(411a) Experimental Verification of Structural Transition and Dissociation Enthalpy Change during CH4 - CO2 Replacement That Occurs in Various Gas Hydrate Structures

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?H_d) 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 DH_d 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 DH_d 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.