Borehole instability in the petroleum industry is mainly because of mechanical, chemical, and thermal factors which result in substantial yearly expenditures. The essential part of analysing borehole stability is choosing an optimum rock strength criterion. Many rock strength criteria have been used for rock failure analysis, however all mechanical, chemical and thermal effects are seldom coupled with rock failure criteria to evaluate borehole stability. This paper presents six rock strength criteria coupled with a poro˗chemo˗thermo˗elastic model to analyse borehole stability in shale formations. This model considers the mechanical, chemical, and thermal effects within the stress distribution around a wellbore to evaluate shear and tensile failure. The effects of time and wellbore configuration (i.e. azimuth and inclination angles) on the stability of borehole in shale formations have also been examined. Furthermore, the minimum mud pressures are estimated by investigating wellbore stability at both the borehole wall and the entire near˗borehole area (i.e. inside the formation). Finally, a sensitivity analysis was performed to quantify and compare the effect of each mechanical, chemical, and thermal parameter as well as their level of importance on wellbore stability analysis. The results of dimensionless sensitivity coefficients indicate that, depending on the applied rock failure criteria, the parameters have different order of importance in analysing borehole stability. The impact of error in determination of input parameters on the magnitude of error in critical mud pressure was also presented. The results of this paper will help to effectively control time-dependent borehole instability in low-permeable shale formations, thereby lowering drilling nonproductive time and costs.

石油工业中的井眼不稳定主要是由于机械，化学和热学因素导致每年的大量支出。分析井眼稳定性的重要部分是选择最佳岩石强度标准。许多岩石强度标准已用于岩石破坏分析，但是，很少将所有机械，化学和热效应与岩石破坏标准结合使用来评估井眼稳定性。本文提出了六个岩石强度标准，并结合了一种化学化学热弹性模型来分析页岩地层的井眼稳定性。该模型考虑了井眼周围应力分布内的机械，化学和热效应，以评估剪切和拉伸破坏。时间和井眼配置的影响（即 还研究了页岩地层中井眼稳定性的方位角和倾角。此外，通过研究井壁和整个近井眼区域（即地层内部）的井眼稳定性，可以估算出最低泥浆压力。最后，进行了敏感性分析，以量化和比较每个机械，化学和热参数的影响以及它们对井眼稳定性分析的重要性。无量纲灵敏度系数的结果表明，根据所应用的岩石破坏准则，这些参数在分析井眼稳定性方面具有不同的重要性顺序。还介绍了确定输入参数时的误差对临界泥浆压力误差大小的影响。