Molecular simulations on the structure and dynamics of water-methane fluids between Na-montmorillonite clay surfaces at elevated temperature and pressure

The structure and dynamics of water-methane fluids between clay surfaces are investigated through the grand-canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The chemical potentials of water and methane at the temperatures and pressures corresponding to different burial depths are calculated. These chemical potentials are used in the GCMC simulations to determine the water and methane contents in the clay interlayer at a burial depth of 6 km. The results are used as initial inputs for further MD simulations to investigate the static and dynamic properties of the confined fluid. Simulation results show that initial clay swelling is dominated by water adsorption into clay interlayer, followed by the formation of methane hydrates as the basal spacing increases. Methane content in clay is found to increase in a step fashion, from initial inner-sphere complex to both inner- and outer-sphere structures. It is found that methane is not fully coordinated by water molecules due to the low density of water content in Na-montmorillonite clay.