CH₄-CO₂ replacement occurring in sII natural gas hydrates for CH₄ recovery and CO₂ sequestration

The CH₄-CO₂ replacement occurring in sII natural gas hydrates for CH₄ recovery and CO₂ sequestration was investigated with a primary focus on thermodynamic, microscopic, and kinetic aspects. The guest-exchange behavior during replacement and the end-state composition analysis of replaced hydrates demonstrated that the extent of the replacement after CO₂ injection into the sII CH₄ + C₃H₈ hydrate was significantly enhanced with an increase in the injected CO₂ pressure (P_CO2). The structure identification using powder X-ray diffraction (PXRD) suggested that the higher extent of replacement at higher P_CO2 was closely related with the higher portion of sI hydrate after replacement. ¹³C NMR spectroscopy confirmed that most of the CH₄ molecules resided in the small cages of the replaced sII hydrate while small amount of them also existed in the CO₂-rich sI hydrate after replacement. The dissociation behavior and dissociation enthalpies of the replaced hydrates examined using an HP μ-DSC also verified the structural coexistence of sI and sII hydrates after replacement. The overall results can offer the first experimental evidence of the relationship between the replacement efficiency and the partial structure-transition in the CH₄ + C₃H₈ − CO₂ replacement, and can provide further insights into the cage-specific occupation of external gas molecules and thermal property changes for the actual replacement occurring in sII natural gas hydrates.