Wettability of Silica Surfaces by Brine vs. CO2: The Nanoscale View

The wetting angles formed by CO2 and water on mineral surfaces are key unknowns in predictions of CO2-brine multiphase flow in porous media. In the case of silica (a well established reference surface in wettability studies and an important component of CO2 storage formations), reported CO2-brine-mineral wetting angles are widely scattered: the wetting angles reported by various groups range from 10 to 60 degrees at low pressure and salinity, and they differ significantly in their pressure and salinity dependence. To investigate the origin of these discrepancies, we used molecular dynamics (MD) simulations carried out on the supercomputers at the National Energy Research Scientific Computing Center (NERSC) to study CO2-brine two-phase mixtures confined between quartz surfaces at conditions relevant to geologic carbon sequestration. Our simulation methodology was previously tested for its ability to predict the pressure and salinity dependence of CO2-water interfacial tension and the structure and dynamics of water on silica surfaces. Our results provide detailed insight into the manner in which the pressure and salinity dependence of silica surfaces arises from the properties of the silica-water, silica-CO2, and water-CO2 interfaces.