Abstract：Most of the existing models for borehole wall stress are based on plane strain hypothesis. So some errors may be generated as compared with the real stress state of rock. Based on the theory of spatial coordinate transformation system，a new three-dimensional FEM model is for ERD borehole stability analysis. It converts the variation of deviation and azimuth of ERD to the variation of in-stiu stress components. So the model is stable for any wellbore trajectory. With the new three-dimensional FEM model，some research is done on the ERD wellbore stability under elastic and elastoplastic conditions. The results show that the elastoplastic formations can assimilate more energy through plastic deformation than the elastic ones. So the former will be more stable than the latter under the same exterior load. Case studies indicate that the lower-than-normal mud weight can be used for drilling the deep elastoplastic formation，which can not only keep the wall stability，but decrease the damage to formation.
