Low-frequency AC electrokinetics

By using a low-frequency AC method based on lock-in amplification, electrok inetic response in porous rock can be accurately measured with high sensiti vity. With this method, we have measured streaming potentials as small as 3 00 nV and electroosmotic pressures as low as 10 Pa. We describe our techniq ue, and discuss the results from a suite of 12 brine-saturated samples, con sisting of six sandstones, two carbonates, and four fused glass bead specim ens. We show that the permeability of the rock may be derived exactly from the electrokinetic transport coefficients and sample conductivity. This res ult is verified experimentally for all NaCl concentrations. The electrokine tic response as a function of concentration and temperature has been measur ed and is compared with predictions from the Debye-Huckel model and Helmhol tz-Smoluchowski equation in the thin-layer limit. (C) 1999 Elsevier Science B.V. All rights reserved.