Experimental Study on the Dissociation Equilibrium of (CH4+ CO2) Hydrates in the (Quartz Sands + NaCl Solution) System

CH4–CO2 swapping has been viewed as a win–win situation for gas hydrate exploitation and geological storage of CO2. This method would form CH4 and CO2 coexist in the hydrate phase after reaction, and the stability condition of (CH4 + CO2) hydrates in the geological environment is significant for better understanding the CH4 recovery and CO2 sequestration. The dissociation points of (CH4 + CO2) hydrates in the (quartz sands + NaCl solution) system were measured with the isochoric pressure-search method in this work. The results indicated that the measured dissociation points shift left compared to the equilibrium curves of bulk hydrate. The equilibrium pressure of (CH4 + CO2) hydrates present a decreasing trend with the increasing of CO2 concentration in the gas phase at a given temperature. The effect of pore water salinity and sediment particle size on the hydrate equilibrium condition was investigated. It was found that the hydrate dissociation condition shifted to a lower temperature with the increase of NaCl concentration. The temperature shifts showed an increasing trend as the sediment particle size decreases. The maximum temperature shift was 1.2 K for the quartz sand system with an average particle size of 8.85 μm. The hydrate dissociation enthalpies were calculated with the Clausius–Clapeyron equation. The dissociation enthalpies of (CH4 + CO2) hydrates are between those of pure CH4 and CO2 hydrate, having an increasing trend with the increase of CO2 content in the gas phase.