Knowledge of the stress conditions in rock formations is important in tectonics and earthquake studies, in the design of structures in rock, and in the extraction of minerals by in situ methods. We developed the hydrofracturing method, which is now the universally accepted technique for measuring stresses through boreholes drilled in both shallow and deep formations.
We search for alternative ways of estimating in situ stress at great depths. Currently, we are investigating the feasibility of using the borehole breakout phenomenon to obtain reliable information on magnitudes and directions of the stress tensor. In experiments with quartz-rich sandstones we discovered breakouts that are long and thin, and appear to be emptied compaction bands generated by the high stress concentration perpendicular to the maximum stress. This discovery may have important implications in the extraction of hydrocarbons.
We have designed and fabricated a true triaxial loading apparatus and have shown that the conventional Mohr-Coulomb failure criterion, which disregards the effect of the intermediate principal stress, is only a conservative estimate of strength. We were able to present the new criterion in the form of an elegant function relating the octahedral shear stress at failure to the mean normal stress acting on the fracture plane.