Objective Water injection into coal seams is recognized as a critical technical measure for dust removal and the prevention and control of coal and gas outbursts and rock bursts in coal mines. The effectiveness of this measure is significantly influenced by the morphological characteristics of fractures, such as their roughness and aperture.

Methods This study prepared coal samples containing fractures with different roughness and apertures using sandpaper and polyimide tape, calculated the fracture roughness parameters accurately using a confocal laser scanning microscope (CLSM), and tested the seepage rate of fractured coal samples.

Results and Conclusions The results indicate that the roughness of a coal profile can be characterized using the surface height deviation (H_a), the maximum surface height (H_z), and the fractal dimension (D_s) of the 3D morphology. The seepage rate of coals decreased exponentially with an increase in H_a, H_z, and D_s. As the profile roughness increased from 4.69 to 18.43, the seepage rate of fractures decreased by up to 84.42%. In the case where fracture apertures ranged from 60 to 90 μm, the fracture roughness exhibited the most significant blocking effect on seepage. With a further increase in the fracture aperture, the blocking effect of roughness elements began to weaken, with the fracture aperture dominating the seepage process. The seepage rate of coals increased exponentially with an increase in the fracture aperture. As the fracture aperture increased from 30 μm to 150 μm, the seepage rate of fractures increased 355.88 times at most. Through a deep exploration into the relationships of the fracture aperture and roughness with seepage rate, this study determines the key mechanisms underlying the impacts of the fracture aperture and roughness on water injection into coal seams. The results suggest that it is necessary to fully consider the morphological characteristics of fractures in water injection practices. Proper water injection methods should be selected based on different fracture apertures and roughness to enhance the water injection efficiency effectively.