Phase Equilibria of the CH 4 -CO 2 Binary and the CH 4 -CO 2 -H 2 O Ternary Mixtures in the Presence of a CO 2 -Rich Liquid Phase

The knowledge of the phase behavior of carbon dioxide (CO 2 )-rich mixtures is a key factor to understand the chemistry and migration of natural volcanic CO 2 seeps in the marine environment, as well as to develop engineering processes for CO 2 sequestration coupled to methane (CH 4 ) production from gas hydrate deposits. In both cases, it is important to gain insights into the interactions of the CO 2 -rich phase—liquid or gas—with the aqueous medium (H 2 O) in the pore space below the seafloor or in the ocean. Thus, the CH 4 -CO 2 binary and CH 4 -CO 2 -H 2 O ternary mixtures were investigated at relevant pressure and temperature conditions. The solubility of CH 4 in liquid CO 2 (vapor-liquid equilibrium) was determined in laboratory experiments and then modelled with the Soave–Redlich–Kwong equation of state (EoS) consisting of an optimized binary interaction parameter k ij (CH 4 -CO 2 ) = 1.32 × 10 −3 × T − 0.251 describing the non-ideality of the mixture. The hydrate-liquid-liquid equilibrium (HLLE) was measured in addition to the composition of the CO 2 -rich fluid phase in the presence of H 2 O. In contrast to the behavior in the presence of vapor, gas hydrates become more stable when increasing the CH 4 content, and the relative proportion of CH 4 to CO 2 decreases in the CO 2 -rich phase after gas hydrate formation.