Positive Zeta Potential in Sandstones Saturated With Natural Saline Brine

It is widely accepted that the zeta potential in natural sandstones is negative in the brine pH and compositions found in the subsurface. The zeta potential is a measure of the electrical potential at a mineral surface. We report new measurements of the zeta potential using a saline natural brine rich in divalent ions. The sandstone samples were quartz dominated but some also contained dolomite, siderite, and clay minerals. The quartz‐dominated sandstones showed behavior consistent with earlier studies, but the dolomite‐ and siderite‐bearing samples returned positive zeta potentials. We suggest that this is caused by a positive zeta potential on dolomite and siderite mineral surfaces, adsorption of divalent ions onto quartz mineral surfaces, and/or ion exchange with clay minerals. The results have broad implications for the zeta potential of natural sandstones, because the magnitude and polarity of the zeta potential impact geophysical monitoring and the transport of the charged contaminants. Plain Language Summary Sandstone is one of the most common porous rocks that form subsurface reservoirs. When the sandstone mineral surfaces are in contact with water, they become electrically charged. The magnitude and polarity of the electrical charge can be characterized by a property called the “zeta potential” which can be measured in the laboratory. Here, we show that the zeta potential of sandstone saturated with saline natural brine can be positive, rather than negative as is usually assumed. We highlight the importance of using realistic rock and water compositions in the experimental measurement of zeta potential. The zeta potential is important because it controls how electrically charged contaminants pass through sandstone aquifers, and how aquifers can be monitored remotely using geophysical methods. Assuming the zeta potential is negative when it is positive will cause serious errors in data interpretation and model predictions. Key Points Zeta potential measured on intact sandstones saturated with natural saline brine Results differ from previously observed trends, including the first reported positive values of zeta potential in natural sandstone Positive zeta potential is attributed to adsorption of divalent ions onto mineral surfaces, ion exchange, and mineral dissolution