Deconvolving CO₂-enhanced coalbed methane processes in subbituminous coals

The evolution of permeability in CO₂-enhanced coalbed methane (ECBM) recovery involves dynamic changes in coal shrinkage/swelling with the reduction/increase in gas sorption/desorption. Injection of CO₂ changes local pore pressures and induces related matrix volume strains, modulated in part by the mechanical boundary conditions; changes in gas saturation and pressure induce changes in permeability. Typically the recovery of methane induces shrinkage and the injection of CO₂ induces swelling - concomitantly permeability decreases where net swelling results and increases where net shrinkage is present. However, permeabilities are also impacted by other important physical phenomena, including water saturations and the sequence of sweeps by different gases used in ECBM. These may be N ₂ or CO₂. To address these issues we report experimental measurements of permeability evolution in subbituminous coal cores from the San Juan basin infiltrated by He, CH₄ and CO₂ under varying pore pressure at constant applied stresses. Experiments are completed with variable water saturations, gas saturations and effective stresses as key parameters modulating permeability evolution. For a subbituminous coal the presence of moisture reduces the sorption capacity, swells the coal volume and offers resistance in flow of other fluids (capillary forces) hence lowering permeability magnitudes. Reduction in permeability with sorption of CH ₄ and CO₂ is lower in wet samples than dry. However, absolute values of permeability for dry samples are higher than the moist (9% moisture content). Swelling induced by sorption of CH₄ and CO ₂ in the coal matrix likely causes aperture reduction including cleat closure. Experimental observations indicate that the magnitude of swelling increases with pore pressure. Preliminary test results demonstrate that the permeability of subbituminous coal (San Juan basin) does not show a significant change with pore pressure for a non-sorbing gas (He), implying the relative stiffness of these coals.