Understanding Calcite Wettability Alteration through Surface Potential Measurements and Molecular Simulations

Mineral wettability and wettability alteration are important factors that determine the distribution and mobility of oil during the recovery process. Because wettability is dependent on many factors (e.g., hydrocarbon composition, mineralogy, and pH), predicting mineral wettability is often difficult. The goal of this study is to look at changes that occur on the mineral itself, specifically changes in the surface structure and surface potential, using experimental methods complemented by quantum-mechanical calculations to better understand the molecular-level processes involved in wettability alteration. Nanoscale surface imaging is combined with Kelvin probe force microscopy (KPFM) to characterize changes in topography and surface potential for water-wet (hydrophilic) and oil-wet (hydrophobic) calcite surfaces, using the surfactant hexamethyldisilazane (NHSi 2 (CH 3 ) 6, HMDS) to render the calcite surface oil-wet. KPFM measurements show that HMDS adsorbs preferentially on step edges of the calcite surface and is coupled by an increase in surface potential, which suggests a decrease in electron density in the valence band wherever HMDS is adsorbed. Density functional theory (DFT) calculations of HMDS adsorption on calcite confirm an increase in the surface potential of oil-wet calcite and show that Ca corner sites are associated with the most favorable HMDS adsorption energies. Coadsorption of H + and OH – with HMDS is more likely to occur at edges and Ca kink sites and indicates that this surfactant may be an effective wettability modifier at a range of pH conditions. This study is the first application of KPFM to mineral wettability and demonstrates that with further development KPFM can be a powerful tool to study interactions between specific functional groups and surface sites modifying the surface’s electronic structure and wettability.