemical capacitance is at least six to eight orders of magnitude larger than electrostatic capacitance associated with bound charge polarization. In other words, the low-frequency ([Formula: see text]) effective permittivity entering Ampère’s law is six to eight orders of magnitude larger than high-frequency dielectric permittivity (measured for instance at 1 GHz). Low-frequency polarization of porous media, with no metallic particles (no electronic conductors and semiconductors) is controlled by polarization of the inner component of the electrical double layer coating the grains. This layer, called the “Stern layer,” plays a strong role in defining the cation exchange capacity of a material. A polarization model based on the polarization of the Stern layer explains a large number of experimental observations and could be used in the interpretation of hydro- and petroleum geophysical measurements.