Gas production from hydrates by CH4-CO2 replacement: Effect of N2 and intermittent heating

Extraction of CH4 gas from Natural Gas Hydrates (NGHs) while storing CO2 by CH4-CO2 replacement method is a promising technique for achieving CO2 emission reduction and CH4 production. However, improving the kinetics and enhancing the replacement efficiency is the fundamental issue for efficient CH4 production and CO2 safe sequestration, which have become the bottlenecks in NGH extraction. In this work, the CO2 replacement characteristics and kinetics process were further elucidated. The effects of small molecule gas (N2) and intermittent heating on the CO2 replacement were quantitatively investigated. And the enhancement mechanism of N2 and intermittent heating on CO2 replacement were deeply analyzed. The results show that as the proportion of N2 increases, the CH4 recovery rate gradually increases, but the CO2 sequestration rate exhibits a pattern of initial increase followed by a decrease. In addition, the intermittent application of heat has been shown to effectively enhance the replacement performance of pure CO2 in porous media. A thicker quasi-liquid layer forms when the temperature of hydrate reservoir approaches the freezing point, making it easier for CO2 gas to form hydrates. Additionally, when intermittent heating is combined with CO2 + N2 replacement, it results in a more significant promotion of CH4 recovery.