Estimation of maximum horizontal principal stress magnitude from stress‐induced well bore breakouts in the Cajon Pass Scientific Research borehole

In this paper we utilize the mechanical properties of core samples, detailed observations of stress‐induced well bore breakouts and estimates of the magnitude of S h min from hydraulic fracturing experiments to construct a vertical profile of the maximum horizontal principal stress S H max to 3.5 km depth in the Cajon Pass borehole. As in essentially all other boreholes, the hydraulic fracturing stress measurements in the Cajon Pass borehole yielded appreciably more (and more reliable) data on the magnitude of the least principal horizontal stress, S h min , than the maximum principal horizontal stress, S H max . To utilize the breakout observations to constrain the magnitude of S H max , a brittle failure criterion was used that is based on the effective strain energy concept. This failure criterion also allows us to account for the polyaxial stress around the borehole and to incorporate the role of pore fluid pressure on rock strength in various ways. A comparison of the S H max profile estimated from the breakouts with estimates of S H max obtained from the hydraulic fracturing tests indicates that they compare fairly well for the case when the effect of pore pressure on hydraulic fracture initiation is negligible. The overall stress state to a depth of 3.5 km in the well corresponds to a normal/strike‐slip faulting regime. The S H max profile indicates several marked decreases in stress magnitude associated with major fault zones, an observation consistent with the S h min profile obtained from hydraulic fracturing tests.