Complementary and competitive dynamics of CO2 and N2 in CH4-Flue gas replacement within natural gas hydrates

To mitigate global warming, the paramount imperative lies in curbing the emission of CO2. 2 . The guest replacement method is a prominent carbon-neutral technological advancement that involves injecting CO2 2 into natural gas hydrate layers to accomplish the dual objectives of energy production and carbon storage. In this study, the guest dynamics in the CH4 4 - flue gas replacement process were examined, and the impacts of the N2 2 concentration of the injected gas were systematically analyzed. A powder X-ray diffraction analysis of the cage-specific guest distributions after CH4 4- CO2 2 (20 %) + N2 2 (80 %) replacement revealed that CH4 4 production increased in both the large and small cages compared to the CH4 4 - CO2 2 replacement. This enhancement was attributed to the N2 2 molecules participating in both cages. However, this simultaneously led to a decrease in CO2 2 storage potential, indicating a 'complementary' relationship for CH4 4 production and a 'competitive' one for CO2 2 storage with respect to CO2 2 and N2. 2 . In situ Raman spectroscopy revealed that the introduction of N2 2 resulted in a deceleration of CO2 2 storage kinetics. Guest composition measurements after replacement showed an upward trend in CH4 4 production and a simultaneous decline in CO2 2 storage as the N2 2 composition increased. Notably, an intriguing correlation was established between the CO2/N2 2 /N 2 ratios for the injected gas and the replaced hydrates, exhibiting a strong alignment with a simple first-order equation. The findings not only contribute to a deeper understanding of the CH4 4- CO2 2 + N2 2 replacement technique but provide practical insights for its application in real-world scenarios.