Permeability evolution of propped artificial fractures in coal on injection of CO₂

Proppants are often utilized during hydraulic fracturing to aid the retention of the fracture aperture. However, for coal the permeability enhancement may be mitigated due to proppant embedment within the natural/artificial fractures of coalbed methane reservoirs. This process may become increasingly complex if CO₂ is injected in the reservoir for enhanced recovery. The reduction in effective fracture aperture occurs under the influence of overburden stress either when CO₂-induced coal softening causes proppant penetration into the coal fracture surface or coal swelling encroaches into the propped facture. Here permeability transformations at simulated in situ conditions were evaluated through a suite of laboratory experiments conducted on split-cores of high-rank coals. A single smooth-surface saw-cut fracture was created and the permeability evolution measured for both non-sorbing (He) and sorbing (CO₂) gases at constant applied confining stress of 10MPa. Permeability was also measured for the idealized case of a uniform monolayer of #70-140 mesh quartz sand proppant sand introduced within the saw-cut fracture for coal. The increase in He permeability was as high as ~10 fold over the unpropped fracture for a monolayer of proppant sandwiched within the coal. A similar increase in permeability with the addition of proppant was observed in the case of sorptive gas (CO₂) for coal. For He there was an exponential increase in permeability with increasing gas pressure (p=1-6MPa) for coal without proppant, as expected, as the effective stress on the core was reduced. However, with CO₂ the permeability decreased in the 1-4MPa pressure range due to either coal swelling or softening or their combination but increased above 4MPa due to reduced effective stress. Optical profilometry pre- and post-exposure was used to quantify any surface deformation due to proppant embedment. Comparison of the fracture surface before and after showed only infrequent new isolated pits, similar to the size of the proppant grains. The slight increase in surface roughness following exposure to CO₂ was presumed due to irreversible rearrangement of the coal structure due to CO₂ uptake then loss. A mechanistic model explains the evolution of permeability in a propped artificial fracture due to interaction with a sorbing gas (CO₂). Permeability evolves with a characteristic "U-shaped" trend with increasing gas pressure at constant confining stress - permeability reduces to a minimum at approximately double the Langmuir pressure flanked by elevated permeabilities at either low sorptive states (low p) or at low effective stress (high p). An excellent fit is recovered between model and experimental observations.