Supercritical CO₂ phase transition and permeability variation characteristic under coupling effect of temperature and confining stress

In the process of CO₂ sequestration in deep coal seam, the pressure of injected supercritical CO₂ gradually decreases with the increase of migration distance, and it turns into gaseous state. This kind of CO₂ with phase change in coal is called mixed-phase CO₂. The pressure distribution characteristics and permeability of mixed-phase CO₂ in coal are important factors affecting the sealing effect. The traditional gas permeability is based on the assumption of linear distribution of gas pressure. However, the physical properties of CO₂ in complex phase state are complicated, and the assumption of linear distribution of pore pressure may no longer be applicable. In this paper, the pressure evolution characteristics of mixed-phase CO₂ were studied on ø100 mm×200 mm anthracite raw coal sample at different temperatures (40, 55, 70, 85 ℃) and confining pressures (25, 35, 45 MPa). The results showed as follows: ① the mixed-phase CO₂ pressure in coal showed a nonlinear decreasing trend of first slow and then sharp. The actual average pore pressure is about 0.6-0.7 times of the injection pressure. ② The difference between the permeability of mixed-phase CO₂ calculated based on the modified average pore pressure and that calculated by the traditional Darcy’s law gradually decreases to below 20% with the increase of confining pressure and temperature. ③ Under the comprehensive influence of coal thermal expansion, CO₂ desorption and viscosity, the mixed-phase CO₂ permeability decreases first and then increases with temperature. ④ When supercritical CO₂ is injected, with the decrease of pore pressure, the internal permeability changes in a U shape, and the minimum value appears in the CO₂ phase transition zone. The permeability of gaseous CO₂ decreases with the increase of pore pressure, while the permeability of supercritical CO₂ increases with the increase of pore pressure, which is the result of adsorption expansion and effective stress.