A study of multi-field coupling continuum numerical method for exploiting CH4 by CO2 replacement in deep-sea formation

The conventional method of gas hydrate exploitation (i.e. depressurization, thermal stimulation, inhibitor injection) could deteriorate the mechanical properties of methane hydrate bearing sediments (MHBS), potentially leading to a series of significant disasters. The carbon dioxide (CO2) replacement method utilizing CO2 to displace methane (CH4) from the hydrate pores, could combine energy extraction with CO2 geological storage and reservoir stability, which is of great significance. Based on TOUGH+HYDRATE (T+H), a numerical simulator T+MixH V1.0 was developed to model CO2 replacement process by expanding the mathematical model and incorporating the Chen-Guo hydrate model to calculate the multi-component hydrate phase equilibrium conditions. The reliability of the simulator had been verified by comparison with laboratory experiment and numerical simulation results from others. Subsequently, condition parameters (L) in the calculation process of a bounding surface constitutive model had been modified in terms of the authors' latest L which considered the influence of CO2 mole fractions. Numerical simulation results of triaxial tests demonstrated that this method can qualitatively capture variation of the soil’s mechanical behavior during the replacement process. Furthermore, by combining with in-house T+H+FLAC3D coupling program, a Thermo-Hydro-Mechanic-Chemical (THMC) multi-field coupling numerical analysis method for CO2 replacement was established. Finally, we simulated gas hydrate exploitation through depressurization and CO2 replacement method in the South China Sea, as well as plate loading tests on methane hydrate-bearing foundations during gas production. The results indicated that CO2 replacement method has increased gas production and mitigated subsidence of the formation. The bearing characteristics of methane hydrate-bearing foundations was mainly affected by two factors: the effective stress variation caused by pore pressure changes, the bond strength variation induced by gas hydrate decomposition/generation.