Polyaxial strength criteria and their use in estimating in situ stress magnitudes from borehole breakout dimensions

Abstract We conducted laboratory simulation tests of borehole breakouts and investigated their potential use as an indicator of in situ stress magnitudes in Westerly granite and Berea sandstone. We also carried out simple triaxial tests and used the results to derive several strength criteria for these rocks. Using the assumption that the state of stress at the breakout boundary on the borehole wall is in a state of limit equilibrium with the rock strength, we attempted to determine the appropriate strength criterion for each of the two rocks. We concluded that the well known Mohr-Coulomb criterion is not compatible with the condition of stress at breakout failure. On the other hand, truly triaxial (polyaxial) strength criteria, which incorporate the effect of the intermediate principal stress on failure, are much more in agreement with the stress at the breakout boundary. One such criterion due to Nadai and another due to Mogi, appear suitable for determining breakout failure in the sandstone and the granite, respectively. Thin-section analysis suggests that breakout failure mechanism may play an important role in determining the appropriate strength criterion for a given rock type. Utilizing the appropriate strength criterion for the corresponding rock type, and having knowledge of the breakout span at the borehole wall and the minimum horizontal and vertical principal stresses, we were able to estimate the maximum far-field principal stress within an average of 14% error.