Thermodynamic and Physico-chemical Analyses of CH4 - CO2 Replacement in Various Clathrate Structures for Application to CH4 Recovery and CO2 Sequestration

Abstract

This study investigated the influence of CO2 or flue gas injection into various hydrate reservoirs — for CH4 recovery with CO2 sequestration — on thermodynamic stability, physicochemical properties, and thermal behavior, to explore a possible structural transition and reveal the CH4 - CO2 replacement mechanism that occurs in sI, sII, and sH hydrates. To verify the influence of CO2 or flue gas injection on the thermodynamic stability of various hydrate reservoirs, the phase equilibrium conditions of the initial sI CH4, sII CH4 + C3H8, and sH CH4 + 2,2-dimethylbutane (neohexane, NH) hydrates and those after replaced with CO2 or flue gas were compared. The shift in the phase equilibrium conditions after replacement implied the substantial extent of the replacement was achievable as the replacement reaction proceeded. In addition, to examine a possible structural transition and to elucidate the cage-dependent guest distribution, the hydrates involved in the swapping process were analyzed via powder X-ray diffractometry (PXRD), carbon-13 nuclear magnetic resonance (13C NMR), and Raman spectroscopy. It was confirmed that the CH4 - CO2 or CH4 - flue gas replacement occurred in the sI-isostructural system and the CH4 + NH - flue gas replacement occurred in the sH-isostructural system, whereas the CH4 + NH - CO2 replacement reaction accompanied the structural transformation (sH → sI). Besides, the partial structure-transition (sII → sI) was also observed in the CH4 + C3H8 - CO2 replacement system. To identify heat generation or absorption during the replacement process and subsequent influence on the thermal properties of the replaced hydrates, changes in heat flow and dissociation enthalpies (ΔHd) were investigated using a high-pressure micro-differential scanning calorimeter (HP μ-DSC). During the CH4 - CO2 or CH4 - flue gas replacement and the CH4 + NH - flue gas replacement, no significant heat flow change from endothermic or exothermic reactions was observed, which indicates that the isostructural replacement proceeded without significant hydrate dissociation or formation. However, a large exothermic peak after the appearance of a significant endothermic peak was observed in the CH4 + NH - CO2 replacement, which indicates that the initial sH hydrate dissociation was followed by the formation of sI hydrate as the replacement reaction proceeded. The Hd value of the replaced hydrate in the complete structure-transitional system (sH → sI) was significantly lower than that of the initial CH4 + NH hydrate and even slightly lower than that of pure CO2 hydrate. However, the Hd values of the replaced hydrates in the isostructural replacement did not change as remarkably as in the structure-transitional replacement. On the other hand, the replacement system accompanying a partial structure-transition (sII → sI) showed gradual decrease of the Hd values as the injected CO2 pressure was increased. The overall experimental results provide further insights into the cage-specific occupation of external gas molecules and thermodynamic stability for the real replacement occurring in NGH reservoirs as a dual function of CH4 recovery and CO2 sequestration.