覆岩结构对卸压瓦斯抽采有重要影响，为探究特厚煤层开采过程中上覆岩层瓦斯运移规律，采用物理相似性模拟实验方法分析覆岩裂隙形态和渗流能力演化规律，研究特厚煤层覆岩裂隙−渗流特征区域演化机理，提出特厚煤层覆岩裂隙−渗流特征区域识别方法，依据该识别方法在实验工作面进行定向钻孔抽采实践。

结果表明：(1) 覆岩中间区域离层量和裂隙密度分别为两侧区域的47%和31%，呈现中间低两边高的分布模式。开采初期，覆岩的裂隙−渗流特征在空间分布具有同步性，随着工作面进一步推进，覆岩受到不同程度的采动影响，覆岩逐渐开始分区演化，中间区域裂隙−渗流特征明显小于四周，两侧区域则随着层位的增加而递减，裂隙率、裂隙熵、连通性系数和渗透率范围分别为1.0%~8.5%、0.15~0.90、0.10~0.65和6.34×10⁻⁹~7.78×10⁻⁷ m²。(2) 基于覆岩裂隙和渗流能力对瓦斯运移的影响，将覆岩分为4个区域，分别是低位湍流区、中位压实区、中位过渡区和高位恒流区，基于裂隙率、裂隙熵、连通性系数和渗透率在不同区域的时空演化规律，建立特厚煤层覆岩裂隙−渗流特征区域演化模型和判定流程。(3) 根据覆岩裂隙−渗流特征区域演化模型，分析中位过渡区和高位恒流区为抽采优势层位，设计钻孔垂距为6~25 m，将钻孔抽采纯量随时间的变化过程分为初始波动阶段、快速增长阶段和缓慢下降阶段，回采期间工作面甲烷体积分数均小于1%，工程实践效果良好，验证了方法的可行性和定向钻孔布置的合理性。研究结果为瓦斯抽采钻孔参数的优化和煤层瓦斯治理提供一定的理论指导。

Given that the overburden’s structure significantly influences pressure relief gas extraction, this study aims to explore the gas migration patterns of the overburden of thick coal seams during coal seam mining. To this end, this study analyzed the evolutionary patterns of the fracture morphology and seepage capacity of the overburden using physical similarity simulation experiments. Accordingly, this study investigated the mechanisms behind the evolution of fracture-seepage characteristic zones of the overburden of extra-thick coal seams and proposed a method for identifying fracture-seepage characteristic zones of the overburden. Using this method, this study conducted gas extraction along the experimental mining face based on directional boreholes.

The results indicated that the bed separation and fracture density of the central part of the overburden were 47% and 31%, respectively, of those of its two sides, indicating that both indicators were low in the central part and high on the two sides. In the early stage of mining, the fracture-seepage characteristics of the overburden displayed synchronous spatial distributions. With the further advancement of the working face, the overburden was affected by mining to varying degrees, gradually displaying evolutionary zoning. Specifically, the central part of the overburden exhibited significantly weaker fracture-seepage characteristics than the surrounding areas, with these characteristics of the two sides gradually weakening with an increase in the horizon. The overburden manifested fracture ratios, fracture entropy, connectivity coefficients, and permeability ranging from 1.0% to 8.5%, from 0.15 to 0.90, from 0.10 to 0.65, and from 6.34×10⁻⁹ m² to 7.78×10⁻⁷ m², respectively. Based on the impacts of the fractures and seepage capacity of the overburden on gas migration, the overburden can be divided into four zones: a low-elevation turbulence zone, a medium-elevation compaction zone, a medium-elevation transition zone, and a high-elevation constant flow zone. Based on the spatiotemporal evolution patterns of the fracture ratios, fracture entropy, connectivity coefficients, and permeability of these zones, this study developed an evolution model and identification flowchart for the fracture-seepage characteristic zones of the overburden of extra-thick coal seams. The analysis of the evolution model reveals that the medium-elevation transition zone and the high-elevation constant current zone were the dominant horizons for gas extraction, with the vertical distances of boreholes designed ranging from 6 m to 25 m. The changes in the pure gas flow extracted from boreholes can be divided into three stages: the initial fluctuation, rapid growth, and slow drop stages. The mining along the mining face yielded methane volume fractions of less than 1%, suggesting encouraging effects in the engineering practice. This verifies the feasibility of the proposed method and the rationality of directional borehole arrangement. The results of this study will provide some theoretical guidance for the optimization of borehole parameters for gas extraction and the gas control of coal seams.