Mining-induced fracture evolution and fractal seepage model of double key strata in deep coal mining

Quantitatively describing the development of mining-induced fractures and the evolution of permeability in the double key strata is crucial for the efficient gas extraction from the gob area and fracture zone. Taking the Dingji Coal Mine in the Huainan mining district as the engineering background, numerical simulations were conducted to study the dynamic evolution of overburden fractures during mining under the double key strata. The fracture density and fracture ratio are quantitatively analyzed using a grid-based image division method. Fractal theory is introduced to describe the propagation and closure processes of mining-induced fractures beneath the deep double key strata. Based on the fracture characteristic parameters, a fractal seepage model for overburden fractures is derived, and an optimized criterion for target layer selection in high-extraction tunnels is proposed. The study results show during the deep coal seam mining under double key strata, the mining-induced fractures develop upward and forward as the working face advances. In the horizontal direction, fractures in the center of the gob area are small and dense, while fractures in the working face and drifts are large and sparse. In the vertical direction, fractures below Key Stratum 2 maintain their integrity for a long period, and the fracture area and fractal dimension are greater than those in other strata, indicating that the key strata play a protective role in fracture distribution. According to the fractal seepage model for mining-induced fractures, the overburden permeability is in the range of 10⁻¹⁴~10⁻¹¹ m². The permeability distribution follows an "unimodal"-"terraced"-"saddle-shaped" evolution pattern as the working face progresses. After mining, a narrow low-permeability zone forms above Key Stratum 1, and the permeability below Key Stratum 2 reaches its maximum value, exceeding 1×10⁻¹¹ m², demonstrating that the double key strata indirectly control the distribution of overburden permeability. The main factors influencing the gas extraction efficiency in high-extraction tunnels include the gas source, gas channels, and tunnel stability. Considering these factors, a concept of an effective gas extraction coefficient for high-extraction tunnels is quantitatively defined from three aspects: the degree of gas accumulation, the duration of high-permeability zones, and tunnel stability. An optimized criterion for selecting target layers for high-extraction tunnels is proposed, and it is applied in a trial working face, achieving good gas extraction results. The research findings provide valuable reference for the study of mining-induced fracture seepage evolution in deep coal seams under double key strata and guide the layout planning of high-extraction tunnel layers in mines.