Wellbore collapse frequently happens in the clay shale formation. To maintain wellbore stability, appropriate mud pressure is a vital factor. When clay formation is opened, drilling unloading occurs, modifying rock structure and strength at the wall of borehole, which affects the selection of mud pressure. Currently, mechanism of drilling unloading is still poorly understood which in return will bring a concern to wellbore stability. Therefore, in this study, a combination of triaxial compressive test and ultrasonic wave test has been used to simulate drilling unloading and analyze its mechanism. Results indicate that more void space is created inside the clay shale sample due to unloading. This structure change leads to a decline of strength and acoustic amplitude. Additionally, unloading influence is depended on varying drilling unloading parameters. Small unloading range and fast unloading rate are able to enhance stability. With various degrees of unloading impact, collapse pressure equivalent density has a clear modification, proving that unloading is a non-negligible influencing factor of wellbore stability. Besides, the unloading effect is much stronger in large confining pressure, implying that more attention should be given to unloading when drilling is in extreme deep or high geostress formation. Findings in this paper can offer theoretical guidance for drilling in the clay shale formation.

中文翻译：

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泥页岩地层中卸钻对井筒稳定性影响的研究

井眼坍塌经常发生在粘土页岩地层中。为了保持井筒的稳定性，适当的泥浆压力是至关重要的因素。当打开粘土层时，发生钻探卸荷，从而改变了井壁的岩石结构和强度，从而影响了泥浆压力的选择。目前，对钻探卸荷的机理仍知之甚少，而这反过来又会引起井筒稳定性的问题。因此，在这项研究中，三轴压缩试验和超声波试验相结合已被用来模拟钻探卸载并分析其机理。结果表明，由于卸载，在泥页岩样品内部产生了更多的空隙空间。这种结构变化导致强度和声振幅下降。另外，卸载影响取决于变化的钻探卸载参数。较小的卸载范围和快速的卸载速度能够增强稳定性。在不同程度的卸载冲击下，塌陷压力等效密度有明显的变化，证明了卸载是井眼稳定性不可忽略的影响因素。此外，在大围压下卸荷效果要强得多，这意味着当钻井处于极深或极高的地应力地层时，应更加注意卸荷。本文的研究结果可为粘土页岩地层的钻探提供理论指导。证明卸载是井眼稳定性不可忽略的影响因素。此外，在大围压下卸荷效果要强得多，这意味着当钻井处于极深或极高的地应力地层时，应更加注意卸荷。本文的研究结果可为粘土页岩地层的钻探提供理论指导。证明卸载是井眼稳定性不可忽略的影响因素。此外，在大围压下卸荷效果要强得多，这意味着当钻井处于极深或极高的地应力地层时，应更加注意卸荷。本文的研究结果可为粘土页岩地层的钻探提供理论指导。