Influences of diffusion-limited transport in the crystals and initial water on the gaseous CO2 dynamic replacement in CH4 hydrate

CO₂–CH₄ dynamic replacement is a process of the CO₂–CH₄ replacement method. In this process, CO₂ is injected into CH₄ hydrate-bearing sediment continuously to recover CH₄. The dynamic replacement process can be divided into the CO₂ displacement stage and the subsequent dynamic replacement stage and is affected by the existence of initial water and diffusion-limited transport caused by the mixed hydrate layer. Effective fugacity is utilized to combine the thermodynamic model and kinetic model to investigate the effects of the above two factors on the CH₄–CO₂ dynamic replacement process. Initial water is assumed to distribute in the pore space. The CH₄ hydrate decomposition due to the decrease of CH₄ partial pressure in the gas phase during the CO₂ displacement stage is considered. Our investigation results show that diffusion-limited transport is the main factor that restricts the replacement percent in the displacement stage, the effect of the existence of initial water on the replacement percent is more obvious than that of the diffusion-limited transport. CO₂ storage efficiency is less than 10% during the entire dynamic replacement and is mainly affected by the existence of initial water rather than the diffusion-limited transport. The temperature increase is mainly due to newly formed hydrate. Finally, more CH₄ hydrate is exploited near the outlet than that near the inlet. Therefore, the CH₄–CO₂ replacement method needs to be enhanced near the inlet. CO₂ is mainly sequestrated in the CO₂ hydrate formed through free water. CO₂ sequestrated in the mixed hydrate is mainly distributed near the inlet, while the CO₂ sequestrated in the CO₂ hydrate is mainly distributed near the outlet.