Hydraulic fracturing technology in coal reservoirs helps increase coalbed methane (CBM) production, and the development of hydraulic fractures (HFs) is influenced by in situ stress and original fractures. Therefore, in this study, triaxial HF tests and computed tomography (CT) were used to determine HF initiation and propagation in coal, considering weak structural planes and in situ stress. The experimental results show that weak structural planes control HF initiation and propagation in coal under a deep-stress environment. Most HFs occurred along bedding planes, followed by exogenetic fractures. The influence of the horizontal stress difference coefficient , which is equal to the maximum horizontal stress minus the minimum horizontal stress and divided by the minimum horizontal stress, on the propagation of the HF was studied. When increases to 1.04, the HF becomes perpendicular to the direction of the minimum principal stress. When < 1.04, the main controlling factor of HF propagation is a weak structural plane, and when ≥ 1.04, the main controlling factor of HF propagation is in situ stress. Based on the CT reconstruction model, the average aperture distributions of the discrete bedding fractures and exogenetic fractures of coal in this experiment were calculated to be 0.0947 mm and 0.1940 mm, respectively. This determines the different strength properties of exogenetic fractures and bedding planes, and determines the priority initiation position of HF in coal. During the HF initiation and propagation, the average energy along the exogenous fracture was lower than that along the bedding planes. On this basis, this study predicts that the main factor controlling HF propagation in a deep coal seam in the Qin-Shui Basin, China, is the weak structural plane in the coal seam.

中文翻译：

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考虑弱面的深层煤层气藏水力裂缝起裂和扩展：CT扫描测试

煤藏水力压裂技术有助于提高煤层气产量，而水力裂缝的发育受到地应力和原始裂缝的影响。因此，在本研究中，考虑到弱结构面和地应力，采用三轴高频试验和计算机断层扫描（CT）来确定煤中高频的引发和传播。实验结果表明，在深应力环境下​​，弱结构面控制着煤中氢氟酸的萌生和传播。大多数 HF 沿层理面发生，其次是外生裂缝。研究了水平应力差系数对HF传播的影响，该系数等于最大水平应力减去最小水平应力再除以最小水平应力。当增加到 1.04 时，HF 变得垂直于最小主应力方向。当<1.04时，高频传播的主要控制因素是弱结构面；当≥1.04时，高频传播的主要控制因素是地应力。基于CT重建模型，计算出本次实验煤体离散层理裂缝和外生裂缝的平均孔径分布分别为0.0947 mm和0.1940 mm。这就决定了外生裂隙和层理面的不同强度特性，并决定了煤中HF的优先起始位置。在高频引发和传播过程中，沿外生裂缝的平均能量低于沿层理面的平均能量。在此基础上，本研究预测，控制沁水盆地深层煤层中高频传播的主要因素是煤层软弱构造面。