Impact of Clay Content on the Zeta Potential of Natural Intact Sandstones

Natural sandstones exhibit a wide range of clay types and content. Whilst these may only make up a few percent of the bulk volume, clays have a relatively large surface area and their distribution within the pore space means they can dominate the surface properties. The zeta potential is a measure of the electrical surface charge and its magnitude and polarity control the electrostatic interactions with other charged species. The zeta potential is commonly measured by electrophoretic methods which require a suspension of particles in an aqueous electrolyte. For natural sandstones, the sample must be crushed to sub grain sized dimensions, thus destroying the natural distribution of minerals within the pore space. Previous work has reported similar zeta potential measurements for different natural sandstones since the dominant bulk mineral species is quartz (typically >90%). The impact of clay type and distribution on the zeta potential is difficult to infer via such destructive methods. The streaming potential method (SPM) can be used to measure the zeta potential on intact rock cores under a range of ionic strengths and electrolyte compositions. We report zeta potential measurements obtained using the SPM for five outcrop sandstones comprising a range of mineralogies from pure quartz to clay rich, saturated with electrolytes representing saline aquifer waters and freshwater. In saline waters, the zeta potential becomes increasingly more negative with increasing clay content due to an increased negative surface charge. However, in fresh waters, the zeta potential becomes more positive with increasing clay content due to increased divalent cation adsorption. The difference between the zeta potential in fresh water and saline waters decreases with increasing clay content and are identical within experimental error for some samples, despite the freshwater being 70x more dilute. These results highlight the importance of measuring the zeta potential on intact rock samples and provide insight into the impact of clay content, challenging conventional assumptions that dilution always yields a more negative zeta potential.