Permeability Evolution of Bituminous Coal Fractures during the Shear Slip Process

The method of enhancing reservoir permeability through volumetric fracturing to create a complex seam network presents a novel approach for extracting coalbed methane. Coal rock joint fractures exhibit low strength, and comprehending the permeability evolution during fracture shear slip is crucial for evaluating the viability of coal bed fracturing and forecasting coal bed methane production capacity. Hence, this study focuses on bituminous coals with varying ash contents in the Ordos Basin to investigate the permeability evolution during fracture shear slip through shear seepage experiments. By analysis of the mineral components of bituminous coals and the three-dimensional morphology changes of fracture surfaces before and after slip, the permeability evolution mechanism of bituminous coal fractures is elucidated. The results show that the permeability of bituminous coal fractures decreases during shear slip, with reductions ranging from 12.89 to 93.65%. On average, the permeability decrease for the three types of bituminous coals is 88.45, 22.63, and 52.24%, respectively. Furthermore, the decrease in permeability of bituminous coal increases with the rise in the laminated silicate content during shear slip. Bituminous coal A, characterized by weaker mechanical properties, experienced an average permeability change of −5.93% during sudden slip rate alterations, significantly higher than that of bituminous coal B (0.59%) and bituminous coal C (−2.59%). Bituminous fractures with weaker mechanical properties and higher laminated silicate content exhibited greater damage under shear stress, resulting in an increased permeability drop.