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<p>Gas pre-extraction and pressure relief through boreholes are essential for safe mining in rockburst and gas outburst-prone coal mines. The double-protection boreholes, comprising pre-extraction boreholes and pressure-relief boreholes, compromises pre-extraction sealing. Clarifying coal permeability around boreholes in fracture fields is critical for detecting air leakage pathways. Triaxial seepage experiments on graded coal under various stress paths were conducted to analyze permeability evolution and its correlation with porosity. A stepwise permeability evolution model was developed to characterize coal permeability in different borehole zones. The result showed that: (1) permeability exhibited a “U-shape” trend during full-stage stress loading, with reduced sample compressibility and narrower effective seepage channels; (2) the generation of branch fractures during the stage of increasing axial pressure and decreasing confining pressure caused some seepage paths to deviate from the main seepage direction, manifested as a special decreasing trend in permeability with increasing porosity; and (3) according to the distribution of the fracture field, a porosity correction coefficient is proposed to characterize the variation in porosity in different fracture zones. The corrected permeability exhibited a stepwise evolutionary trend. Meanwhile, flow-enhanced regions formed within the fracture region. This elevated the risk of borehole leakage in fractured zones. Guided by the permeability zoning, the gas leakage sensitive degree of double-prevention boreholes was further investigated. This study provides a theoretical basis for identifying dominant air seepage pathways and accurately determining air leakage areas in double-protection boreholes.</p>

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Stepwise Evolution of Coal Permeability Under Full-Stage Stress with Double-Prevention Boreholes Structure

  • 期刊论文

  • 发表日期：

    2025年12月12日

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  • Tianjun Zhang,
  • Jiawei Tian,
  • Lei Zhang,
  • Hang Zhang,
  • Yan Cao,
  • Yuhang Chu,
  • Hongyu Pan

摘要

Gas pre-extraction and pressure relief through boreholes are essential for safe mining in rockburst and gas outburst-prone coal mines. The double-protection boreholes, comprising pre-extraction boreholes and pressure-relief boreholes, compromises pre-extraction sealing. Clarifying coal permeability around boreholes in fracture fields is critical for detecting air leakage pathways. Triaxial seepage experiments on graded coal under various stress paths were conducted to analyze permeability evolution and its correlation with porosity. A stepwise permeability evolution model was developed to characterize coal permeability in different borehole zones. The result showed that: (1) permeability exhibited a “U-shape” trend during full-stage stress loading, with reduced sample compressibility and narrower effective seepage channels; (2) the generation of branch fractures during the stage of increasing axial pressure and decreasing confining pressure caused some seepage paths to deviate from the main seepage direction, manifested as a special decreasing trend in permeability with increasing porosity; and (3) according to the distribution of the fracture field, a porosity correction coefficient is proposed to characterize the variation in porosity in different fracture zones. The corrected permeability exhibited a stepwise evolutionary trend. Meanwhile, flow-enhanced regions formed within the fracture region. This elevated the risk of borehole leakage in fractured zones. Guided by the permeability zoning, the gas leakage sensitive degree of double-prevention boreholes was further investigated. This study provides a theoretical basis for identifying dominant air seepage pathways and accurately determining air leakage areas in double-protection boreholes.