The Role of Water and Carbon Dioxide Intercalation on Na-Montmorillonite Swelling Behavior at Geological Carbon Sequestration Conditions*

Swelling of Na-montmorillonite in the environment relevant to geological CO$_{2}$ sequestration in deep underground formations is investigated by conducting classical Monte Carlo and molecular dynamics simulations. Both the binary (clay-water or clay- CO$_{2})$ and the ternary (clay-water- CO$_{2})$ systems containing the clay, water and carbon dioxide phases are simulated, and the free energy for clay swelling is calculated as a function of the interlayer distance. The calculations indicate that while water intercalates into the clay layer and forms stable monolayer and bilayer hydration states, in the absence of interlayer water adsorption of dry carbon dioxide is thermodynamically unfavorable. In the ternary system, two hydration states are observed with interlayer spacings corresponding closely to those of the pure water binary system. In addition, the simulations of the ternary system show that the incomplete first hydration state is more effective at adsorbing CO$_{2}$ molecules than is the incomplete second hydration state.