The dynamic permeability of propped and non-propped artificial fracture in granite and bituminous coal with changes in efective stress

Low permeability coalbed reservoirs are often fractured to aid productivity. Typically a proppant-slurry is injected during the fracturing process to induced fractures and place proppants to retain the enhanced permeability. However, the dynamics of effective stresses in the reservoir may result in proppant embedment. This occurs wither when the coal surfaceis softer than the proppant or in the case of enhanced coalbed methane recoveryCO ₂ induced coal softening or coal swelling may cause coal penetration into the propped fracture. The resulting permeabilityis expected to be dynamic with effective stress and the pore gas pressure. We investigate permeability transformations through a suite of laboratory experiments conducted on selected granite (non-sorbing hard rock), and bituminous coal cores sawn into two halves with a thin diamond coated blade. The permeability of the artificial "fracture/cleat" was measured for a non-sorbing gas (He) and a sorbing gas (CO₂) at constant confining stress of 10MPa. Permeability was also determined with a uniform monolayer of #70-140 mesh proppant sand within the fracture. The presence of proppant increases the gas permeability in granite and coal by more than an order of magnitude, depending on the gas injected for the conditions evaluated. When a sorbing gas (CO₂) was injected, there was a reduction in permeability for the non-propped and propped coal experiments. However the gas permeability of the propped fracture was always higher than that for the non-propped fracture. Surprisingly the relative reduction in permeability, due to sorbing gas exposure, for the propped fracture was higher than that for the non-propped fracture. The helium permeability of non-propped and propped artificial fracture in granite decreases with effective stress. This decrease may be 2-3 times if the effective stress increases from 4 MPa to 6 MPa. The permeability of propped fracture was 10-15 times higher than that of non-propped fracture in granite. Similar observations are made for artificial fracture in bituminous coal. The increase in He permeability may be as high as ∼10 folds if monolayer proppant is sandwiched in the coal fracture. Similar increase is observed in the case of sorptive gas (CO ₂) permeability. These observations are consistent with a combination of coal swelling and/or an enhanced coal softening in the presence of this sorbing gas allowing proppant penetration into the coal or coal swelling encroaching into the fracture.