Molecular Simulation Studies of CO2-CH4-H2O Ternary Geological Fluids in Clay Confinements.

Molecular understanding of the CO2-CH4-H2O mixture in clay interlayers provides crucial insights into many geochemical processes in CO2-enchanced oil/gas recovery and CO2 subsurface sequestrations. In this work, we developed new molecular models by optimizing the unlike-pair Lennard-Jones (LJ) parameters for the CO2-H2O, CH4-H2O and CO2-CH4 binary mixtures. The optimization utilizes the coupling parameter method in combination with Gibbs ensemble Monte Carlo (GEMC) simulations. The transferability of binary-derived parameters to ternary CO2-CH4-H2O system is further demonstrated. These optimized parameters are used to study CO2-CH4-H2O mixtures in Na-montmorillonite interlayers at 323 K and 90 bar, through grand-canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. Chemical potentials of CO2, CH4, and H2O in clay interlayer are calculated through the equation of state (EOS) with the Lennard-Jones referenced Statistical Associating Fluid Theory (SAFT-LJ), which is suitable for aqueous mixtures. The equilibrium basal spacing distances and species concentrations under different relative humidity (RH) and CO2/CH4 mole fractions are determined through extensive GCMC simulations. Importantly, using the new molecular models, we find that the trend of the sorbed CO2 content versus the sorbed H2O content shows good agreement with in situ infrared (IR) spectroscopy data by Loring et al. [Environmental Science& Technology, 2021, 55, 11192-11203]. While the predicted CO2 concentrations in the monolayer hydration state (1W) is higher than the experimental results due to the heterogeneous expansion of the clay mineral in experiments, the CO2 content in the bilayer hydration state (2W) compares remarkably well with the experimental data. The CH4 adsorption in clay interlayers is found substantially lower than that of CO2. In general, GCMC results show that water intercalation under high RH suppresses the sorption of CO2 and CH4, and that Na-montmorillonite preferentially adsorbs CO2 over CH4.