Study on Multiscale Structural Evolution Mechanism of Coal with Horizontal Fractures under Stress and Gas Relief

This work aims at solving the problem of prevention and control of coal and gas outburst disasters that are dominated by stress in a deep coal seam. It is found that hydraulic slotting and gas extraction lead to double pressure relief for the coal seam stress and gas. A double pressure relief coefficient is proposed. Mechanical seepage experiments of horizontal fracture coal samples with different double pressure relief coefficients are designed (K = 0.5, 1.0, 1.5). This study determined the evolution mechanism and mathematical model of the macro-fine-microstructure of coal seam fracture under the action of stress and gas relief. The results show that the fracture of horizontal slot coal has a tendency to weaken gradually with the increase of the double pressure relief coefficient K. The macrofracture plays a more dominant role than the microfracture. When K = 1.0 (Δσ = ΔP), the macrofracture is the most developed. When K = 0.5 (Δσ = 0.5ΔP), the mesofracture is the most developed. Before and after the experiment, the proportion of micropores and small pores is the largest, followed by the proportion of large pores, and the proportion of middle holes is the smallest. After the experiment, the proportion of medium and large pores increases, while the proportion of micro- and small pores decreases. The relationship between the fracture parameters and the double pressure relief coefficient K is quadratic or cubic. The relationship between pore size and cumulative volume ratio is a logarithmic function. This study provides a scientific basis for double pressure relief and permeability improvement of stress and gas in coal seams, efficient extraction, and effective outburst prevention.