Evolution of coal permeability to gaseous CO₂ and supercritical CO₂, N₂ and He under in-situ stresses

Gaseous CO2 becomes a supercritical liquid when temperature and pressure transit the critical point (31.10 ℃ and 7.39 MPa). During CO2 storage for carbon sequestration and CO2 injection in enhanced coalbed methane/shale (ECMB) recovery, CO2 can readily reach its critical point and thereby become supercritical. Therefore, it is essential to define the evolution of coal permeability inclusive of this phase transition. This study presents experimental measurements of coal permeability in response to CO2/ SCCO2, Helium (He) and Nitrogen (N2) over the typical pore pressures range of 1-13 MPa under a 15 MPa confinement, at a constant temperature of 40 ℃. The results show a W-shaped curve of permeability versus pressure instead of the typical U-shaped curve due to the CO2 phase change. When gaseous CO2 enters the supercritical region, coal permeability is significantly reduced compared with that in subcritical region. This reduced permeability to SCCO2 permeability is up to 70 times smaller than its initial permeability to subcritical CO2, due to the enlarged sorption-induced swelling. Changes in coal mechanical properties and CO2 fluid properties induced by the phase transition are the main causative factors of the permeability reduction after the critical point. Three distinctly different permeability evolution types are summarized based on the experimental permeability observations to different fluids.