Borehole breakout, which is shear failure of the borehole wall, is one of the serious problems in drilling wells for oil/gas exploration and production. Investigation of stress condition causing breakout helps to avoid this problem. Conventionally, simple failure criteria, such as Mohr-Coulomb, which disregard the effect of the intermediate principal stress, were used to evaluate the stress condition. However, many researchers have reported that the intermediate principal stress affects the stress condition causing breakout. In this paper, the authors conducted true triaxial experiments using cubic rock specimens with a borehole to investigate the effect of the intermediate principal stress. In these experiments, measurements of tangential strains around the borehole wall and a weight of failed rock fragments by the scale, and observation of the wall using video camera were conducted to detect the failure development process. The effect of the intermediate principal stress was evaluated in five points along failure development detected by the measurements and the observations described above. The experimental results elucidated that the magnitudes of stress causing breakout clearly increase with increase of the intermediate principal stress in the all points along the failure development. However, when the authors compared the results with the three failure criteria, Modified-Lade, Drucker-Prager and Mogi, which consider the effect of the intermediate principal stress, the three failure criteria underestimated the stress condition causing breakout.
