Low permeability coalbed reservoirs are often fractured to aid productivity. Typically a proppant-slurry is injected during thefracturing process to induced fractures and place proppants to retain the enhanced permeability. However, the dynamics of effectivestresses in the reservoir may result in proppant embedment. This occurs wither when the coal surfaceis softer than the proppant or inthe case of enhanced coalbed methane recoveryCO_2 induced coal softening or coal swelling may cause coal penetration into thepropped fracture. The resulting permeabilityis expected to be dynamic with effective stress and the pore gas pressure. We investigatepermeability transformations through a suite of laboratory experiments conducted on selected granite (non-sorbing hard rock), andbituminous coal cores sawn into two halves with a thin diamond coated blade. The permeability of the artificial “fracture/cleat” wasmeasured for a non-sorbing gas (He) and a sorbing gas (CO_2) at constant confining stress of 10MPa. Permeability was also determinedwith a uniform monolayer of #70-140 mesh proppant sand within the fracture. The presence of proppant increases the gas permeabilityin granite and coal by more than an order of magnitude, depending on the gas injected for the conditions evaluated. When a sorbinggas (CO_2) was injected, there was a reduction in permeability for the non-propped and propped coal experiments. However the gaspermeability of the propped fracture was always higher than that for the non-propped fracture. Surprisingly the relative reduction inpermeability, due to sorbing gas exposure, for the propped fracture was higher than that for the non-propped fracture. The heliumpermeability of non-propped and propped artificial fracture in granite decreases with effective stress. This decrease may be 2-3 timesif the effective stress increases from 4 MPa to 6 MPa. The permeability of propped fracture was 10-15 times higher than that of nonproppedfracture in granite. Similar observations are made for artificial fracture in bituminous coal. The increase in He permeabilitymay be as high as ～10 folds if monolayer proppant is sandwiched in the coal fracture. Similar increase is observed in the case ofsorptive gas (CO_2) permeability. These observations are consistent with a combination of coal swelling and/or an enhanced coalsoftening in the presence of this sorbing gas allowing proppant penetration into the coal or coal swelling encroaching into the fracture.
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