A transient dual porosity/permeability model for coal multiphysics

Conventional dual porosity/permeability models for coal cannot capture the true transient nature of matrix-fracture mechanical interactions because these interactions are normally characterized through two equilibrium systems within the same REV (representative elementary volume). In this study, the transient process in the matrix system is included through the embedment of a local REV structure into the overall multiphysics formulation. This inclusion transforms conventional dual porosity/permeability equilibrium models into non-equilibrium ones. Consequently, coal permeability evolves from initial to final equilibrium within the REV. Equilibrium models represent two end points (initial and final equilibrium) while our new model represents the evolution of coal permeability between these two end points. The model is verified against experimental observations of coal permeability under common experimental conditions of constant confining pressure and constant effective stress. Our results show that conventional equilibrium models underestimate the role of coal matrix or matrix-fracture mechanical interactions, that current experimental observations represent only a small portion of the complete evolution process, and that as a tool of knowledge extension our model extends the experimental observations to a representation of the coal permeability whole evolution process from initial to ultimate equilibrium. Article Highlights Through the embedment of a local REV structure into the overall multiphysics formulation, the transient/non-equilibrium process in the matrix system is considered. Conventional dual porosity/permeability models for coal are modified to capture the true transient nature of the matrix-fracture interaction. The whole evolution process of coal permeability from initial to ultimate equilibrium is represented.