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

The CH4 center dot 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. C-13 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 mu-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.