Crustal stress determination from boreholes and rock cores: Fundamental principles
Obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling. The borehole cavity, however, disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences. At sufficiently low stress magnitudes, the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress. At higher levels, the stresses damage then fracture the rock near the wellbore wall or within a growing core stub. Indeed, almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses. This contribution reviews, at a fundamental level, the concentration of stresses by the borehole, the effects on the materials in the borehole's vicinity, and how these are used to infer stress states. Stress concentrations applied to rocks, which are generally nonlinear elastic materials, induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods. Hydraulic fractures initiate, propagate, and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes. At higher stress levels, the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes. Similarly, the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered. Unfortunately, no method currently allows for complete determination of the stress tensor; using a number of different but complementary techniques is necessary to best constrain in situ stress states.