Coexistence of Methane Hydrate and Gas Associated with the Difference in Thermodynamic Stabilities of Hydrate in the Pores of Sediments from Shenhu, South China Sea

In recent field investigations and laboratory experiments, the coexistence of natural gas and gas hydrate in an electrolyte solution has been observed. The intrinsic mechanism for the coexistence of natural gas in gas hydrate stability zones in sediment is crucial for comprehending the physical and chemical properties and gas production potential in a reservoir but has not yet been thoroughly elucidated yet. In this study, the dissociation process of hydrates formed in fine marine sediments from the Shenhu area in the South China Sea was observed across a temperature range of ∼6–12 °C. It was found that the hydrates formed in the sediment pores exhibit varying stability, and their stability boundaries can be further defined by two equilibrium curves in this study. The results obtained can be the basis for the explanation of the stable coexistence of natural gas and hydrate in the gas hydrate stability zone, which can result from the stability difference of hydrate in the sediment pores with different sizes.