Numerical Simulation of Gas Production from Marine Hydrate Reservoir by Depressurization Assisted CO₂ Replacement

Natural gas hydrate (NGH) is a solid clathrate compound formed by water and natural gas at low temperatures and high pressures that is characterized by abundant resources and wide distribution. The production methods of NGH mainly include depressurization, thermal stimulation, chemical injection, CO₂ replacement, and combination methods. Among them, the depressurization assisted CO₂ replacement method has gained wide attention and become a research hotspot due to its capability of producing CH₄ and sequestering CO₂ simultaneously in hydrate reservoirs. Numerous experiments have been developed based on the different sequences of CO₂ replacement and depressurization. However, a comparative analysis of these two production sequences remains ambiguous. In this study, a numerical model for depressurization assisted CO₂ replacement was established to clarify the production sequences. Results indicated that the formation of CO₂ hydrate promoted the dissociation of CH₄ hydrate. The depressurization followed by CO₂ replacement represented the optimal production sequence, with cumulative gas production (V_p) and CO₂ sequestration ratios (R_CO2) higher than those of CO₂ replacement followed by depressurization by 3.17% and 1.61%, respectively. This study provides advantageous information for actual engineering applications.