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 (Vp) and CO2 sequestration ratios (RCO2) 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.