Estimation of hydraulic parameters of shaly sandstone aquifers from geoelectrical measurements

Abstract

Electrical and hydraulic conductivities of shaly sandstones are described using a capillary approach for a granular, clay bearing material. The clays are assumed to occur as shale shells uniformly coating the insulated sand grains. The real and imaginary components of the complex electrical conductivity for this material model are written in terms of a unique solid matrix conductivity, treated as a volumetric property. Above a critical salt concentration the conductive contribution of the shells is independent of the electrolyte salinity. Under this condition the equation for the bulk conductivity of the sandstone can be expressed in a simplified form. However, below the critical concentration the matrix conductivity is dependent on the conductivity of the water saturating the shaly component and can be expressed only by the complete equation. The hydraulic conductivity for this model is expressed by a modified Kozeny-Carman equation. A new semi-empirical equation relates the hydraulic conductivity for such rocks, to their porosity, formation resistivity factor and the electrical conductivity of its solid matrix. These combined properties are described as the lithoporosity factor. In this new formulation the petrophysical parameters involved are easily determined from the electrical geophysical measurements. The performance of this equation is firmly tested with experimental laboratory data available in the literature. Its application is then extended to estimate the hydraulic parameters of a shaly sandstone aquifer in Bahia-Brazil, using either the borehole or the surface geoelectrical data. Examples are given to emphasize the combined use of electrical resistivity and induced polarization measurements in computing hydraulic properties.