Borehole breakout and stress measurements

Elongated breakout cross sections for different rock stresses and strengths have been produced using a numerical simulation based on boundary element methods and a micromechanics model of extensile splitting. Although these breakout cross sections are completely stable for an elastic-brittle rock they are not related uniquely to the stresses and strengths, and depend upon the sequence in which holes are drilled and the stresses applied. The depth of any breakout depends mainly on the initial breakout angle. The same initial breakout angle can result from many different combinations of stress. We conclude that breakout shapes cannot be used to infer the magnitudes of the stresses orthogonal to a borehole. The interpretation of hydraulic fracturing measurements based on the stress around a circular hole does not result in large errors if the hole has moderate breakouts. Large breakouts result in tensile fractures that preclude measurements of breakdown pressures.