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Natural gas hydrate has attracted worldwide attention for its abundant availability and clean and nonpolluting combustion products. The actual environment of marine methane hydrates is water‐saturated, but present researches on water‐saturated hydrates are relatively few. This study prepares water‐saturated hydrate sediment of ≈70% saturation and mainly focuses on the thermodynamic and kinetic characteristics of decomposition process using different depressurization rates. Compared with gas‐saturated hydrates, both the thermodynamic evolution and the decomposition kinetics of water‐saturated hydrates get weakened due to the inhibition of water molecules surrounding hydrate crystals on heat and mass transfer. In all experimental cases of water‐saturated hydrates, the temperature still decreases at most 0.5 °C in several minutes after the pressure has become constant, indicating the delay effect of depressurization in water phase. The results show that the increase in depressurization rate strengthens both temperature decrease and pressure delay of water‐saturated hydrate‐bearing sediment. In addition, the decomposition rate of water‐saturated hydrates in depressurization stage is constant and positive to the depressurization rate, while the decomposition rate in constant pressure stage is a function of residual hydrate amount. This study is significant for decomposition control of hydrate mining under actual marine environment. [ABSTRACT FROM AUTHOR]

Gas permeability and the depressurization path are decisive factors affecting the efficiency of gas hydrate reservoir exploitation. However, there was a lack of quantitative studies on the gas permeability and dynamic optimization of real-time depressurization paths under different in-situ conditions. Therefore, hydrate-bearing cores within the core holder were remolded using South Sea clay under different in-situ conditions. Then, the gas permeability was continuously measured using a constant flow rate. Next, a gas permeability model coupling effective pressure, hydrate saturation, and absolute permeability were established. It was used to predict the gas permeability under different in situ conditions, with deviations mainly in the range of 1–2 %. In addition, the optimum outlet pressure and positive outlet pressure range during depressurization under different in-situ conditions were proposed. The real-time gas flow rate of the optimal depressurization path was larger than other depressurization paths. Also, the logic reasoning process for real-time optimal outlet pressure and positive outlet pressure range was given. Finally, the inlet pressure and the optimum outlet pressure were also accurately predicted. None of the predicted values deviated by more than 4 %. The dynamic optimization of the outlet pressure behavior during depressurization can provide some scientific guidance for hydrate development and utilization. [Display omitted] • Permeability model including hydrate saturation, effective pressure was established. • Optimum outlet pressure and pressure range during depressurization were found. • Real-time inlet pressure of the core was accurately predicted. • Real-time optimal outlet pressure and pressure range were accurately predicted. [ABSTRACT FROM AUTHOR]