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

Abstract

Abstract Simultaneous CO 2 sequestration and CH 4 production has been considered as a possible way to form and restore CO 2 hydrates in sediments of natural gas hydrates in seabeds. The phase equilibrium data for the upper-quadruple (H–W–L–V) phase states of CO 2 CH 4 mixed hydrates are needed for the application of the replacement of CH 4 hydrate with CO 2 . In this paper, the upper-quadruple (Q 2 ) phase equilibrium properties of CO 2 CH 4 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 Q 2 phase coexistence increases with the methane concentration until the retrograde of the quadruple phase is reached. The start and end melting points of the CO 2 CH 4 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 CO 2 CH 4 vapor–liquid equilibrium. The existence of the Q 2 phase equilibrium region and its extension are clarified.