Spectral induced polarization of rocks with electronically conductive inclusions: a semi-empirical capillary model

Abstract

Induced polarization (IP) method is widely used in different applications including mining geology, hydrogeology, contaminant hydrology, geodynamics and geothermal research. For this reason, petrophysical investigations of IP parameters present an increased interest. Last decades IP behaviour of rocks and sediments containing metallic particles (MP) was extensively studied. Petrophysical investigations were performed mostly with synthetic granular models presented mixtures of sands and MP. In this paper, for the first time, we present IP data measured on a synthetic capillary model. The model made of Plexiglas contains a bundle of capillaries: some active (i.e. blocked by MP), others passive (open). In the experiments, we varied (i) the volumetric content of MP (ii) the water electrical resistivity, (iii) the model porosity and (iv) the MP resistance. We found that the chargeability primarily depends on the ratio of the active to total surface of the pore cross-sections. Consequently, when the model contains only one active pore, and no passive pores, the chargeability is equal to 100 per cent. This is the case even if the volumetric content of MP is very small. This behaviour of the capillary model is contrary to that of sand-MP mixture where the chargeability is approximately proportional to the volumetric MP content. We also found that the relaxation time of the capillary model depends on the bulk resistivity, the MP resistivity and the specific capacitance of the electrical double layer at the ‘MP–pore water’ interface. We explain the experimental data based on a new semi-empirical model. The model presents an extension of Pelton’s model with explicit consideration of electrical and structural parameters of the capillaries.