Can borehole observations characterize crustal stress?

Knowledge of the in situ stress state is of great importance to understanding a wide range of geomechanical processes in the Earth’s crust, and to addressing many practical problems in the subsurface. The in situ stress characterization in boreholes through the classic hydraulic fracturing tests and borehole failure observations has enabled fundamental knowledge of the state of stress in the brittle upper crust. Compiling borehole observations and other stress indicators at much larger scales, a coherent and consistent stress orientation and relative stress magnitude over appreciable depths and between boreholes at the regional scale becomes evident. Stress magnitudes determined from hydraulic fracturing method and borehole failure observations are consistent with the classic Anderson and Coulomb faulting theories, with the empirical Byerlee’s law, which is useful in constraining the in situ stress state and quantifying fault stability. The general state of frictional equilibrium in the upper crust is present, although stress variations at local scales due to discontinuities, lithology contrasts and other factors are practically ubiquitous. To date, the hydraulic fracturing method and borehole failure observations, and their evolved variants, remain extremely useful. However, our progress in better characterizing and understanding the in situ stress is out of sync with the latest technological advances in instrumentation, modeling and data science. The experimental and theoretical efforts in characterizing in situ stress and understanding its heterogeneity are not well coordinated and integrated, thus hindering stress data collection and interpretation. It is imperative to fundamentally revolutionize how we collect, interpret and share the stress data with innovative developments in crustal stress characterization.