Multifrequency Inversion of Complex Electrical Conductivity Measurements for Simultaneous Assessment of Wettability, Porosity, and Water Saturation

Abstract

Summary Multifrequency complex electric measurements are influenced by the joint effects of mineralogy, spatial distribution of fluids and solids, properties of fluids and minerals, and interfacial polarization mechanisms. The interpretation of multifrequency electrical conductivity and dielectric permittivity dispersion can yield information about the dominant dielectric polarization mechanisms. The dielectric polarization mechanisms detected in sedimentary rocks are linked to grain size, shape, and orientation relative to the externally applied electric field, electrochemical adsorption/desorption complexation reactions, wettability, porosity, and fluid saturations. In this work, we introduce a new workflow for multifrequency interpretation of complex permittivity measurements. The introduced workflow enables simultaneous assessment of dominant wettability, porosity, and hydrocarbon reserves. These properties are estimated by minimizing a cost function using a combination of downhill gradient-descent and an evolutionary algorithm. The cost function quantifies the difference between measured and numerically calculated multifrequency complex dielectric permittivity through a recently introduced rock-physics model. We successfully applied the new interpretation workflow to multifrequency electrical resistivity/permittivity measurements obtained from two water-wet and two hydrocarbon-wet sandstone core samples. Simultaneous estimates of wettability, water saturation, and porosity obtained by applying the new interpretation workflow were in agreement with experimental measurements. Assessment of hydrocarbon reserves, porosity, and dominant wettability solely from the interpretation of multifrequency complex-valued electrical measurements is a unique contribution of the introduced workflow. Additionally, the new workflow honors rock fabric and, thus, it minimizes the necessity of extensive calibration efforts. All the parameters needed as inputs to the new interpretation workflow are associated with physical mechanisms at microscopic- and pore-scale domains or realistic and quantitative pore geometry features of the rock.