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Borehole breakouts are used to constrain the magnitude of maximum horizontal stress. However, when the borehole wall strength is higher than the in situ tangential stress, borehole wall failure does not develop. Additional compressive stress can be induced by heating borehole walls. To validate this concept experimentally, we conducted room-temperature and elevated temperature true-triaxial tests on Berea sandstone and Niagaran dolomite samples. We used acoustic emission sensors to capture the onset of breakout development, and we measured the temperature close to borehole wall to assess the magnitude of induced thermal hoop stress. The test results show that within a specific rock type, the breakouts develop in similar manner in room-temperature and elevated-temperature tests. Therefore, the maximum horizontal stress can be constrained from the following dataset: critical tangential stress at which breakout develops, minimum horizontal stress, elastic and thermal properties, and temperature change at the borehole wall. 

We investigate the mode 1 fracture toughness and its anisotropy of Poorman Schist rocks recovered from the Enhanced Geothermal Systems Collaboration (EGS Collab) Experiment 1 site. The EGS Collab team is conducting a series of intermediate (10-20m) scale stimulation and inter-well flow tests with comprehensive instrumentation and characterization at the Sanford Underground Research Facility to validate existing theories and description of hydraulic fractures propagation and associated fluid flow. An important parameter to constrain is how the fracture toughness varies depending on the orientation of the fracture and the direction of fracture propagation, which may have controls on hydraulic fracture propagation. Fracture toughness relative to foliation orientation was determined through the utilization of Cracked Chevron Notched Brazilian Disk (CCNBD) samples in three different orientations (Divider, Arrester, and Foliation Splitting/Short Transverse). Each sample group contains at least three 25.4 mm diameter and 12.7 mm thick CCNBD samples, one of each sample type. Arrester and Foliation Splitting samples were obtained from the same sub-core while Divider samples were obtained from a separate sub-core obtained in close proximity. We found fracture toughness to be weakest in the Foliation Splitting orientation and strongest in the Divider orientation, similar to findings from anisotropic fracture toughness measured in shale rocks. Our findings on the influence of foliation orientation on fracture toughness are presented here.