Experimental Verification of Structural Transition and Dissociation Enthalpy Change during CH4 - CO2 Replacement that Occurs in Various Gas Hydrate Structures

Abstract

This study investigated the CH4-CO2 replacement that occurs in various gas hydrate structures (sI, sII, and sH), primarily focusing on thermodynamic stability, structural transition, and dissociation enthalpy change for its dual function of CH4 recovery and CO2 sequestration. To verify the influence of CO2 injection on the thermodynamic stability of various gas hydrate structures, phase equilibria of the initial CH4 hydrate (sI), CH4 + C3H8 hydrate (sII), and CH4 + 2,2-dimethylbutane (neohexane, NH) hydrate (sH) were compared with those of corresponding gas hydrates replaced with CO2. The shift in the phase equilibrium conditions after replacement implied that the substantial extent of the replacement was achievable after the replacement reaction proceeded. In addition, to examine a possible structural transition and to elucidate cage-dependent guest distributions, the initial and replaced hydrates were analyzed via carbon-13 nuclear magnetic resonance (13C NMR). The 13C NMR spectroscopic results confirmed that the CH4 - CO2 replacement occurred in the sI-isostructural system and the CH4 + C3H8 - CO2 replacement occurred in the sII-isostructural system, whereas the CH4 + NH - CO2 replacement underwent the structural transformation from sH to sI. To reveal heat generated or absorbed during the replacement process and its influence on the thermal properties of the replaced hydrates, changes in heat flow and dissociation enthalpies (Î?Hd) were measured using a high-pressure micro-differential scanning calorimeter (HP μ-DSC). During the CH4 - CO2 replacement and the CH4 + C3H8 - CO2 replacement, no significant changes in heat flow emanating from endothermic or exothermic reactions were observed, indicating that the isostructural replacement occurred without significant hydrate dissociation or reformation. However, a significant endothermic peak followed by a large exothermic peak was observed in the CH4 + NH - CO2 replacement, indicating that the initial sH hydrate dissociation was followed by the formation of sI hydrate as the replacement reaction proceeded. The DHd value of the replaced hydrate in the structure-transitional system was significantly lower than that of the initial CH4 + NH hydrate and even slightly lower than that of pure CO2 hydrate. However, the DHd values of the replaced hydrates in the isostructural replacement did not change as remarkably as in the structure-transitional replacement. 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 natural gas hydrate reservoirs for CH4 recovery and CO2 sequestration.