Evolution of Coal Permeability under Constant Effective Stresses: Direct Measurements and Numerical Modeling

Coal permeability is normally measured under the assumed condition of local equilibrium, i.e., the fracture pressure is equalized with the matrix pressure. When the local equilibrium condition is not met, the coal is in a transient state. In this study, we define coal permeability at a transient state as nonequilibrium permeability. A gas expansion-based experimental technique is developed to directly measure both the overall strain and the...