J.L. Desmond, P.M. Rodger, T.R. Walsh
Keywords: interfaces, saline, enhanced oil recovery, adsorption
Summary:The term ‘low-salinity effect' (LSE) describes the phenomenon where oil recovery can be substantially enhanced via use of low-salinity solutions to sweep sandstone reservoirs. Exploitation of the LSE is widely practiced. However, the underlying molecular-scale mechanisms by which this process occurs remain unresolved. Here, we elucidate the role of Ca2+ by combining chemical force microscopy (CFM) and molecular dynamics (MD) simulations, using amorphous silica  as a model for the sandstone substrate. 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. Our CFM data reveal stronger adhesion at 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 Ca2+ 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 provide new practical insights into how surface wettability may be manipulated by electrolyte concentration to achieve enhanced oil recovery.