Experimental Study on the Distribution Characteristics of CO 2 in Methane Hydrate-Bearing Sediment during CH 4 /CO 2 Replacement.

CH₄/CO₂ replacement is of great significance for the exploitation of natural gas hydrate resources and CO₂ storage. The feasibility of this method relies on our understanding of the CH₄/CO₂ replacement efficiency and mechanism. In this study, CH₄/CO₂ replacement experiments were carried out to study the distribution characteristics of CH₄ and CO₂ in hydrate-bearing sediments during and after replacement. Similar to previously reported data, our experiments also implied that the CH₄/CO₂ replacement process could be divided into two stages: fast reaction and slow reaction, representing CH₄/CO₂ replacement in the hydrate-gas interface and bidirectional CH₄/CO₂ diffusion caused replacement, respectively. After replacement, the CO₂ content gradually decreased, and the methane content gradually increased with the increase of sediment depth. Higher replacement percentage can be achieved with higher replacement temperature and lower initial saturation of methane hydrate. Based on the calculation of CO₂ consumption amounts, it was found that the replacement mainly took place in the fast reaction stage while the formation of CO₂ hydrate by gaseous CO₂ and water almost runs through the whole experimental process. Thus, the pore scale CH₄/CO₂ replacement process in sediments can be summarized in the following steps: CO₂ injection, CO₂ diffusing into sedimentary layer, occurrence of CH₄/CO₂ replacement and CO₂ hydrate formation, wrapping of methane hydrate by mixed CH₄-CO₂ hydrate, continuous CO₂ hydrate formation, and almost stagnant CH₄/CO₂ replacement. [ABSTRACT FROM AUTHOR]