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

Abstract

In this work, we experimentally simulated the production process of sandy hydrate systems in water-rich and gas-rich environments, and the hydrate samples with a saturation of ~35% were prepared based on the reservoir conditions of the Shenhu area. The hydrate decomposition, fluids flow and heat transfer characteristics of the reservoirs induced by depressurization were analyzed by combining the multiphase flow mechanism in porous media. Results showed that compared with the gas-rich hydrate system, the consumption time for depressurization operation in the water-rich system could be shortened by 4/9, but its total production cycle was significantly longer. During depressurization, the decomposition rate of hydrates in water-rich system was greater than that in gas-rich system, which consumed relatively more sensible heat from the reservoir. While during constant pressure, the rate of hydrate decomposition was reversed, and the constant k of the former decreased to half that of the latter, mainly due to the difference in water content in the reservoirs. For the water-rich system, the temperature rebound was delayed due to the large specific heat of water, almost half of hydrates decomposed during unstable depressurization, and the duration accounted for only ~5% of the total production cycle, indicating that its decomposition efficiency was significantly higher than that of gas-rich reservoirs during this period. At the end of production, water-rich reservoirs have lower gas recovery rates but higher water recovery rates than gas-rich reservoirs, that is, the existence of free gas is conducive to actual reservoir production.

Keywords: Natural gas hydrates, water-rich environment, sandy hydrate systems, depressurization, Shenhu area, free gas