Phase equilibria of carbon dioxide + sulfur hexafluoride mixed gas hydrate as fundamental data toward improving the mechanical properties of marine sediments

Abstract

Isothermal phase equilibria of carbon dioxide (CO₂) + sulfur hexafluoride (SF₆) mixed gas hydrate at temperatures of 281.85 K, 284.05 K, 288.05 K, 291.05 K, 291.74 K, and 292.20 K were measured so as to improve the mechanical properties of marine sediment by hydrate cementation. The addition of SF₆ significantly reduces the equilibrium pressure of CO₂-containing mixed gas hydrate at each temperature. At temperatures above the quadruple point Q₂ (pure CO₂ hydrate + aqueous + CO₂-rich liquid + vapor phases) of 283.22 K, the four-phase (mixed gas hydrate + aqueous + guest-rich liquid + vapor phases) equilibrium point(s) exists(exist) on the isotherms of the CO₂+SF₆ mixed gas hydrate system. The four-phase equilibrium curve was connected from the quadruple point Q₂ of pure CO₂ hydrate to that of pure SF₆ hydrate and had a maximum temperature point at 292.0 ± 0.2 K, which is higher than both the Q₂ temperatures of pure CO₂ hydrate and pure SF₆ hydrate. Therefore, the addition of SF₆ to CO₂ brings a significant effect to expand the thermodynamically stable region of CO₂-containing mixed gas hydrate in order for simultaneous CO₂ storage and sediment improvement to be realized at marine sediment.