Experimental and theoretical investigation of borehole breakouts

INTRODUCTION Many boreholes enlarge in cross section from circular to elliptical shape. Such changes are defined as breakouts when the shorter diameter remains equal to the drill-bit diameter. A remarkable uniformity in the orientation of wellbore breakouts has been repeatedly observed (Cox, Bell and Gough) . For many years, these breakouts have been attributed to the state of stress around the borehole under anisotropic horizontal stress. Following this assumption, the longer axis of breakouts will be aligned with the minimum horizontal in situ stress. Breakouts may thus provide an inexpensive method of determining the principal stress directions. At the present time, there are research works to know whether the size or the existence of breakouts may also provide an evaluation of stress magnitude.