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

The surface:(zeta) potentials of sandstone rock cores in aqueous and non-aq ueous systems were investigated under flow conditions. Rock surface potenti als determined by streaming potential measurements can be in very considera ble error as a result of surface conductivity, whereas streaming currents a re free from this disadvantage. Streaming current measurements do however n eed a knowledge of the ratio area/length (AIL) of the effective capillaries . A new method was developed to determine the AIL ratio for porous systems by combining streaming potential and current measurements so that the unamb iguous streaming current method can be used. The impact of pore (fluid) pre ssure on streaming potential and current measurements was investigated. It was observed that the change in the behaviour of streaming potential and cu rrent measurements as a function of pore pressure is direction dependent, e xhibiting a strong hysteresis.: This phenomenon of hysteresis is important because of its direct link with the estimation of area to length ratio of t he capillaries and the related rock surface (zeta) potential measurements. (C) 1999 Elsevier Science B.V. All rights reserved.