A Non-Equilibrium multiphysics model for coal seam gas extraction

Abstract

Current permeability models are normally derived on the assumption of local equilibrium between coal matrixes and fracture within the representative elementary volume (REV) during gas extraction/injection. Under this assumption, the gas pressure and its associated swelling strain will distribute uniformly throughout the entire REV irrespective of the equilibration process between coal matrixes and fracture. This uniform distribution has long been considered as the reason why current permeability models cannot explain permeability data as widely reported. Significant efforts have been made to resolve this issue for the last decade but all these efforts ignore the transient nature of local equilibration evolution from initial to ultimate equilibrium states. In this study, we developed a concept of local non-equilibrium index (LNEI) to define a complete permeability model under the influence of gas extraction/injection. The application of this concept transforms equilibrium permeability models to non-equilibrium ones. Equilibrium models represent only two end points (before gas extraction/injection and after the completion of gas extraction/injection) while our non-equilibrium one represents the complete evolution of coal permeability between two end points. Our non-equilibrium model is degenerated to replicate all equilibrium models as reported in the literature and used as a key cross-coupling relation to formulate the non-equilibrium multiphysics model. Our non-equilibrium multiphysics model is verified against two rare experimental data sets and applied to predict the effects of the local equilibration process on both the evolution of coal permeability and the gas production under field conditions.