Zonal Evolution and Fractal Characterization of Coal Fracture Networks Around Gas Drainage Boreholes

To elucidate the structural evolution of fracture networks around gas drainage boreholes and their controlling mechanisms on grouting sealing performance, a coal seam in the Zhongmacun Mine (No. 2-1 coal seam) was selected as the engineering background. An integrated approach combining borehole camera observation, gray-level co-occurrence matrix (GLCM) texture analysis, and fractal theory was adopted to systematically characterize the development behavior, spatial heterogeneity, and fractal evolution of fracture networks under different stress zones surrounding the borehole. Furthermore, the quantitative relationship between fracture structure characteristics and grouting parameters was explored. Results indicate that three axial stress-related zones are formed, including a stress-relief zone (0–4 m), a post-peak stress concentration zone (4–20 m), and a pre-peak stress concentration zone (>20 m), with fracture development strongly dependent on stress state. Quantitative analysis based on GLCM parameters and their coefficients of variation reveals a progressive transition from highly complex and strongly heterogeneous fracture structures in the stress-relief zone to simpler and weakly heterogeneous characteristics in the pre-peak stress concentration zone. The fractal dimension (D) decreases from 1.847–1.907 to 1.676–1.713 across these zones, consistent with the evolution trends of GLCM metrics. Based on the relationship between fractal dimension and fracture connectivity, a prediction model for the equivalent permeability of fracture networks based on fractal dimension was established, and the quantitative relationship between grouting pressure and fractal dimension, slurry viscosity, and diffusion radius was derived, providing a theoretical method for analyzing the correlation between fracture structure characteristics and grouting parameters. On this basis, a zonal differentiated grouting sealing optimization scheme was proposed, and a theoretical calculation method for grouting pressure and sealing section length based on fractal parameters was established, providing theoretical references and technical support for precise sealing of gas drainage boreholes.