Abstract: Enhancing coal seam water injection capacity is an urgent problem to be solved, and the difficulty of coal seam water injection lacks accurate criteria. By considering the characteristics of the seepage fractal structure of the pores and cracks of the coal and the chemical wetting characteristics under capillary action, a model of the seepage flow of coal seam water injection is established. Based on the methods of probability theory and mathematical statistics, a set-pair connection degree interval evaluation system is constructed, and the difficulty of water injection into the coal seam is divided. The results show that: 1) The effect of water injection is affected by the simplest parameters such as porosity, pore radius, contact angle, liquid density, surface tension, viscosity coefficient, gravitational acceleration and other seepage structural parameters as well as chemical wetting characteristics. The system model characterizes the relationship between the seepage structure and chemical wetting characteristics of coal and the seepage flow. 2) Under combined weighting, the porosity weight is 0.32, which has a greater contribution to the water injection effect, followed by the maximum pore throat radius and contact angle, and the minimum pore throat radius weight is 0.17, which has the least impact on the water injection effect. 3) The evaluation index values of coal samples X1, X2 and X5 are between (0, 0.25), which indicate that the coal seam belongs to the coal seam with difficult water injection; the evaluation index values of coal samples X3 and X4 are between (0.25, 0.50), which imply the coal seam belongs to the one relatively difficult to inject water.

Key words: water injection, characteristics of physicochemical, degree of difficulty, model of evaluation