| Summary: | CH<sub>4</sub>/CO<sub>2</sub> replacement is of great significance for the exploitation of natural gas hydrate resources and CO<sub>2</sub> storage. The feasibility of this method relies on our understanding of the CH<sub>4</sub>/CO<sub>2</sub> replacement efficiency and mechanism. In this study, CH<sub>4</sub>/CO<sub>2</sub> replacement experiments were carried out to study the distribution characteristics of CH<sub>4</sub> and CO<sub>2</sub> in hydrate-bearing sediments during and after replacement. Similar to previously reported data, our experiments also implied that the CH<sub>4</sub>/CO<sub>2</sub> replacement process could be divided into two stages: fast reaction and slow reaction, representing CH<sub>4</sub>/CO<sub>2</sub> replacement in the hydrate-gas interface and bidirectional CH<sub>4</sub>/CO<sub>2</sub> diffusion caused replacement, respectively. After replacement, the CO<sub>2</sub> 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 CO<sub>2</sub> consumption amounts, it was found that the replacement mainly took place in the fast reaction stage while the formation of CO<sub>2</sub> hydrate by gaseous CO<sub>2</sub> and water almost runs through the whole experimental process. Thus, the pore scale CH<sub>4</sub>/CO<sub>2</sub> replacement process in sediments can be summarized in the following steps: CO<sub>2</sub> injection, CO<sub>2</sub> diffusing into sedimentary layer, occurrence of CH<sub>4</sub>/CO<sub>2</sub> replacement and CO<sub>2</sub> hydrate formation, wrapping of methane hydrate by mixed CH<sub>4</sub>-CO<sub>2</sub> hydrate, continuous CO<sub>2</sub> hydrate formation, and almost stagnant CH<sub>4</sub>/CO<sub>2</sub> replacement.
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