Borehole instabilities during drilling are more common in bedding plane rocks than in most other rock formations. Bedding plane rocks make up more than 80% of rocks in siliciclastic environments and about three quarters of borehole problems are caused by bedding plane rocks instability. The assessment of in situ stress and analysis of borehole failure due to instability and weak bedding plane represents one of the most critical factors when evaluating borehole stability that causes borehole failure. This paper is based on elasto-plastic and isotropic model for stresses around the wellbore, with the aim of trying to understand the general behavior of inclined boreholes due to anisotropy. It was found that borehole collapse was caused predominantly mainly by shear but also by tensile failure. It is seen that bedding exposed is not only depends on inclination but also depends on dip of the formation, attack angle and azimuth. The numerical analyses presented in this paper were carried out using a 3-Dimensional numerical program. The effects of several dips and dip directions of rock mass layering and angles of well and three different field stress conditions have been investigated. It is known that the differential stress has an important influence on wellbore instability. Also, the effect of a high differential stress is exacerbated by the layer geometry and well angles. In other words, some dip and dip direction of bedding plane causes maximum displacement toward wellbore and significantly affect wellbore stability during drilling.
