A permeability model for anisotropic coal masses under different stress conditions

Abstract

The heterogeneous and anisotropic nature of coal can have an important impact on its permeability. Coal permeability is usually directly related to the stress and hence, it is of great significance to understand the evolution of anisotropic coal permeability under different stress conditions. In this work, based on the stress-strain relationship of coal and the interaction between coal fractures and matrix, an anisotropic permeability model was constructed considering different stress conditions. The proposed model is further improved by considering the combined influence of the effective stress, pore pressure and slippage effect. The proposed permeability model is verified using available experimental data from previously published studies, which has shown close agreement with the experimental results in predicting coal permeability under different stress conditions. Cleat compressibility exhibited anisotropy and was dependent on the orientation of gas flow. The variation rule between coal permeability and effective stress was determined to be in accordance with a negative exponential function. The model clearly illustrates strong anisotropy in both magnitude and variation of directional permeability.