| Abstract |
As part of a study relating fractured rock hydrology to in-situ stress and recent deformation within the Dixie Valley Geothermal Field, borehole televiewer logging and hydraulic fracturing stress measurements were conducted in a 2.7-km-deep geothermal production well (73B-7) drilled into the Stillwater fault zone. The borehole televiewer logs from well 73B-7 show numerous drilling-induced tensile fractures, indicating that the direction of the minimum horizontal principal stress, Shmin, is S57oE + 10o. As the Stillwater fault at this location dips S50oE at -53o, it is nearly at the optimal orientation for normal faulting in the current stress field. Analysis of the hydraulic fracturing data shows that the magnitude of Shmin is 24.1 and 25.9 MPa at 1.7 and 2.5 km, respectively. Analysis of a hydraulic fracturing test from a shallow water well (24W-5) 1.5 km northeast of 73B-7 indicates that the magnitude of Shmin is 5.6 MPa at 0.4 km depth. Given the calculated vertical stress, Coulomb failure analysis indicates that the magnitude of Shmin measured in these wells is close to that predicted for incipient normal faulting on the Stillwater and other subparallel faults, using laboratory-derived coefficients of friction of 0.6 to 1 .O and estimates of the in-situ formation fluid pressure. |