Pore-scale modeling of electrical conductivity in unsaturated sandstones

Non-invasive techniques are becoming common tools for site characterization. These techniques include electrical and electromagnetic methods, and are generally referred to as hydro-geophysical techniques. This approach relies upon the correct translation of geophysical properties into hydrological ones, and particularly moisture content (in the vadose zone) and solute concentration (in both the vadose and saturated zones). In this note, we present a. pore-scale approach for the investigation of the relationships that link electrical properties of porous media with water content and the solid interfacial areas responsible for surface conductivity. In this approach we create a digital porous medium on the basis of experimental grain size distribution data and apply a previously developed drainage simulator based on pore-morphology analysis. Then we simulate electrical flow within the pore-scale distributions of the phases and derive the bulk resistivity of the porous medium. Simulation results are compared with resistivity data from a UK Permo-Triassic Sandstone (Sherwood S.); resistivity of the solid phase, or alternatively surface conductivity, has been derived from calibration against measured bulk resistivity. The agreement between measured and simulated bulk conductivities, at different saturation values, is very good.