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Borehole stability evaluation in overbalanced and underbalanced drilling: based on 3D failure criteria

Wellbore instability implies a lot of cost to petroleum industry. Because the demand for more directional and multi lateral wells increased, borehole instability during overbalanced drilling (OBD) and underbalanced drilling condition (UBD) becomes more difficult issue to handle. Usually applied stresses around the borehole were compared with rock strength by using an appropriate failure criterion to obtain a safe mud window during drilling. Mohr–Coulomb failure criterion is the most common failure criterion in the literature, but it is conservative. In this research, a computer program is developed to analyze the mechanical stability of deviated wellbores. It used three-dimensional failure criteria to obtain critical bottomhole pressure in both OBD and UBD situations. Also optimized well trajectory is determined during OBD and UBD conditions. The results of this study indicate that in situ stress regimes play a key role in wellbore stability. In addition, it shows that optimum well trajectory is the same for both OBD and UBD wells in different stress regimes.