Of particular relevance to hydrate exploitation by gas replacement strategy, the composition and selectivity of the mixed carbon dioxide/methane hydrates confined in porous rocks remain to be explored in detail. We here employ molecular modeling tools to assess the confinement and surface effects on the gas composition and selectivity of mixed hydrates. First, Monte Carlo simulations are performed in the grand canonical ensemble to determine the gas compositions in both mixed gases and mixed hydrates under realistic thermodynamic conditions. With these calculations, the gas selectivity of carbon dioxide over methane in the bulk hydrate phase is then determined. Beyond the well-identified influences of temperature and pressure, the gas selectivity in mixed hydrates is found also to be dependent on the composition of mixed gases. Meanwhile, the density profiles are shown to be able to compute the gas selectivity well in mixed hydrates. Finally, the density profiles in mixed hydrates confined in nanoporous carbons are determined. The surface force/confinement decreases the carbon dioxide density but increases the methane density in mixed hydrates. As a result, the mixed hydrates under nanoconfinement exhibit the reduced selectivity of carbon dioxide over methane compared to that of the bulk counterpart. In addition, the gas adsorption in porous solids is found to have an influence on the gas composition and selectivity in mixed hydrates confined to the nanoscale.
Fang et al. (Wed,) studied this question.