Study on the Mechanisms of Electrochemical Logging Response in Argillaceous Sandstone Based on Capillary Model

The microscopic mechanisms of induced polarization potential involved in the spontaneous potential and the induced polarization potential logging response are mainly based on the hypothesis of the electrical double‐layer deformation and the concentration difference polarization. There is no mathematical model to describe these mechanisms up to now. In this paper, we formulate the flux of ion and the electric current from the theory of electrical double‐layer and the relationship between the cation exchange capacity and Zeta potential based on the microscopic capillary model of porous media. We derived an analytical expression of ion concentration distribution and the mathematical model of the induced polarization potential and the spontaneous potential for water‐bearing argillaceous sandstone from the conservation law of charge and the conservation law of matter. The work rigorously proved that the induced polarization of water‐bearing argillaceous sandstone is caused by the ion concentration differential polarization potential and the deformation potential of electrical double‐layer under the joint influence of the electric current field and the concentration gradient field. And it proved that the mechanism and the equation describing spontaneous potential in argillaceous sandstone are consistent with that of induced polarization potential. The results show that the induced polarizability decreases with the increase of porosity, permeability and concentration of solution but increases with the increase of the cation exchange capacity. It is noted that the concentration of formation water and the cation exchange capacity are the main factors that influence the spontaneous potential.