Experimental Study on the Formation and Dissociation Characteristics of Mixed CO2 + CH4 Hydrates in Quartz Sand

Naturalgas has been considered as a transiting fossil fuel intothe future low-carbon world, improving air quality and reducing carbonintensity. The efficiency of purifying unconventional gas resourcesand recovering gas from hydrates via a CO2-CH4 exchange has attracted widespread attention in the scientificfields. However, the associated fluid flow, heat transfer, and hydratereaction kinetics involving the mixed gas formation and dissociationare strongly needed in understanding the fundamental thermophysicalproperties. In this study, we designed a series of experiments employingdepressurization and thermal stimulation to dissociate mixed CO2 + CH4 hydrates. We examined the kinetics, fluidproduction behavior, heat transfer characteristics, and separationfactors during the hydrate formation and dissociation processes. Thegas consumption and each phase saturation show a good consistencyin all cases, however, the induction time was stochastic, rangingfrom 38 to 58 min. The separation factors were all less than 1.0,and the hydrate shows a strong selectivity to CH4 duringthe gas mixture hydrate formation process. The gas production increasedwith the decrease of the depressurization pressure. The peak valueof the decomposition rate at 0.5 MPa is 0.2147 mol/min, which is 1.13times that at 1.0 MPa and 13.5 times that at 1.5 MPa. Increasing decompositiontemperature led to the rapid end of hydrate decomposition. The CO2 fraction was larger than that of CH4 during thehydrate dissociation process, and CO2 was easier to escapefrom the hydrate cage structure during the mixed gas hydrate decompositionprocess with decreasing pressure and increasing temperature.