Replacement in CH4-CO2 hydrate below freezing point based on abnormal self-preservation differences of CH4 hydrate

CH4-CO2 replacement for natural gas hydrate exploitation is a win-win method to simultaneously realize CH4 recovery and CO2 sequestration. However, the exchange progress is severely blocked by the reformation of compact CO2 hydrate film. Constructing porous CO2 hydrate could be a promising approach to solve this annoyance. In consideration of the difference in ice layer defects and decomposition rate of CH4 hydrates at different temperatures in its anomalous self-preservation region, in this study, we proposed a concept of CH4-CO2 hydrate replacement by utilizing the multi-defective ice generated from dissociated CH4 hydrate to form porous CO2 hydrate. The CH4-CO2 hydrate exchange experiments occurred at 268.15 K and 272.15 K with rapid CH4 exhaust-CO2 injection were conducted. The results showed that the replacement efficiency at 272.15 K was much higher than at 268.15 K. In addition, we examined the replacement at 274.15 K and replacement with slow gas sweep at 268.15 K for comparison. These experimental results confirmed the possibility of the replacement concept.