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 Mg2+? 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 Mg2+ ions increases decay length. These results imply that Na+ ions reduce the affinity of calcite surfaces for water whereas Mg2+ 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 Mg2+ 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.