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Using Borehole Induced Structure Measurements at Fallon FORGE Combined with Numerical Modeling to Estimate In-Situ Stresses
Derrick J. Blanksma1, Kelly Blake2, Will Pettitt1, Andrew Sabin2, Varun Varun1 and Branko Damjanac1
1Itasca Consulting Group 111 Third Ave South Suite 450 Minneapolis MN 55401
2U.S. Navy Geothermal Program Office
Keywords: EGS, FORGE, borehole breakouts, 3DEC, in-situ stresses
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 at depth. 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 at depth.
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 79% of the vertical stress and maximum horizontal stress that lies in a range of 79% to 88% 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 80% of the vertical stress, respectively.
Maximum and minimum stresses on the borehole wall as a function of its azimuth (𝜽) at 1900 m vertical depth below the collar. The borehole oreintation and in-situ stress state is given in Table 1. The maximum effective stress on the borehole wall is rotated at 𝜽𝒕𝒎𝒂𝒙 = 𝟗𝟕° and has a magnitude of 41.1 MPa, which does not exceed the compressive strength of 55 MPa, thus a breakout does not occur at this depth.
PROCEEDINGS, 43rd Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 12-14, 2018 SGP-TR-213