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首页> 外文期刊>Journal of natural gas science and engineering >Shear mechanism of fracture initiation from a horizontal well in layered shale
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Shear mechanism of fracture initiation from a horizontal well in layered shale

机译：分层页岩中水平井骨折启动的剪切机制

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Prediction of fracture initiation pressure (FIP) is an essential issue for safety drilling, fracturing evaluation and efficient production in shale gas exploitation. To quickly and accurately evaluate the FIP of a horizontal borehole drilled in anisotropic rocks, an analytical solution for borehole stress was first deduced based on the complex potential method and superposition principle. In combination with anisotropic tensile and shear fracturing criteria for the bedding planes and shale matrix, the FIP for a particular case was then evaluated using the analytical solution, and the initiation mechanism was also revealed. Finally, parametric studies were conducted to explore the main factors influencing the initiation pressure and fracture mode. The results showed that the elastic anisotropy significantly alters the borehole stress and thus influences the fracture pressure and initiation orientation. The fluid pressure needed for fracture initiating in shear mode is much lower than that in tensile mode. This is mainly resulted from the weak shear strength of the bedding planes. Fracture will probably initiate from the bedding plane in shear mode under certain in situ stress conditions, regardless of the bedding inclination. A lower in situ stress difference appears to induce fracture initiating in shear mode; a larger one, however, is more likely to produce tensile failure. By incorporating the elastic anisotropy, fracture all initiates in shear mode with the increase in elastic anisotropy. However, when the elastic anisotropy degree is larger than 2, fracture will be produced from the rock matrix due to shear failure. The weaker the bedding's shear strength, the easier the fracture will initiate from the bedding planes due to shear failure. The analytical solution developed in this study is extremely useful and convenient to assess the influence of mechanical anisotropy on borehole stress, borehole collapse, wellbore stability and fracture generation of horizontal wells in anisotropic rocks.
机译：裂缝起始压力（FIP）预测是页岩气开采中安全钻井、压裂评价和高效生产的关键问题。为了快速、准确地计算各向异性岩石中水平钻孔的FIP，首先基于复势法和叠加原理推导了钻孔应力的解析解。结合层理面和页岩基质的各向异性拉伸和剪切破裂准则，然后使用解析解评估特定情况下的FIP，并揭示了起爆机理。最后，对影响起爆压力和破裂方式的主要因素进行了参数研究。结果表明，弹性各向异性显著改变了钻孔应力，从而影响了破裂压力和起爆方向。在剪切模式下引发裂缝所需的流体压力远低于拉伸模式下的流体压力。这主要是由于层理面的抗剪强度较弱。在一定地应力条件下，无论层理倾角如何，断裂都可能以剪切模式从层理面开始。较低的地应力差似乎会导致断裂以剪切模式启动；然而，更大的一个更容易产生拉伸破坏。考虑到弹性各向异性，随着弹性各向异性的增加，断裂均以剪切模式开始。然而，当弹性各向异性程度大于2时，由于剪切破坏，岩石基质将产生断裂。层理的剪切强度越弱，由于剪切破坏，从层理面开始断裂就越容易。本研究开发的解析解对于评估各向异性岩石中力学各向异性对水平井应力、井眼坍塌、井眼稳定性和裂缝生成的影响非常有用和方便。

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