Investigation of the effects of ions on short-range non-DLVO forces at the calcite/brine interface and implications for low salinity oil-recovery processes.

A primary question investigated in this study is the influence of brine chemistry on the behavior of short-range non-DLVO (e.g., hydration, discrete ion charge effects, and so on) forces at calcite surfaces. Specifically, how do wetting films containing Na differ from those containing Mg? Force-distance spectroscopy as measured by atomic force microscopy is used to probe short-range non-DLVO forces in various single-salt and multiple-salt electrolyte solutions. Experimental results reveal that, in single-component solutions, a greater concentration of Na ions decreases the decay length of short-range repulsion while a greater concentration of Mg ions increases decay length. These results imply that Na ions reduce the affinity of calcite surfaces for water whereas Mg ions make calcite more hydrophilic. Importantly, the relationship between the behavior of non-DLVO forces at small separations and concentrations of ions is not monotonic in multiple-component brines. Our observations support the hypothesis that Na ions disturb the interfacial water structure of calcite while Mg ions are arranged farther away from the surface. Results obtained here may have implications for the design of so-called low salinity waterfloods where the composition of brines used for crude oil recovery is manipulated to increase oil recovery.