Numerical Simulation of Gas Production from Natural Gas Hydrate Reservoirs in the South China Sea via Staged Depressurization

Abstract

Depressurization is generally regarded as the most practical method for extracting marine natural gas hydrates. However, conventional single-stage depressurization often leads to rapid temperature drops and excessive water production, which can increase the risk of formation collapse. Therefore, considering both safety and economic efficiency, this study established a numerical model based on geological data from the SH7 site in the Shenhu area of the South China Sea and designed multiple depressurization strategies. The dynamic gas production behavior under both single-stage and staged depressurization schemes over a 1000 day period was investigated. The depressurization amplitude coefficient (C_DA) and the depressurization return ratio (R_DR) are introduced to quantify the trade-off between production efficiency and depressurization risks. The results indicate that although a larger depressurization in single-stage depressurization enhances early gas production and improves the gas-to-water ratio, it also results in higher water production and reduced long-term recovery. When the initial depressurization and production timing are properly configured, the staged depressurization strategy can effectively stimulate reservoir potential, increasing cumulative gas production by approximately 18.6% compared with conventional single-stage depressurization. Furthermore, staged strategies achieve higher gas production rates while maintaining a lower C_DA, demonstrating a favorable balance between economic benefit and operational safety.