Elastoplastic finite element analysis of inclined wellbores

An effective finite element model which can be used for elastoplastic analysis of inclined wellbores is presented. The same discretised model can be used for any wellbore inclination and azimuth by varying only the applied initial stress field or boundary conditions. The model is applied for calculating the optimum drilling mud-pressure assuming an elastoplastic rock behaviour and different failure criteria. The rock material parameters were derived from triaxial compression tests. The results show that the difference in mud- pressure predictions between elasticity and plasticity depends strongly on the employed failure criteria. The difference between elasticity and plasticity becomes more pronounced with a criterion which allows the material near the borehole wall to reach a critical plastic strain determined from the calibration tests. In such a case differences from 45% to 80% were encountered. Calibration factors given by the ratio of plasticity predictions over the elasticity predictions have been derived and can be used with analytical models for quick field wellbore stability computations.