Gas Production from Three-Phase Coexisting Sandy Hydrate Systems Induced by Depressurization: Insights into Water- and Gas-Rich Environments

Abstract

Natural gas hydrates, a new mineral resource distinct from traditional coal, oil, and gas, are considered a potential successor energy source in the post-oil era. In this work, we experimentally simulated the production process of sandy hydrate systems in both water- and gas-rich environments. Hydrate samples with a saturation of approximately 35% were prepared based on the reservoir conditions of Shenhu area in the South China Sea. Hydrate decomposition, fluid flow, and heat transfer characteristics induced by depressurization were analyzed by incorporating the multiphase flow mechanism in porous media. Results showed that compared to the gas-rich system, the depressurization duration in the water-rich system could be reduced by approximately 4/9, although the overall production cycle was significantly longer. During the depressurization stage, hydrate decomposition in the water-rich system proceeded at a higher rate than in the gas-rich system, consuming more sensible heat from the reservoir. In contrast, during the constant pressure stage, the rate of hydrate decomposition reversed, with the decomposition rate constant (k) in the water-rich system decreasing to half that of the gas-rich system, primarily due to the difference in water content. For the water-rich system, the temperature rebound during constant pressure was delayed due to water’s high specific heat capacity. Nearly half of the hydrates decomposed during stable depressurization, with this phase accounting for only ∼5% of the total production cycle, indicating significantly higher decomposition efficiency compared with gas-rich reservoirs during this period. In water-rich systems, intensive gas production primarily occurred during the stable depressurization and constant stages, while in gas-rich systems, gas production was more evenly distributed across all stages. By the end of production, water-rich reservoirs exhibited lower gas recovery rates but higher water recovery rates than gas-rich reservoirs, demonstrating that the presence of free gas is beneficial for actual reservoir production.