Organic-Silica Interactions in Saline: Elucidating the Structural Influence of Calcium in Low-Salinity Enhanced Oil Recovery.

Enhanced oil recovery using low-salinity solutions to sweep sandstone reservoirs is a widely-practiced strategy. The mechanisms governing this remain unresolved. Here, we elucidate the role of Ca(2+) by combining chemical force microscopy (CFM) and molecular dynamics (MD) simulations. We probe the influence of electrolyte composition and concentration on the adsorption of a representative molecule, positively-charged alkylammonium, at the aqueous electrolyte/silica interface, for four electrolytes: NaCl, KCl, MgCl2, and CaCl2. CFM reveals stronger adhesion on silica in CaCl2 compared with the other electrolytes, and shows a concentration-dependent adhesion not observed for the other electrolytes. Using MD simulations, we model the electrolytes at a negatively-charged amorphous silica substrate and predict the adsorption of methylammonium. Our simulations reveal four classes of surface adsorption site, where the prevalence of these sites depends only on CaCl2 concentration. The sites relevant to strong adhesion feature the O(-) silica site and Ca(2+) in the presence of associated Cl(-), which gain prevalence at higher CaCl2 concentration. Our simulations also predict the adhesion force profile to be distinct for CaCl2 compared with the other electrolytes. Together, these analyses explain our experimental data. Our findings indicate in general how silica wettability may be manipulated by electrolyte concentration.