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Anomalous Zeta Potential Trends in Natural Sandstones

It is widely accepted that the zeta potential in natural sandstones is negative and becomes larger in magnitude with decreasing electrolyte concentration. In our experimental measurements of zeta potential, an almost pure quartz sandstone saturated with NaCl electrolyte followed this expected concentration dependence. However, the zeta potential in a clay‐ and dolomite‐bearing sandstone initially increased but then decreased in magnitude with decreasing NaCl concentration. The effluent contained Ca2+ cations, although none were injected. At the lowest NaCl concentration tested, the zeta potential was identical within experimental error to that measured using CaCl2 electrolyte with the same concentration. We argue that Ca2+ cations can be released from natural sandstones by mineral dissolution or ion exchange and these ions are preferentially adsorbed onto quartz mineral surfaces, dominating the behavior of the zeta potential at low NaCl concentration. The results have broad implications for the zeta potential of natural sandstones in subsurface settings.