Mining-induced fracture network reconstruction and anisotropic mining-enhanced permeability evaluation using fractal theory

In the concurrent extraction of coal and gas, the quantitative assessment of evolving characteristics in mining-induced fracture networks and mining-enhanced permeability within coal seams serves as the cornerstone for effective gas extraction. However, representing mining-induced fracture networks from a three-dimensional (3D) sight and developing a comprehensive model to evaluate the anisotropic mining-enhanced permeability characteristics still pose significant challenges. In this investigation, a field experiment was undertaken to systematically monitor the evolution of borehole fractures in the coal mass ahead of the mining face at the Pingdingshan Coal Mining Group in China. Using the testing data of borehole fracture, the mining-induced fracture network at varying distances from the mining face was reconstructed through a statistical reconstruction method. Additionally, utilizing fractal theory, a model for the permeability enhancement rate (PER) induced by mining was established. This model was employed to quantitatively depict the anisotropic evolution patterns of PER as the mining face advanced. The research conclusions are as follows: (1) The progression of the mining-induced fracture network can be classified into the stage of rapid growth, the stage of stable growth, and the stage of weak impact; (2) The PER of mining-induced fracture network exhibited a typical progression that can be characterized with slow growth, rapid growth and significant decline; (3) The anisotropic mining-enhanced permeability of the reconstructed mining-induced fracture networks were significant. The peak PER in the vertical direction of the coal seam is 6.86 times and 4446.38 times greater than the direction perpendicular to the vertical thickness and the direction parallel to the advancement of the mining face, respectively. This investigatione provides a viable approach and methodology for quantitatively assessing the anisotropic PER of fracture networks induced during mining, in the concurrent exploitation of coal and gas.