Study on the Seepage Model and Evaluation System of Coal Seam Water Injection Considering Fracture Aperture and Surface Roughness

As an environmentally friendly means of mine disaster prevention and control, the existing technology of coal seam water injection has not fully integrated the seepage theoretical model and mathematical evaluation method, which makes it difficult to accurately quantify and classify the difficulty of water injection. First, a seepage model of coal seam water injection was constructed by introducing the corrected actual length of the seepage capillary channel and taking the fracture aperture and surface roughness into consideration. The main controlling factors were screened by Sobol sensitivity analysis, and a water injection capacity evaluation system associated with all main controlling factors was constructed by pair analysis, with the key index defined as <i>P</i> to rank the water injection difficulties of coal seams. Finally, the reliability of the constructed evaluation system was assessed using the permeabilities of six coal samples measured by seepage experiment. The results show that the water injection seepage quantity increases with the increases of fracture aperture, porosity, and pore throat radius and decreases with the increase of surface roughness, dynamic viscosity coefficient of the fluid, and contact area. Porosity has the greatest impact on seepage quantity, with the sensitivity of 0.1006, and its influence is much greater than those of other seven factors, followed by fracture aperture with the sensitivity of 0.0905. The minimum pore throat radius and fluid dynamic viscosity coefficient are the least important influencing factors of seepage quantity, showing the sensitivity of 0.0005 and 0.0109, respectively. The water injection difficulties of coal seams can be ranked into four levels, nonwater injection (0 < <i>P</i> < 0.25), difficult water injection (0.25 ≤ <i>P</i> < 0.5) interval), relatively easy water injection (0.5 ≤ <i>P</i> < 0.75), and easy water injection (0.75 ≤ <i>P</i> < 1). The evaluation system orders the water injection difficulties of the six coal samples from easy to difficult as TC > SM > YC1 > NM > ZG > YC2. Through the test results of permeability of six coal samples, it is found that the order of permeability is basically consistent with the evaluation system. The seepage model and difficulty evaluation system of coal seam water injection constructed in this paper offer a novel strategy to identify the water injection capacities of coal seams and provide support for further improving the theoretical understanding of coal seam injectability.