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

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

Swelling Properties of Montmorillonite and Beidellite Clay Minerals from Molecular Simulation: Comparison of Temperature, Interlayer Cation, and Charge Location Effects

The swelling properties of smectite clay minerals are relevant to many engineering applications including environmental remediation, repository design for nuclear waste disposal, borehole stability in drilling operations, and additives for numerous industrial processes and commercial products. We used molecular dynamics and grand canonical Monte Carlo simulations to study the effects of layer charge location, interlayer cation, and temperature on intracrystalline swelling of montmorillonite and beidellite clay minerals. For a beidellite model with layer charge exclusively in the tetrahedral sheet, strong ion–surface interactions shift the onset of the two-layer hydrate to higher water contents. In contrast, for a montmorillonite model with layer charge exclusively in the octahedral sheet, weaker ion–surface interactions result in the formation of fully hydrated ions (two-layer hydrate) at much lower water contents. Clay hydration enthalpies and interlayer atomic density profiles are consistent with the swelling results. Water adsorption isotherms from grand canonical Monte Carlo simulations are used to relate interlayer hydration states to relative humidity, in good agreement with experimental findings