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首页> 美国卫生研究院文献>Proceedings of the National Academy of Sciences of the United States of America >Branching of hydraulic cracks enabling permeability of gas or oil shale with closed natural fractures
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Branching of hydraulic cracks enabling permeability of gas or oil shale with closed natural fractures

机译：水力裂缝的分支使天然气或油页岩具有封闭的天然裂缝可渗透

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While hydraulic fracturing technology, aka fracking (or fraccing, frac), has become highly developed and astonishingly successful, a consistent formulation of the associated fracture mechanics that would not conflict with some observations is still unavailable. It is attempted here. Classical fracture mechanics, as well as current commercial software, predict vertical cracks to propagate without branching from the perforations of the horizontal well casing, which are typically spaced at 10 m or more. However, to explain the gas production rate at the wellhead, the crack spacing would have to be only about 0.1 m, which would increase the overall gas permeability of shale mass about 10,000×. This permeability increase has generally been attributed to a preexisting system of orthogonal natural cracks, whose spacing is about 0.1 m. However, their average age is about 100 million years, and a recent analysis indicated that these cracks must have been completely closed by secondary creep of shale in less than a million years. Here it is considered that the tectonic events that produced the natural cracks in shale must have also created weak layers with nanocracking or microcracking damage. It is numerically demonstrated that seepage forces and a greatly enhanced permeability along the weak layers, with a greatly increased transverse Biot coefficient, must cause the fracking to engender lateral branching and the opening of hydraulic cracks along the weak layers, even if these cracks are initially almost closed. A finite element crack band model, based on a recently developed anisotropic spherocylindrical microplane constitutive law, demonstrates these findings [Rahimi-Aghdam S, et al. (2018) arXiv:1212.11023].
机译：虽然水力压裂技术（即压裂（或压裂，压裂））已得到高度发展并取得了惊人的成功，但仍无法获得与某些观察结果不冲突的相关断裂力学的一致表述。在这里尝试。经典的裂缝力学以及当前的商业软件都预测垂直裂缝的传播不会从水平井套管的孔眼分支出来，而水平井套管的孔眼通常间隔10 m或更大。但是，为了解释井口处的产气速率，裂缝间距必须仅约为0.1 m，这将使页岩质量的总气体渗透率增加大约10,000 × 。这种渗透率的增加通常归因于预先存在的正交自然裂缝系统，其间距约为0.1 m。但是，它们的平均年龄约为1亿年，最近的分析表明，这些裂缝肯定在不到一百万年的时间内就被页岩的二次蠕变完全封闭了。在这里，认为在页岩中产生天然裂缝的构造事件必定也形成了具有纳米裂缝或微裂缝破坏的薄弱层。数值表明，沿薄弱层的渗透力和渗透率大大提高，同时具有较大的横向比奥特系数，必须引起压裂引起侧向分支和沿薄弱层的水力裂缝开裂，即使这些裂缝最初是几乎关闭。基于最近开发的各向异性球面微平面本构律的有限元裂纹带模型证明了这些发现[Rahimi-Aghdam S等。 （2018）arXiv：1212.11023]。

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