Stress cages and fracture cages in stress trajectory models of wellbores: Implications for pressure management during drilling and hydraulic fracturing

This study visualizes in high resolution the stress trajectory patterns around a wellbore resulting from the interaction between the induced, effective borehole pressures with and without far-field tectonic stresses. Near-wellbore stress trajectory maps are produced using non-dimensional solutions of the stress function for balanced, overbalanced and underbalanced borehole sections. Particular attention is paid to the isotropic points, where no deviatoric stress occurs, as these mark the locations where principal stress reversals occur, an aspect that has not been highlighted before. When wellbores are overbalanced, stress cages develop, and the region occupied by a stress cage grows when the net pressure (positive) on the wellbore is much larger than the far-field principal stress. Fracture cages are a complementary phenomenon occurring in underbalanced wellbores. Fracture cages occupy larger areas when the absolute value of the (negative) net pressure grows larger relative to the far-field (native) stress state. A novel nomogram visualizes the continuous range of stress trajectories around overbalanced as well as underbalanced borehole sections, and is scalable by a single dimensionless number, 1/ζ, coined the Frac number. A proper understanding of the principal stress trajectories is extremely useful for predicting the planes of least resistance for fracture initiation. The Frac number can thus contribute to the optimization of well design and hydraulic fracture placement.