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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.

Plain Language Summary Porous rocks such as sandstone are often important underground reservoirs for water and other fluids such as oil or gas. When the mineral surfaces of the rock are in contact with water, they become electrically charged. A measure of this electrical charge is provided by the so-called "zeta potential." Understanding of the zeta potential is important for a number of reasons. For example, the zeta potential is an important control on how the mineral surfaces chemically react with fluids in the pore space. Moreover, when water flows through the rock, the zeta potential gives rise to electrical signals that can be used to monitor the flow. The zeta potential can be measured in laboratory experiments on rock samples. In this paper, we show that reactions between water and minerals in the rock can modify the chemical composition of the water, which, in turn, modifies the zeta potential. Ignoring this effect could lead to erroneous estimates of the zeta potential in many subsurface reservoirs. Such errors could lead to inaccurate interpretations of monitoring data and incorrect predictions of chemical reactions, with significant environmental and economic consequences.