Multi-field coupling continuum numerical method for exploiting CH₄ by CO₂ replacement in deep-sea formation

The extraction method that induces hydrate dissociation may deteriorate the mechanical properties of methane hydrate bearing sediments (MHBS), potentially leading to a series of significant disasters. The carbon dioxide (CO₂) replacement method can effectively improve the sediment strength, but few methods have considered its mechanical response. So, a numerical method for the CO₂ replacement method as well as its application is studied. Based on TOUGH+HYDRATE, a numerical simulator, T+MixH V1.0 is developed to simulate the CO₂ replacement process by incorporating the Chen-Guo hydrate model to calculate the multi-component hydrate phase equilibrium conditions. The reliability of the simulator is validated by comparing with other experimental and numerical results. Then, a thermo-hydro-mechanic-chemical multi-field coupling numerical method for the CO₂ replacement is established by coupling with FLAC^3D. Finally, the gas hydrate exploitation is simulated through depressurization and CO₂ replacement methods in the South China Sea, as well as the plate loading tests on MHBS ground during gas production. The results indicate that the CO₂ replacement increases gas production and mitigates subsidence of the ground, which comes from that depressurization causes energy loss, limiting the decomposition of CH₄ hydrate, whereas the CO₂ replacement method occurs spontaneously, and the generated CO₂ hydrate provides support in the ground. Besides, the bearing characteristics of MHBS ground are mainly affected by two factors: the effective stress variation caused by pore pressure changes, and the bond strength variation induced by gas hydrate decomposition/generation.