Experimental Study on the Dissociation Equilibrium of (CH4 + CO2 + N2) Hydrates in the Mixed Sediments

The recovery of CH4 from gas hydrate by injecting flue gas would inevitably lead to coexistence of CH4, CO2, and N-2 in the hydrate phase. To simulate the actual situation and investigate the stability condition of (CH4 + CO2 + N-2) hydrates, their dissociation equilibrium in the mixed sediments of quartz sand and kaolin clay was measured with the isochoric pressure search method. Six gas samples were tested and analyzed with respect to the CH4 content and the mole ratio of CO2/N-2 in the gas phase. The results indicated that the hydrate dissociation equilibrium pressure at a given temperature shows a decreasing trend as the CH4 content increases at a lower CO2/N-2 ratio, which was contrary to that at a higher CO2/N-2 ratio. The dissociation equilibrium temperature of (CH4 + CO2 + N-2) hydrates for the mixed sediments is shifted lower than those for bulk phase at the same pressure. The effect of the mixed sediment composition and water saturation on the hydrate dissociation equilibrium was investigated. The results showed that the temperature shift increases with the increase of kaolin content in the mixed sediments, while it decreases as the water saturation increases. The maximum temperature shift observed in this work was 1.56 K in the mixed sediments (the mass ratio quartz sand and kaolin clay was 1:0.20) with a water saturation of 20%. The hydrate dissociation enthalpies are calculated with the Clausius-Clapeyron equation. It was observed that the hydrate dissociation enthalpies exhibited a decreasing trend with the increase of CH4 content in the gas phase.