Evolution of coal permeability under unconstrained swelling: Contribution of matrix-fracture interaction

Coal permeability model is needed to define the transient characteristics of coal permeability evolution due to adsorbing gas injection. A broad variety of models have evolved to represent the effects of sorption, swelling/shrinkage and effective stresses on the evolution of permeability. This work evaluated the performance of these models against analytical solutions for both unconstrained swelling and constrained swelling conditions. Constrained model predictions are apparently consistent with typical laboratory measurements. Nevertheless, this consistency is due to the mismatch between model boundary condition (constrained) and experiment boundary condition (unconstrained), demonstrating that current permeability models can hardly explain net reductions in coal permeability where swelling is unconstrained. To explore the possible reason, a full coupling approach was applied to explicitly model the interactions of coal matrix-fracture and translate these interactions into the permeability change. In this approach, the important non-linear responses of coal matrix to the effective stress are quantified through the incorporation of heterogeneous distributions of coal properties into complex mechanical coupling with gas transport. When swelling coefficient and modulus vary spatially relative to the fracture void, a net reduction in coal permeability is achieved from the initial no-swelling state to the final equilibrium state. These modeling results are consistent with the observed responses under the unconstrained swelling conditions, which demonstrates the contribution of this work.