Low-frequency AC electrokinetics

By using a low-frequency AC method based on lock-in amplification, electrokinetic response in porous rock can be accurately measured with high sensitivity. With this method, we have measured streaming potentials as small as 300 nV and electroosmotic pressures as low as 10 Pa. We describe our technique, and discuss the results from a suite of 12 brine-saturated samples, consisting of six sandstones, two carbonates, and four fused glass bead specimens. We show that the permeability of the rock may be derived exactly from the electrokinetic transport coefficients and sample conductivity. This result is verified experimentally for all NaCl concentrations. The electrokinetic response as a function of concentration and temperature has been measured and is compared with predictions from the Debye-Huckel model and Helmholtz-Smoluchowski equation in the thin-layer limit. (C) 1999 Elsevier Science B.V. All rights reserved.