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

Abstract

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 Shmin from hydraulic fracturing experiments to construct a vertical profile of the maximum horizontal principal stress SHmax 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, Shmin, than the maximum principal horizontal stress, SHmax. To utilize the breakout observations to constrain the magnitude of SHmax, 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 SHmax profile estimated from the breakouts with estimates of SHmax 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 SHmax profile indicates several marked decreases in stress magnitude associated with major fault zones, an observation consistent with the Shmin profile obtained from hydraulic fracturing tests.

The Mohr–Coulomb criterion for intact rock strength and friction – a re-evaluation and consideration of failure under polyaxial stresses

“…Initial approaches to this problem applied the 2-D Mohr-Coulomb criterion (Barton et al, 1988) assuming the circumferential stress at the onset of failure corresponded to the axisymmetric, unconfined compressive strength. It was quickly realized that the uniaxial compressive strength would underestimate the strength of the borehole wall and that a polyaxial failure criterion was required (Vernik and Zoback, 1992). The vertical stress parallel to the borehole wall is due largely to the depth of burial but is modulated by the circumferential stress through the Poisson ratio effect so that it too varies sinusoidally around the borehole wall.…”

“…It is easy to adapt to many geomechanical modelling problems and it is widely applied to problems that involve polyaxial loading (e.g. Vernik and Zoback, 1992;Castillo et al, 2000), often because nothing otherwise is known about the behaviour of particular rocks under polyaxial stress conditions. There are many applications that demand knowledge of failure or frictional sliding under generalized stress conditions.…”

Compression directions in southern California (from Santa Barbara to Los Angeles Basin) obtained from borehole breakouts

“…The direction of least horizontal principal stress, S h, will be orthogonal to this. Note that in this case, information on stress magnitudes cannot be recovered unless other assumptions are made [e.g., Moos and Zoback, 1990;Vernik and Zoback, 1992].…”

A Failure Criterion for Rocks Based on True Triaxial Testing

“…The experiment-based criterion was successfully employed in two major international research projects in conjunction with in situ stress measurements using a hybrid method involving hydraulic fracturing and borehole breakouts (Vernik and Zoback 1992). The true triaxial failure criteria and the logged breakout spans were used to derive the in situ maximum principal stress in the KTB amphibolite, Germany (Haimson and Chang 2002), and in the siltstone adjacent to Chelungpu Fault in Taiwan (Haimson et al 2010).…”