Experimental Verification of Structural Transition and Dissociation Enthalpy Change during CH4 Recovery Induced by CO2 Injection into CH4 + C3H8 hydrate

Abstract

The CH4-CO2 replacement occurring in natural gas hydrates has attracted significant attention due to its double function for CH4 recovery and CO2 sequestration. In this study, CO2 was injected into the CH4 + C3H8 hydrate to identify the influence of the replacement on the hydrate structure and dissociation enthalpy (ΔHd). The shift of the three-phase (H-LW-V) equilibrium line after CO2 injection indicated that a significant portion of CH4 and C3H8 in the initial hydrate was replaced with CO2. The extent of the replacement was measured using a gas chromatograph, and was gradually increased as the driving force (ΔPCO2) increased. The NMR confirmed that the CH4 + C3H8 hydrate is structure II and no structural transition was observed after the replacement. The influence of the replacement on ΔHd was examined using a high-pressure micro-differential scanning calorimeter (HP μ-DSC). During the replacement, there was no significant heat flow associated with the hydrate dissociation or formation. This study revealed that the CH4 recovery induced by CO2 injection into CH4 + C3H8 hydrate could be achieved in a sII isostructural system without significant hydrate cage destruction.