Study on the transportation and permeability characteristics of mixed-phase CO2 at different temperatures in deep coal seams

With respect to Carbon Capture and Storage (CCS) technology in deep coal seams, owing to changes in temperature and pressure, CO2 will exist in supercritical and gaseous states during transportation, resulting in the formation of mixed-phase CO2. To evaluate this, through theoretical analysis, a calculation model of mixed-phase CO2 permeability is established. Combined with large-scale mixed-phase CO2 experimental results, the phase transition and its transportation characteristics in deep coal seams are calculated and analysed. The pressure decreases gradually with increasing CO2 transportation distance and is nonlinearly distributed along the direction of CO2 transportation, and the phase state of CO2 also changes with increasing pressure. Compared with the traditional Peng−Robinson (P−R) equation, the “Span-Wagner + Vesovic model” is more scientific and accurate for calculating the physical properties of mixed-phase CO2 in the near-critical and supercritical zones. The permeability of mixed-phase CO2 decreases but then increases with increasing temperature. The minimum permeability of mixed-phase CO2 occurs at the phase transition point in the transition zone, and the internal permeability and full-length permeability of mixed-phase CO2 change in a “U” shape. The “temperature inflection point” is 328.15 K for the change in mixed-phase CO2 permeability with temperature, and the “distance weight inflection point” is 0.75 for the change in the supercritical state deposition distance. The mixed-phase permeability is similar to that of the gaseous state within the range of 0.62–0.75. The range of 0.75–0.81 is close to the supercritical state. This study has important theoretical significance and application value for evaluating the CO2 injection capacity and CCS geological storage potential in deep coal seams.