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.