中文摘要：

中国页岩储层钻探方面处于起步阶段，钻井过程中复杂情况频发，井壁坍塌问题严重。借鉴北美地区页岩气钻井经验，对比中国四川盆地与北美Haynesville的页岩气水平井井身结构。发现随井限尺寸减小，钻井复杂情况有所降低。研究过程中，结合岩石强度的尺寸效应理论。比较不同强度尺寸模型的优缺点，优选Vutukuri模型分析微小井眼井壁围岩的强度变化规律，并根据McLamore各向异性破坏准则确定岩石单轴抗压强度随主应力与层理面法向之间夹角的关系。以此作为页岩储层强度准则：根据定向井坍塌压力计算方法分析井周应力状态，结合尺寸效应与强度准则，分别得出标准井眼和微小井眼在层理性地层中的坍塌压力随井眼轨迹的变化规律，同时探究了固定井眼方位下的坍塌压力随井斜角变化规律。对比不同井斜角下两者的差距，结果表明，不论直并或定向井，采用微小井眼均能显著降低钻井坍塌压力，降低幅度约为10％，研究结论揭示了小井眼减少钻井复杂的内在原因。

英文摘要：

Drilling and exploitation of the gas shale is just in an inilial stage in China. Drilling practice shows that drilling accidents are frequent, and major cause is wellbore instability. From the experience of the shale-gas drilling in North America, and the casing programs in Sichuan Basin and Haynesville, it is shown thai the drilling accidents can be reduced by the ,lecrease of the size of the borehole. Based on the size effect theory, the advantages and disadvantages of diffrent size cffect models are compared. The Vulukturi＇s model is chosen to analyze the micro-borehole surrounding rock strength and the McLamnre＇s anisotmpic failure prediction model is adopted to demonstrate the strength anisotropy of the shale. Based on the collapse pressure prediction model, lhe collapse pressure of a normal horehole and a micro-bnrehole in the bedding formations are ealculated, respectively, furthermore, the trajech）ry sensitivity analysis is also conducted. The collapse pressure changes with the well declination are also considered. Comparing the difference hetween the two sides, it is shown that the micro-borehole can significantly reduce the collapse pressure by approximately 10%, therefore rcduce the shale-gas drilling accidents.