Isothermal Phase Equilibria of the CH4+CO2 Mixed-Gas Hydrate System for CO2 Capture and Storage in a Reservoir after CH4 Hydrate Exploitation

Isothermal phase equilibria of methane (CH₄) + carbon dioxide (CO₂) mixed-gas hydrate were investigated at 281.98, 284.17, 286.02, and 287.17 K for the carbon dioxide capture and storage (CCS) using gas hydrates in a reservoir after CH₄ hydrate exploitation. At temperatures higher than the upper quadruple point Q₂ (hydrate + aqueous + CO₂-rich liquid + vapor phases) of pure CO₂ hydrate, the phase behavior in the CH₄ + CO₂ mixed-gas hydrate system becomes very complicated due to two characteristic behaviors. One is the four-phase coexistence curve of hydrate + aqueous + CO₂-rich liquid + vapor phases in the CH₄ + CO₂ mixed-gas hydrate system, which originates from Q₂ and extends to the critical end point, where it intersects with the vapor–liquid critical locus of the CH₄ + CO₂ binary system. The other is the negative azeotropic-like retrograde (local pressure minimum) behavior observed at 286.02 and 287.17 K, where the equilibrium pressure of the CH₄ + CO₂ mixed-gas hydrate is lower than that of either pure CO₂ hydrate or pure CH₄ hydrate. When CO₂ is injected into a CH₄-remaining reservoir after CH₄ hydrate exploitation, the CO₂ composition in the resulting in CH₄ + CO₂ mixed-gas hydrate phase can increase up to either compositions at the local pressure minimum point (if it exists) or the four-phase equilibrium point at sediment temperatures above Q₂ temperature.