Interfacial polarization of disseminated conductive minerals in absence of redox-active species — Part 1: Mechanistic model and validation

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Electrically conductive mineral inclusions are commonly present in organic-rich mudrock and source-rock formations such as veins, laminations, rods, grains, flakes, and beds. Laboratory and subsurface electromagnetic (EM) measurements performed on geomaterials containing electrically conductive inclusions generally exhibit frequency dispersion due to interfacial polarization phenomena at host-inclusion interfaces. In the absence of redox-active species, surfaces of electrically conductive mineral inclusions are impermeable to the transport of charge carriers, inhibit the exchange of charges and behave as perfectly polarized (PP) interfaces under the influence of an externally applied EM field. Interfacial polarization phenomena involving charge separation, migration, accumulation/depletion, and relaxation around PP interfaces is referred to as PP interfacial polarization; it influences the magnitude and direction of the electric field and charge carrier migration in the geomaterial. We have developed a mechanistic model to quantify the complex-valued electrical conductivity response of geomaterials containing electrically conductive mineral inclusions, such as pyrite and magnetite, uniformly distributed in a fluid-filled, porous matrix made of nonconductive grains possessing surface conductance, such as silica and clay grains. The model first uses a linear approximation of the Poisson-Nernst-Planck equations of dilute solution theory to determine the induced dipole moment of a single isolated conductive inclusion and that of a single isolated nonconductive grain surrounded by an electrolyte. A consistent effective-medium formulation was then implemented to determine the effective complex-valued electrical conductivity of the geomaterial. Model predictions were in good agreement with laboratory measurements of multifrequency complex-valued electrical conductivity, relaxation time, and chargeability of mixtures containing electrically conductive inclusions.