The layered shale formation is an extremely troublesome formation of borehole instability, with a significant influence on drilling. This paper studies the effect of joint weak planes in the rock mass and proposes a combined model, which combines both the bedding plane model and the intact rock model to describe the characteristics of the layered formation, especially the characteristics of the bedding plane. The finite difference software FLAC3D is used to establish the numerical model of the layered shale formation and to simulate the drilling process. To compare with the isotropic shale formation, the distributions of the failed area, the borehole displacement and the secondary stress in the layered shale formation are obtained. The failure process of the bedding plane and characteristics of the deformation are well simulated. The difference of the displacements on the two sides of the bedding plane is up to 0.8 mm and the slip displacement is 8.13 times the shift displacement. The results agree with the drilling practice. The slip behavior of the bedding plane is shown to be the central cause of the borehole instability in the layered shale formation. The effect of the geo-stress, the occurrence of the bedding plane, the parameters of the well track and the borehole size on the well deformation in inclined and horizontal layered formations are also simulated.
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