Seepage forces, important factors in the formation of horizontal hydraulic fractures and bedding‐parallel fibrous veins (‘beef’ and ‘cone‐in‐cone’)
Seepage forces, important factors in the formation of horizontal hydraulic fractures and bedding‐parallel fibrous veins (‘beef’ and ‘cone‐in‐cone’)
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DOI：
10.1111/j.1468-8123.2007.00183.x
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发表时间：
2007-08
期刊：
影响因子：
1.7
通讯作者：
P. Cobbold;N. Rodrigues
中科院分区：
文献类型：
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作者：
P. Cobbold;N. Rodrigues
层理平行的纤维脉（“牛肉”和“锥中锥”）在许多沉积盆地中很常见，尤其是那些含有黑色页岩的盆地。模式产地是英格兰西南部。纤维中最常见的矿物质是方解石。纤维表明垂直开口，抵抗重力。过去，这被归因于液体超压。然而，基于 Von Terzaghi 的概念的简单分析得出这样的结论：为了使裂缝是水平的，岩石要么必须是各向异性的，要么必须受到水平压缩。通过物理模型支持的更完整的分析，我们表明，即使岩石是各向同性且不存在构造应力，水平裂缝也是可以预料的。响应超压梯度，向上的流体流动向固体框架的所有元件施加渗透力。渗流力抵消甚至超过岩石的重量，产生有效拉应力。该过程可能导致拉伸水力压裂或膨胀剪切破坏。我们认为这两种失效模式以及合适溶质的可用性分别解释了“牛肉”和“锥中锥”的频繁发生。
Bedding-parallel fibrous veins (‘beef’ and ‘cone-in-cone’) are common to a number of sedimentary basins, especially those containing black shale. The type locality is SW England. The commonest mineral in the fibres is calcite. The fibres indicate vertical opening, against the force of gravity. In the past, this has been attributed to fluid overpressure. However, a simple analysis, based on Von Terzaghi's concepts, leads to the conclusion that, for the fractures to be horizontal, either the rock must be anisotropic, or it must be subject to horizontal compression. By means of a more complete analysis, supported by physical models, we show that horizontal fractures are to be expected, even if the rock is isotropic and there are no tectonic stresses. Upward fluid flow, in response to an overpressure gradient, imparts seepage forces to all elements of the solid framework. The seepage forces counteract the weight of the rock, and even surpass it, generating a tensile effective stress. The process may lead, either to tensile hydraulic fracturing, or to dilatant shear failure. We suggest that these two failure modes, and the availability of suitable solutes, explain the frequent occurrence of ‘beef’ and ‘cone-in-cone’ respectively.