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.

裂缝起始压力（FIP）的预测对于页岩气开采中的安全钻井，压裂评估和有效生产至关重要。为了快速，准确地评价各向异性岩石中水平井的FIP，首先根据复势法和叠加原理推导了井壁应力的解析解。结合层理面和页岩基质的各向异性拉伸和剪切断裂准则，然后使用分析溶液评估特定情况下的FIP，并揭示引发机理。最后，进行了参数研究，以探讨影响初始压力和断裂模式的主要因素。结果表明，弹性各向异性显着改变了井眼应力，从而影响了裂缝压力和初始取向。在剪切模式下引发裂缝所需的流体压力远低于在拉伸模式下所需的流体压力。这主要是由于垫层的抗剪强度较弱所致。无论层理的倾角如何，在一定的原位应力条件下，断裂都可能以剪切模式从层理平面开始。较低的原位应力差似乎会导致剪切模式下的裂缝萌生。然而，较大的一个更可能产生拉伸破坏。通过合并弹性各向异性，随着弹性各向异性的增加，所有断裂均以剪切模式发生。但是，当弹性各向异性度大于2时，由于剪切破坏，岩石基质将产生裂缝。垫层的剪切强度越弱，由于剪切破坏，从垫层平面开始的断裂就越容易。本研究开发的解析解决方案对于评估各向异性对各向异性岩石中井眼应力，井壁塌陷，井筒稳定性和水平井裂缝产生的影响非常有用且方便。