Carbon Dioxide in Montmorillonite Clay Hydrates: Thermodynamics, Structure, and Transport from Molecular Simulation

We report a Monte Carlo and molecular dynamics simulations study of carbon dioxide in hydrated sodium montmorillonite, including thermodynamical, structural, and dynamical properties. In order to simulate the behavior of a clay caprock in contact with a CO2 reservoir, we consider clays in equilibrium with H2O-CO2 mixtures under conditions close to relevant ones for geological storage, namely a temperature T = 348 K, and pressures P = 25 and 125 bar, and under which two bulk phases coexist: H2O-rich liquid on the one hand and CO2-rich gas (P = 25 bar) or supercritical fluid (P = 125 bar) on the other hand. We first use grand canonical MC simulations to determine the number of stable states in clay, their composition, and the corresponding equilibrium interlayer distances. The vertical, horizontal, and radial distribution functions of the confined mixture, subsequently obtained using molecular dynamics, reveal some structural feature induced by the presence of CO2. Finally, the simulations indicate that carbon dioxide considerably influences the diffusion of mobile species in clays. We discuss these results by comparing them with those obtained for the bulk mixtures, as well as for Na-montmorillonite in equilibrium with a pure water reservoir water at the same temperature and pressure.