Determination of the upper-quadruple-phase equilibrium region for carbon dioxide and methane mixed gas hydrates

Simultaneous CO2 sequestration and CH4 production has been considered as a possible way to form and restore CO2 hydrates in sediments of natural gas hydrates in seabeds. The phase equilibrium data for the upper-quadruple (H–W–L–V) phase states of CO2CH4 mixed hydrates are needed for the application of the replacement of CH4 hydrate with CO2. In this paper, the upper-quadruple (Q2) phase equilibrium properties of CO2CH4 mixed hydrates were investigated by using a visual experimental apparatus in the temperature range of 273.16–297.15 K and pressure up to 10 MPa and verified by calculation with thermodynamic models. The coexisting of upper-quadruple phase region was identified in 4.46–8.4 MPa, 283.09–287.9 K and 0–0.225 methane mole fractions. It is found that the range of temperature and pressure for Q2 phase coexistence increases with the methane concentration until the retrograde of the quadruple phase is reached. The start and end melting points of the CO2CH4 mixed hydrates in the upper quadruple states were tested in constant volume condition. The tested results had also been compared with the predictions with a combination of the thermodynamic models for the mixed gas hydrate and CO2CH4 vapor–liquid equilibrium. The existence of the Q2 phase equilibrium region and its extension are clarified.