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. The guest replacement method is a prominent carbon-neutral technological advancement that involves injecting CO2 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 – flue gas replacement process were examined, and the impacts of the N2 concentration of the injected gas were systematically analyzed. A powder X-ray diffraction analysis of the cage-specific guest distributions after CH4 − CO2 (20 %) + N2 (80 %) replacement revealed that CH4 production increased in both the large and small cages compared to the CH4 – CO2 replacement. This enhancement was attributed to the N2 molecules participating in both cages. However, this simultaneously led to a decrease in CO2 storage potential, indicating a ‘complementary’ relationship for CH4 production and a ‘competitive’ one for CO2 storage with respect to CO2 and N2. In situ Raman spectroscopy revealed that the introduction of N2 resulted in a deceleration of CO2 storage kinetics. Guest composition measurements after replacement showed an upward trend in CH4 production and a simultaneous decline in CO2 storage as the N2 composition increased. Notably, an intriguing correlation was established between the CO2/N2 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 − CO2 + N2 replacement technique but provide practical insights for its application in real-world scenarios.