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In this study, thermodynamic stability and cage occupation behavior in the CH₄ – CO₂ replacement, which occurs in natural gas hydrate reservoirs by injecting flue gas, were investigated with a primary focus on phase equilibria and composition analysis. The phase equilibria of CH₄/N₂/CO₂ mixed hydrates with various compositions were measured to determine the thermodynamic stability of gas hydrate deposits replaced by N₂/CO₂ gas mixtures. The fractional experimental pressure differences (Δp/p) with respect to the CSMGem predictions were found to range from −0.11 to −0.02. The composition analysis for various feed gas mixtures with a fixed N₂/CO₂ ratio (4.0) shows that CO₂ is enriched in the hydrate phase, and the N₂/CO₂ ratio in the hydrate phase is independent of the feed CH₄ fractions. Moreover, ¹³C NMR measurements indicate that N₂ molecules preferentially occupy the small 5¹² cages of sI …