Experimental Investigation into the Dissociation Behavior of CH4-C2H6-C3H8 Hydrates in Sandy Sediments by Depressurization

The dissociation kinetics of gas hydrate formed from binary CH4 C3H8 and ternary CH4 C2H6 C3H8 gas mixtures were studied by a gas-collection-analysis method at constant back-pressure and different temperatures. During hydrate dissociation, the gas produced was first collected by sample bags consecutively and then analyzed by gas chromatography. It was found that the gas production of the mixed hydrates was quite different from that of methane hydrate. Interestingly, the molar composition of C3H8 in the gas mixture produced changed little as hydrate dissociation proceeded. The retainment of C3H8 in hydrates was confirmed with the calculation results of composition of hydrates remained during gas production. The preferential of CH4 and C2H6 over C3H8 molecules released from hydrate decomposition was attributed to the difference in guest-to-cavity size ratios. The heterogeneous dissociation of the multiple guests complicates the gas production process as the remaining hydrate rich in C3H8 may act as a barrier to gas diffusion. The self-preservation model for CH4 hydrate dissociation below ice point was applied to describe the heterogeneous dissociation of the mixed hydrates, suggesting that the dissociation kinetics of the mixed hydrates containing C3H8 above ice point was similar to that of CH4 hydrate below ice point. These results are of interest for the gas recovery from hydrates, and for natural gas storage and transportation in the hydrate state.