Experimental study on the influence of different hydrate saturation and occurrence morphology on CH4-CO2 replacement behavior from hydrate-bearing sediments: Implications for CH4 recovery and CO2 storage

CO2 replacement method is a promising technology to extract CH4 from natural gas hydrates (NGHs) within permafrost regions, while achieves for both CH4 extraction and CO2 geologic sequestration. In this study, the replacement process of CO2-CH4 hydrates was investigated under different hydrate saturations and occurrence morphologies. The replacement behavior of CO2-CH4 hydrate was deeply analyzed in porous media under different conditions and the influence mechanism of hydrate saturations and occurrence morphologies on the replacement characteristics of CO2-CH4 hydrates were demonstrated. The results indicate that, the lower hydrate saturation obtains the better replacement outcomes under the same initial conditions. Under different conditions of occurrence morphologies, when the hydrate saturation is 16.5%, the replacement effect of patchy hydrate is the best. Meanwhile, when the hydrate saturation is 14.2% and 11.5%, the replacement effect of dispersed hydrate is the best. Additionally, the dispersed hydrate reservoir with a hydrate saturation of 11.5% is most suitable for CO2 replacement, achieving CH4 replacement and CO2 sequestration rates of 20.933% and 75.948%, respectively. The relevant results provide a scientific guidance and theoretical foundation for the efficient extraction of gas hydrates in permafrost regions by CO2 replacement method.