Geomechanical Behavior Analysis of Methane Hydrate Production via Depressurization with CO ₂ Hydrate Sealing: A Coupled THMC Numerical Study

Exploiting natural gas hydrate (NGH) through depressurization combined with the CO2 hydrate sealing method can obtain high gas production and realize CO2 sequestration simultaneously. However, this process induces competing geomechanical effects: while NGH dissociation destabilizes the reservoir structure, the formation of CO2 hydrate enhances formation stability. Therefore, a coupled thermal-hydrologic-mechanical-chemical (THMC) model was developed to systematically analyze both production performance and geomechanical behavior. Three CO2 hydrate sealing configurations were evaluated: Type A (sealing only the overburden), Type B (sealing only the underburden), and Type C (sealing both the overburden and underburden), with parametric studies conducted on sealing length (40–200 m) and CO2 hydrate saturation (0.2–0.95) to assess their impacts on production performance and geomechanical stability. Results demonstrated that Type A optimally balances the gas production and subsidence control. Specifically, a 160 m sealing length with 0.8 CO2 hydrate saturation maximized gas production while minimizing seafloor subsidence, demonstrating superior performance versus depressurization alone. These findings establish practical guidelines for achieving high-efficiency NGH extraction with enhanced reservoir stability.