Molecular Simulation of Carbon Dioxide, Brine, and Clay Mineral Interactions and Determination of Contact Angles

Abstract

Capture and subsequent geologic storage of CO₂ in deep brine reservoirs plays a significant role in plans to reduce atmospheric carbon emission and resulting global climate change. The interaction of CO₂ and brine species with mineral surfaces controls the ultimate fate of injected CO₂ at the nanoscale via geochemistry, at the pore-scale via capillary trapping, and at the field-scale via relative permeability. We used large-scale molecular dynamics simulations to study the behavior of supercritical CO₂ and aqueous fluids on both the hydrophilic and hydrophobic basal surfaces of kaolinite, a common clay mineral. In the presence of a bulk aqueous phase, supercritical CO₂ forms a nonwetting droplet above the hydrophilic surface of kaolinite. This CO₂ droplet is separated from the mineral surface by distinct layers of water, which prevent the CO₂ droplet from interacting directly with the mineral surface. Conversely, both CO₂ and H₂O molecules interact directly with the hydrophobic surface of kaolinite. In the presence of bulk supercritical CO₂, nonwetting aqueous droplets interact with the hydrophobic surface of kaolinite via a mixture of adsorbed CO₂ and H₂O molecules. Because nucleation and precipitation of minerals should depend strongly on the local distribution of CO₂, H₂O, and ion species, these nanoscale surface interactions are expected to influence long-term mineralization of injected carbon dioxide