Measurement of the electrokinetic potential at reservoir rock surfaces avoiding the effect of surface conductivity

Abstract

The surface (zeta) potentials of sandstone rock cores in aqueous and non-aqueous systems were investigated under flow conditions. Rock surface potentials determined by streaming potential measurements can be in very considerable error as a result of surface conductivity, whereas streaming currents are free from this disadvantage. Streaming current measurements do however need a knowledge of the ratio area/length ( A/ L) of the effective capillaries. A new method was developed to determine the A/ L ratio for porous systems by combining streaming potential and current measurements so that the unambiguous streaming current method can be used. The impact of pore (fluid) pressure on streaming potential and current measurements was investigated. It was observed that the change in the behaviour of streaming potential and current measurements as a function of pore pressure is direction dependent, exhibiting a strong hysteresis. This phenomenon of hysteresis is important because of its direct link with the estimation of area to length ratio of the capillaries and the related rock surface (zeta) potential measurements.