Anisotropic Permeability Evolution of Pre-Existing Natural Fractures in Cubic Coal Under True-Triaxial Stresses: An Experimental and Modelling Study

Prediction of coal permeability is very important as it is directly related with coal bed methane (CBM) production, underground coal mining, and CO₂ sequestration in deep coal seams. However, the anisotropy of both internal fractures of coal and outer stress environment brings difficulty in replicating the gas transport characteristics of coal at the laboratory scale. There is limited research in this area. Thus, in this paper, we performed a series of permeability measurements on cubic high-rank coal samples using the true-triaxial apparatus. The principal stresses and flow directions were systematically varied during the test. The results show that differential stress of 20 MPa compressed to coal resulted in an order of magnitude drop in permeability. For the pre-existing natural fractures in coal, the butt cleat plane was more sensitive to variations in the principal stress. By using an exponential equation containing different mean cleat compressibility and stress terms, the anisotropic coal permeability data obtained under true-triaxial stress conditions can be accurately predicted. The experimental and theoretical results of this study can help better understanding gas transport in coal seams and optimizing the layout of the gas drainage boreholes and well design during CBM production or CO₂ sequestration.