Understanding the in-situ stress state is an important component for all subsurface engineering disciplines. In Enhanced Geothermal Systems (EGS) the behavior of joints, fractures and faults during hydraulic stimulation depends on the principal stress field. In this study, we take an alternative approach to interpreting the stress field by generating a three-dimensional (3D) distinct element model in 3DEC that simulates a borehole at the Fallon FORGE site where breakouts have been measured at the depth of the proposed EGS reservoir. The simulated breakouts that develop in the 3D geomechanical model depend on the input principal stress magnitudes, strength of the rock, and orientation of the borehole. Assuming the strength of the rock is known, we compare the locations of simulated breakouts to the observed breakouts at different input stresses to produce a range for the likely in-situ stress magnitudes. The range is further informed by an analysis of observed stability on joints, fractures and faults under in-situ stress conditions. The approach illustrates that an ability to simulate where a borehole breakout occurs can provide confidence in estimating the magnitude of the principal stresses that supports traditional measurements of stress orientations. Estimates for the in-situ stresses at Fallon FORGE were produced for different rock strengths to test uncertainties. The results indicate a minimum horizontal stress that lies in a range of 53% to 72% of the vertical stress. Using this approach, the best estimate of the minimum and maximum horizontal principal stresses at the Fallon FORGE site are 56% and 82% of the vertical stress, respectively.

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