Molecular Simulation Studies of CO 2 –CH 4 –H 2 O Ternary Geological Fluids in Clay Confinements

Molecular understanding of the CO 2 –CH 4 –H 2 O mixture in clay interlayers provides crucial insights into many geochemical processes in CO 2 -enchanced oil/gas recovery and CO 2 subsurface sequestrations. In this work, we developed new molecular models by optimizing the unlike-pair Lennard-Jones (LJ) parameters for the CO 2 –H 2 O, CH 4 –H 2 O and CO 2 –CH 4 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 CO 2 –CH 4 –H 2 O system is further demonstrated. These optimized parameters are used to study CO 2 –CH 4 –H 2 O 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 CO 2, CH 4, and H 2 O 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 CO 2 /CH 4 mole fractions are determined through extensive GCMC simulations. Importantly, using the new molecular models, we find that the trend of the sorbed CO 2 content versus the sorbed H 2 O 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 CO 2 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 CO 2 content in the bilayer hydration state (2W) compares remarkably well with the experimental data. The CH 4 adsorption in clay interlayers is found substantially lower than that of CO 2 . In general, GCMC results show that water intercalation under high RH suppresses the sorption of CO 2 and CH 4, and that Na-montmorillonite preferentially adsorbs CO 2 over CH 4 .