Numerical Study on Thermal–Hydraulic–Mechanical–Chemical (THMC) Coupled Processes of Submarine CO2 Hydrate Sequestration via Horizontal Well Injection

To ensure the safe and efficient implementation of subsea CO2 hydrate sequestration, it is essential to investigate the stability of sediment reservoirs during the sequestration process. Thermal–hydraulic–mechanical–chemical (THMC) coupled numerical simulation can systematically reveal the migration, transformation, and geomechanical responses of CO2 during horizontal well injection. However, most existing studies rely on simplified cross-sectional models, which fail to accurately capture the coupled spatial evolution between hydrate formation and reservoir deformation. In this study, a THMC-coupled numerical simulation framework suitable for subsea horizontal well sequestration was developed using COMSOL Multiphysics. Based on typical geological parameters of the Shenhu area in the South China Sea and considering the constraints of horizontal well injection, the study systematically investigated the formation and spatial distribution of CO2 hydrates, the evolution of pore pressure and temperature fields, reservoir deformation, and sequestration stability. The results provided theoretical support for the engineering design and risk assessment of deep-sea CO2 hydrate sequestration. The simulation results showed that, under an injection rate of 4320 kg/d and a well length of 100 m, approximately 1.15 × 107 kg of CO2 can be sequestered after 5000 days. CO2 hydrate formation through horizontal well injection induced significant seabed uplift, with a maximum vertical displacement of 0.19 m after 5000 days. During long-term sequestration, the principal effective stress remained below the yield threshold, ensuring operational safety. Compared with vertical wells, horizontal well injection produced a wider hydrate cap, extended the injection duration from 9110 days to 16535 days, and achieved a greater overall sequestration potential.