标题： 具有封闭天然裂缝的气体或油页岩中水力裂缝的分支：如何掌握渗透率 显示英文标题

标题： Branching of Hydraulic Cracks in Gas or Oil Shale with Closed Natural Fractures: How to Master Permeability

摘要： 尽管水力压裂技术，即压裂（或压裂，压裂），已经高度发展并取得了惊人的成功，但尚未有与一些观察结果不冲突的相关的断裂力学的一致性表述。这里进行了尝试。经典断裂力学以及当前的商业软件预测垂直裂缝从水平井套管的射孔处无分支地传播，这些射孔通常间距为10米或更多。然而，为了解释井口的气体产量，裂缝间距必须仅为约0.1米，这将使页岩体的整体气体渗透率增加约10,000$\times$。这种渗透率的增加通常归因于一个现有的正交天然裂缝系统，其间距约为0.1米。但它们的平均年龄约为一亿年，最近的分析表明，这些裂缝必须在不到一百万年内被页岩的次级蠕变完全关闭。在这里认为，产生页岩中天然裂缝的构造事件也必定创造了具有纳米或微裂纹损伤的弱层。数值上证明，沿弱层的渗透率大大增强，并且横向比奥系数大大增加，这必然会导致压裂产生横向分支，并沿弱层开启水力裂缝，即使这些裂缝最初几乎关闭。基于最近开发的各向异性球柱状微平面本构定律的有限元裂纹带模型，证明了这些发现。 显示英文摘要

摘要： While the 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 the current commercial softwares, 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$\times$. This permeability increase has generally been attributed to a preexisting system of orthogonal natural cracks, whose spacing is about 0.1 m. But 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 nano- or micro-cracking damage. It is numerically demonstrated that 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 recently developed anisotropic spherocylindrical microplane constitutive law, demonstrates these findings.