Simulation of Gas Production and Seafloor Subsidence during the Development of Natural Gas Hydrates in the South China Sea

In the process of developing offshore hydrate reserves, the stability of the subsea formations emerges as a crucial determinant, exerting a significant influence on the feasibility of prolonged extraction. Addressing the challenge of seafloor subsidence induced by pressure reduction during extraction, a numerical simulation method was employed that integrates multiphase seepage, heat transfer, and mechanical considerations. It delves into the dynamic evolution of mechanical parameters in shallow seabed formations, considering the depth-dependent changes. An in-depth analysis is conducted to discern the impact of reservoir permeability, hydrate saturation, and geothermal gradient on both formation subsidence and the overall efficiency of the extraction process. Key findings highlight that the permeability of the reservoir plays a pivotal role in enhancing the extraction efficiency. Extracting from high-permeability reservoirs not only yields greater gas production but also avoids significant subsidence compared to that from low-permeability counterparts. Furthermore, higher hydrate saturation levels are found to substantially contribute to the increased gas production. The cohesive effects of the hydrates further elevate the stability of the surrounding strata near the wellbore. When dealing with reservoirs characterized by higher geothermal gradients, the expanded range of hydrate decomposition results in a heightened formation subsidence. Consequently, the ratio of gas-extraction-induced subsidence exhibits a discernible downward trend. The results of this research provide valuable theoretical insights, serving as a reference for effectively controlling risks during the process of the depressurization extraction of natural gas from shallow offshore reserves.