Fracture toughness anisotropy in shale
Fracture toughness anisotropy in shale
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DOI：
10.1002/2015jb012756
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发表时间：
2016-03-01
影响因子：
3.9
通讯作者：
Crawford, Brian R.
中科院分区：
文献类型：
--
作者：
Chandler, Michael R.;Meredith, Philip G.;Crawford, Brian R.
利用水力压裂开采页岩气已将人们的注意力集中在含气页岩的基本断裂特性上，但有关其机械和物理特性的可用实验数据仍然很少。此类页岩具有强各向异性，其裂缝扩展轨迹取决于其各向异性力学性质与浅部地壳各向异性地应力场之间的相互作用。在这里，我们报告了使用改进的短杆方法在所有三个主要裂缝方向（分隔器、短横向和止动器）确定的 Mancos 页岩的断裂韧性测量结果。出于比较目的，还报告了一系列其他沉积岩和碳酸盐岩的实验结果。环境条件下页岩断裂韧性测量观察到显着的各向异性，当裂纹面垂直于层理时，各向异性高达0.72MPam(1/2)；当裂纹面平行于层理时，各向异性低至0.21MPam(1/2)。对于不平行于层理扩展的裂纹，我们观察到偏离层理平行方向的趋势。应用最大能量释放率标准，我们确定了在更广义的混合模式加载条件下或多或少可能发生此类偏差的条件。我们发现，对于 Mancos 页岩，无论加载条件如何，裂缝都应偏向韧性最低的平面。
The use of hydraulic fracturing to recover shale gas has focused attention on the fundamental fracture properties of gas-bearing shales, but there remains a paucity of available experimental data on their mechanical and physical properties. Such shales are strongly anisotropic, so that their fracture propagation trajectories depend on the interaction between their anisotropic mechanical properties and the anisotropic in situ stress field in the shallow crust. Here we report fracture toughness measurements on Mancos shale determined in all three principal fracture orientations: Divider, Short Transverse, and Arrester, using a modified short-rod methodology. Experimental results for a range of other sedimentary and carbonate rocks are also reported for comparison purposes. Significant anisotropy is observed in shale fracture toughness measurements at ambient conditions, with values, as high as 0.72MPam(1/2) where the crack plane is normal to the bedding, and values as low as 0.21MPam(1/2) where the crack plane is parallel to the bedding. For cracks propagating nonparallel to bedding, we observe a tendency for deviation toward the bedding-parallel orientation. Applying a maximum energy release rate criterion, we determined the conditions under which such deviations are more or less likely to occur under more generalized mixed-mode loading conditions. We find for Mancos shale that the fracture should deviate toward the plane with lowest toughness regardless of the loading conditions.