Abstract Coal permeability is a key issue in CO2 injection and enhanced coalbed methane (CBM) recovery, and it is determined by the fracture network, which is strongly influenced by mining-induced stress evolutions. For a comprehensive understanding of the size and spatial distribution characteristics of coal fracture networks under different mining-induced stress conditions, a series of laboratory experiments, computed tomography (CT) scans and image analyses focusing on 3D coal fracture systems have been conducted considering the stress conditions induced by three typical mining layouts, i.e., top-coal caving mining (TCM), non-pillar mining (NM) and protective coal-seam mining (PCM). The size and spatial distributions of mining-induced coal microfractures and the anisotropic tortuosity characteristics of coal fracture networks have been quantitatively determined based on fractal theory. The results show that the size distributions of microfracture geometries, the fractal dimensions of fracture size distribution and the tortuosity of the mining-induced coal fracture networks vary according to the mining-induced stress conditions, resulting in differences in the seepage capacity of coal masses. The coal samples subjected to PCM conditions have the highest percentage of large microfractures, and those exposed to NM conditions have the lowest percentage. The fractal dimensions (Da and Dd) of the microfracture size distributions of the typical coal specimens decrease in the order of PCM, NM and TCM conditions, and the Da and Dd of coal specimens without pre-mining unloading-expansion simulation (PUES) are much lower than those of specimens with PUES. The tortuosity fractal dimensions (δ) of the mining-induced fracture network are spatially anisotropic. The ranges of δ for the coal masses exposed to PCM, NM and TCM conditions are 1.0–1.2, 1.2–1.3 and 1.25–1.4, respectively. Using fractal theory and the measured fracture network data, the anisotropic spatial distribution of coal permeability can be theoretically estimated.

中文翻译：

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不同采动应力条件下煤体裂缝网络尺寸及空间分形分布

摘要 煤炭渗透率是 CO2 注入和提高煤层气（CBM）采收率的关键问题，它由裂缝网络决定，裂缝网络受采矿应力演化的强烈影响。为了全面了解不同开采应力条件下煤裂隙网络的大小和空间分布特征，已经进行了一系列以 3D 煤裂隙系统为重点的实验室实验、计算机断层扫描 (CT) 扫描和图像分析。三种典型的采矿布局诱导的条件，即放顶煤开采（TCM）、无柱开采（NM）和保护性煤层开采（PCM）。基于分形理论定量确定了采煤微裂缝的尺寸和空间分布以及煤裂缝网络的各向异性曲折特征。结果表明，微裂缝几何形状的尺寸分布、裂缝尺寸分布的分形维数和采动煤裂缝网络的曲折度随采动应力条件的不同而不同，导致煤体的渗流能力存在差异。经受 PCM 条件的煤样具有最高百分比的大微裂缝，暴露于 NM 条件的煤样百分比最低。典型煤样微裂缝尺寸分布的分形维数（Da和Dd）按PCM、NM和TCM条件的顺序减小，未经开采前卸载-膨胀模拟（PUES）的煤样的Da和Dd远低于采用PUES的煤样。采矿诱导裂缝网络的曲折分形维数 (δ) 是空间各向异性的。暴露于 PCM、NM 和 TCM 条件的煤体的 δ 范围分别为 1.0-1.2、1.2-1.3 和 1.25-1.4。利用分形理论和实测裂缝网络数据，可以从理论上估算煤层渗透率的各向异性空间分布。分别。利用分形理论和实测裂缝网络数据，可以从理论上估算煤层渗透率的各向异性空间分布。分别。利用分形理论和实测裂缝网络数据，可以从理论上估算煤层渗透率的各向异性空间分布。