Gas seepage in underground coal seams: Application of the equivalent scale of coal matrix-fracture structures in coal permeability measurements

•The concept of equivalent scales of dual-porosity coal structures was put forward.•A permeability model in the complete stress-strain process of coal was established.•A new method for measuring the permeability in coal seams was put forward. Coal is a complex dual-porosity porous medium that consists of both fractures and coal matrices containing pores. The gas seepage characteristics in coals are closely related to the coal matrix-fracture structures. However, the elements that compose the dual-porosity structure of coal (i.e., the coal matrix and the fracture) are often simplified to geometries with regular shapes in theoretical research investigations due to the complexity of coal structures. Therefore, the geometrical size of the simplified geometries can be regarded as the equivalent scale of the coal matrices and the fractures. The equivalent scale characteristics can optimize the properties of the complex fracture system in coal bodies. Based on this, a simplified permeability evolution model in the complete stress-strain process of coals was proposed. The new model takes into account the impacts of the equivalent scale characteristics of the coal matrix-fracture structures, the effective stress and the matrix sorption induced strain on the evolution laws of coal permeability. Then, a simple and convenient method for measuring the permeability in coal seams based on the new permeability evolution model was put forward, and the reliability of the new method was verified by comparing its results with field test data. This research enriches the theory of gas seepage in coal seams and lays a foundation for studying the evolution mechanism of coal permeability during plastic deformation occurs in coal bodies.