Comparison of permeability in cataclastic coals along the face and butt cleat directions: experiment and fractal analysis

Abstract Deep coalbed methane reservoirs often develop extensive cleat systems, resulting in significant structural complexity and permeability anisotropy. However, understanding of the permeability characteristics and directional dependence along face and butt cleat directions is still limited. To address this gap, a comparative investigation was conducted to examine the directional permeability behaviour of cataclastic coals under different effective stress conditions. Permeability experiments were performed on three sets of cataclastic coal samples using the transient pressure pulse method. A face–butt cleat fractal permeability model was developed and validated against the experimental data. The study primarily focuses on assessing the influence of cleat orientation on the permeability evolution of cataclastic coals. Results indicate that the coal permeability decreases significantly with increasing effective stress. A heavy hysteresis behaviour is also observed during unloading. The permeability of cataclastic coal exhibits strong directional dependence, with lower permeability along the butt cleat direction than along the face cleat direction. The directional permeability difference exhibits an exponential decline as effective stress increase. Face cleats are characterized by larger maximum apertures but lower fractal dimensions and tortuosity compared with butt cleats. With stress increasing, the maximum aperture decreases significantly, while both fractal dimension and tortuosity increase. By integrating experimental measurements and theoretical modelling, the study investigates how cleat systems affect permeability and anisotropy in cataclastic coal.