Objective Coal seam water injection is a key technical measure for dust removal, prevention and control of coal and gas outbursts, and rockburst disasters in mines. The morphological characteristics such as fracture roughness and aperture have a significant impact on the effectiveness of coal seam water injection.

Methods Fractured coal samples with different roughness and aperture were prepared by means of sandpaper and polyimide high temperature tape. The fracture roughness parameters were accurately calculated by laser spectral confocal microscope, and the seepage flow of fractured coal samples was tested.

Results and Conclusions  The results showed that the surface height deviation (H_a), the maximum surface height (H_z) and the fractal dimension (D_s) of 3D morphology can be used as parameters to characterize the roughness of coal profile. The seepage flow of coal decreases exponentially with the increase of H_a, H_z and D_s. As the roughness of the section increases from 4.69 to 18.43, the seepage flow in the fracture decreases by 84.42%. When the fracture aperture is 60~90 μm, the roughness has the highest blocking effect on the seepage. After that, the blocking effect of the rough element begins to weaken, and the fracture aperture plays a leading role in the seepage process. The seepage flow of coal increases exponentially with the increase of fracture aperture. When the fracture aperture is increased from 30 μm to 150 μm, the seepage flow of fracture can be increased by 355.88 times. This study conducted an in-depth exploration of the relationship between fracture aperture, roughness and seepage flow, clarifying their key mechanisms in the process of coal seam water injection. It emphasizes that in coal seam water injection practice, fracture morphology should be fully considered, and water injection methods should be reasonably selected based on different fracture opening and roughness conditions to effectively improve the effectiveness of coal seam water injection.