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, CO(2)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 CO2-CH4 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.

Automated summary

CO2-CH4 dynamic replacement is a method for recovering CH4, which is mainly affected by diffusion-limited transport and initial water. The study results indicate that diffusion-limited transport is the main factor inhibiting replacement, while the influence of initial water is more significant. The CO2 storage efficiency is low and is mainly influenced by initial water rather than diffusion-limited transport.