Wellbore instability due to natural fractures is a significant issue impeding drilling operations in shale formations. However, most of the existing literature on wellbore stability analysis focuses on the coupled effects of fluid flow and deformation, and the impact of three-dimensional natural fracture characteristics on wellbore instability is not yet closely examined. This paper carried out a series of experiments to analyze the shale sample's fracture characteristics and shear strength. A fully coupled 3D hydro-mechanical model was then developed using the distinct element method (DEM) to investigate the wellbore instability. Finally, a comprehensive parametric study was performed to analyze the effects of characteristics of fractures (e.g., distribution density, fracture roughness and associated shear strength, dip angle, and seepage time) on wellbore stability under different in-situ stress states. The results show that the shear strength of shale increases with the increase in normal pressure (4 MPa, 6 MPa, and 8 MPa); the shear strength increases by 64.9% when the roughness is 0.325 mm (the largest increase among the three roughness cases). The maximum displacement around the wellbore is 1.3–1.5 times greater under the presence of weak planes than under no weak planes. Fracture roughness poses an important impact on wellbore stability because larger fracture roughness is associated with higher fracture shear strength. As the dip angle of the weak plane increases, the number of yield zones (shear failure) and maximum wellbore displacement increases and then decreases, and the peak occurs at the dip angle of 45°. The seepage time of drilling fluid is positively correlated with the number of yield zones around the wellbore, and the damage rate of drilling fluid to wellbore stability gradually decreases with the seepage time.

中文翻译：

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天然裂缝地层中的井眼不稳定性：实验研究和 3D 数值研究

天然裂缝导致的井筒不稳定是阻碍页岩地层钻井作业的一个重要问题。然而，现有的井壁稳定性分析文献大多侧重于流体流动与变形的耦合效应，而三维天然裂缝特征对井壁稳定性的影响尚未得到深入研究。本文开展了一系列实验来分析页岩样品的断裂特征和剪切强度。然后使用离散元法 (DEM) 开发了完​​全耦合的 3D 流体力学模型来研究井眼不稳定性。最后，进行综合参数研究，分析不同地应力状态下裂缝特征（如分布密度、裂缝粗糙度和相关剪切强度、倾角和渗流时间）对井壁稳定性的影响。结果表明，页岩的剪切强度随着常压（4 MPa、6 MPa、8 MPa）的增加而增加；当粗糙度为0.325 mm时，剪切强度增加了64.9%（三种粗糙度情况下增加幅度最大）。有弱面存在时井筒周围最大位移是无弱面情况下的1.3~1.5倍。裂缝粗糙度对井眼稳定性具有重要影响，因为较大的裂缝粗糙度与较高的裂缝剪切强度相关。随着弱面倾角的增大，屈服区（剪切破坏）数量和最大井眼位移先增加后减小，在倾角45°处出现峰值。钻井液渗流时间与井筒周围屈服区数量呈正相关，随着渗流时间的延长，钻井液对井壁稳定性的损害率逐渐减小。