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 degrees 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 degrees 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 sub critical 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 (N-2) 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.