A new well path optimization model for increased mechanical borehole stability

Borehole collapse is the commonly encountered wellbore stability problem during drilling. The precursor of such borehole failure can be often eliminated by a proper determination of the critical mud pressure. Recently, Al-Ajmi and Zimmerman developed a new 3D analytical model to estimate the mud pressure required to avoid shear failure (i.e., borehole collapse) at the wall of an arbitrarily oriented borehole. This was achieved by using linear elasticity theory to calculate the stresses around the borehole, in conjunction with the fully-polyaxial Mogi-Coulomb criterion to predict failure. This model was used to study the behaviour of the collapse pressure under different in situ stress regimes. We found that the optimal drilling trajectory is directly related to the principal in situ stresses. Accordingly, we now develop a new analytical solution to optimize well path. The stability model has been applied to several field cases, from the North Sea, Indonesia and the Arabian Gulf, and the new model in each case is consistent with the field experience. We conclude that the optimal wellbore profile can be ensured by employing this new analytical stability model.