Interactions of multiple processes during CBM extraction: A critical review

Coal permeability models are required to define the transient characteristics of permeability evolution in fractured coals during CBM recovery. A broad variety of models have evolved to represent the effects of sorption, swelling and effective stresses on the dynamic evolution of permeability. In this review, we classify the major models into two groups: permeability models under conditions of uniaxial strain and permeability models under conditions of variable stress. The performance of these models is evaluated against analytical solutions for the two extreme cases of either free shrinking/swelling or constant volume. For the case of free shrinking/swelling none of the swelling/shrinking strain contributes to the change in coal permeability because effective stresses do not change. Conversely, for the case of constant volume the full swelling/shrinking strain contributes to the change in coal permeability because the coal is completely constrained from all directions. Therefore, these two solutions represent the lower bound and the upper bound behaviors of permeability evolution, respectively.Review of laboratory observations concludes that although experiments are conducted under conditions of free shrinking/swelling the observed response is closest to that for constant volume condition. Similarly, review of in-situ observations concludes that coal gas reservoirs behave close to the constant volume condition although these observations are made under undefined in-situ stress and constraint conditions anticipated to be intermediate between free swelling and constant volume (i.e. for uniaxial strain). Thus comparison of these laboratory and field observations against the spectrum of models indicates that current models have so far failed to explain the results from stress-controlled shrinking/swelling laboratory tests and have only achieved some limited success in explaining and matching in situ data. Permeability models under uniaxial strain are more appropriate for the overall behavior of coal gas reservoirs under typical in situ conditions while models representing variable stress conditions are more appropriate for behavior examined under typical laboratory conditions. Unlike permeability models under the uniaxial strain condition, models under the constant volume condition are effective-stress based and can be used to recover the important non-linear responses due to the effective stress effects when mechanical influences are rigorously coupled with the gas transport system. Almost all the permeability models are derived for the coal as a porous medium, but used to explain the compound behaviors of coal matrix and fracture. We suggest that the impact of coal matrix-fracture compartment interactions has not yet been understood well and further improvements are necessary.