Pressure-relief mining is the primary method to remove gas and coal outbursts and realize coal and gas simultaneous extraction in deep, high-gas, and low-permeability coal seams. Numerical modeling has been widely used to evaluate the effectiveness of protective coal seam (PCS) longwall mining, which usually employs experiment-derived permeability models to describe permeability changes and coalbed methane (CBM) flow. Due to the difficulty of tests and data available, published models and parameters were commonly utilized in simulations, which, however, may limit the accuracy of the results because of their case-dependent nature. In this paper, a series of laboratory tests were performed on different damage degree rock mass, i.e., broken rock samples (caving zone), fracture coal samples (fracture zone and swelling deformation zone), and raw coal samples (original zone), to obtain permeability models for different damage zones surrounding the longwall face. Besides the stress states, the experiments also highlight that the damage degree and stress path play vital roles in the permeability changes. The stress sensitivity of permeability in unloading stages is found to be significantly lower than that in loading stages, indicating the importance of the stress path when simulating longwall mining-induced permeability changes. A stress path-dependent permeability law was proposed, and the associated algorithm was designed and embedded into the FLAC3D code. For the first time, the stress path-dependent permeability law was employed to simulate the pressure relief, permeability enhancement, and CBM flow in the protected coal seam (PDCS) when longwall mining is executed in the upper PCS. Field measurements proved the feasibility and accuracy of the new numerical method. Numerical results also suggest the common loading permeability model overestimates the effectiveness of the pressure relief mining. This study highlights the significant role of stress path-dependent permeability laws to accurately predict the permeability changes and CBM flows in pressure relief mining.

中文翻译：

---

利用应力路径相关的渗透率定律评价保护煤层的渗透率增加和煤层气流量：一个案例研究

卸压开采是消除瓦斯和煤层突出并实现深，高瓦斯，低渗煤层同时采煤和瓦斯的主要方法。数值模型已被广泛用于评估保护煤层（PCS）长壁开采的有效性，通常采用实验得出的渗透率模型来描述渗透率变化和煤层气（CBM）流量。由于测试和可用数据的困难，通常在模拟中使用已发布的模型和参数，但是由于它们的大小相关性，可能会限制结果的准确性。本文针对不同破坏程度的岩体进行了一系列的实验室测试，即碎石样品（崩落带），碎煤样品（断裂带和溶胀变形区），和原煤样品（原始区域），以获得长壁工作面周围不同破坏区域的渗透率模型。除了应力状态，实验还强调了破坏程度和应力路径在渗透率变化中起着至关重要的作用。发现卸载阶段渗透率的应力敏感性显着低于加载阶段，表明模拟长壁开采引起的渗透率变化时应力路径的重要性。提出了应力路径相关的渗透率定律，并设计了相关算法并将其嵌入到FLAC3D代码中。首次采用了应力路径相关的渗透率定律来模拟压力释放，渗透率增加，当在上部PCS中进行长壁开采时，煤层气在保护煤层（PDCS）中流动。现场测量证明了新数值方法的可行性和准确性。数值结果还表明，常用的载荷渗透率模型高估了泄压开采的有效性。这项研究强调了应力路径相关的渗透率定律在准确预测卸压开采中的渗透率变化和煤层气流量方面的重要作用。