Modeling of anisotropic coal permeability under the effects of matrix-fracture interaction

Abstract

Research relating to changes in coal's permeability has a positive significant effect for coal mining safety. The construction of an accurate permeability model through appropriate methods would help to understand the flow characteristics of coalbed methane (CBM). However, it is worth noting that it is necessary to include the influence of anisotropy and heterogeneity when analyzing changes in coal's permeability, otherwise the results would distort the actual conditions. In this work, we considered, fully, the interaction of coal matrix-fractures, and established a coal permeability model. In addition, the effects of stress compaction, sorption-induced deformation, including the slippage effect have been incorporated into the permeability model. With respect to this proposition, the anisotropic strain law for coal has been analyzed based on the generalized Hooke constitutive model. And the anisotropic sorption-induced deformation mechanism in coal has been analyzed through an improved sorption-induced deformation equation. Following experimental results, an anisotropic coal permeability model was constructed. Through the constructed permeability model, we described further the change characteristics relating to anisotropic permeability in coal. In addition, the permeability model was verified through an anisotropic seepage laboratory test. The influence of the elastic modulus reduction coefficient, and the ratio of the elastic modulus to adsorption expansion modulus on coal's permeability have also been analyzed. Further, the elastic modulus reduction coefficient that determines the fracture width and porosity, thereby affecting coal's permeability, has also been considered. Moreover, the ratio of the elastic modulus during adsorption expansion that determines sorption-induced deformation, and indirectly affects permeability, has also been calculated. This work aims to further understand changes in gas' permeability under the combination of multiple fields during coal mining through the theoretical results from the proposed anisotropic permeability model.