Influence of Divalent Electrolytes on the Zeta Potential of Porous Geological Materials
Main Article Content
Abstract
Self-potential (SP) signals arising from electrokinetic coupling induced by fluid flow in porous media are widely used in hydrogeophysical and geophysical applications. These signals are fundamentally controlled by the electrical double layer (EDL) at the mineral-fluid interface, in which the zeta potential plays a key role. Accurate determination of zeta potential is therefore essential for quantitative interpretation of SP measurements. In this study, the zeta potential of three porous rock samples (Bentheim sandstone, Berea sandstone, and an artificial ceramic) saturated with divalent electrolytes (CaCl2, CaSO4, and MgSO4) was investigated over a concentration range from 10-4 to 10-2 M. The zeta potential was derived by combining measurements of the streaming potential coefficient and electrical conductivity. The results show that the magnitude of the zeta potential systematically decreases with increasing electrolyte concentration for all samples, reflecting compression of the EDL. Significant dependence on both rock type and electrolyte composition is observed: The zeta potential depends strongly on both rock type and electrolyte conditions. In most cases, the natural sandstones exhibit more negative zeta potentials than the artificial ceramic, and MgSO4 generally produces more negative values compared to calcium-based electrolytes, indicating stronger Mg2+ surface interactions. In addition, an empirical linear relationship between zeta potential and the logarithm of electrolyte concentration is proposed for divalent electrolytes.
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