W-shaped permeability evolution of coal with supercritical CO₂ phase transition

Gaseous CO₂ becomes a supercritical liquid when temperature and pressure transit the critical point (31.1 °C and 7.4 MPa) – a typical state for CO₂ storage for carbon sequestration and CO₂ injection in enhanced coalbed methane/shale recovery. Therefore, it is essential to define the evolution of coal permeability inclusive of this phase transition. This study presents experimental measurements of coal permeability across the CO₂/SCCO₂ transition over typical ranges of confining stress (3–15 MPa) and pore pressures (1–13 MPa) at a constant temperature of 40 °C. The results show that contrary to the typical U-shaped permeability -vs- pressure evolution in the subcritical region, a second permeability minimum occurs in the vicinity of the critical pressure – even after the impacts of viscosity transition are accommodated. Permeability to SCCO₂ is almost two-orders-of-magnitude smaller than its initial permeability to subcritical-CO₂. The phase transition from subcritical to supercritical state controls the W-shaped coal permeability profile. Permeability in the same coal is indexed relative to non-sorbing Helium (He) and slightly-adsorbing Nitrogen (N₂) to probe the origins of the W-shaped permeability profile around the SCCO₂ phase transition. Observed irreversible changes in mechanical properties driven by the SCCO₂ phase transition are linked to the observed permeability reduction. First, observed reductions in coal strength and stiffness after SCCO₂ saturation indicate a significant weakening effect - SCCO₂ acts as a strong plasticizer enabling rearrangement of the physical structure and adding molecular mobility. The softened coal responds to increased compressional deformation and cleat closure under the same effective stress, resulting in a net permeability reduction in the supercritical region countering any influences of permeability-enhancing microcrack formation. Second, the SCCO₂ permeability is further reduced by a large increase in sorption-induced swelling observed after the transition. Measured swelling strains indicate a > 90% increase in SCCO₂ sorption-induced swelling relative to Langmuir response at 13 MPa. The enlarged SCCO₂ adsorption capacity presumably results from the slightly polar nature of the SCCO₂ dimer and the increased SCCO₂ density. Permeability reduction due to plasticization and increased swelling is shown to dominate over permeability increases driven by brittle damage.