ABSTRACT
This paper summarizes some of the lessons learned from a multi-year research project on the mechanics of vertical borehole failure, and the potential use of borehole breakouts to provide estimates of the directions and magnitudes of the in situ principal stresses. It has long been established based on field observations and laboratory verification that the orientation of stress-induced breakouts around the borehole wall coincides with the direction of the in situ minimum horizontal stress. Dog-eared breakouts, typical of granites and limestones, also provide estimates of the far-field stress magnitudes from the relationship between these stresses upon breakout initiation. In addition, their span at the borehole wall is a function of the far-field stress conditions, and a simple model relating the span to the true-triaxial strength criterion enables the actual computation of one of the principal stresses. Fracture-like breakouts occur in quartz-rich high-porosity sandstones, which are common oil-bearing strata. The length of these breakouts varies with the far-field stress and that is the source of another potential means of in situ stress magnitude estimation.
