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.