Lithologies, Hydrothermal Alteration, and Rock Mechanical Properties in Wells 15-12 and BCH-3, Bradys Hot Springs Geothermal Field, Nevada

In the Bradys Hot Springs geothermal field of northwestern md) for the rhyolites. The metamorphic samples have less than 2% porosity and negligible permeability. Rock mechanical tests on the core were conducted to determine mechanical properties of the various lithologies including: radial versus axial volumetric strain, stress-strain relationships, dynamic versus static Young's moduli, and frictional strengths and failure responses under a vari- ety of confining conditions and temperatures. The mechanical test results indicate moderately high rock strengths; with unconfined compressive strength estimates of 240-275 MPa for the more siliceous lithologies, and 70-194 MPa for argillically-altered rhyolites and chloritic metavolcanic rocks. Effective compres- sive strengths range from 220 Mpa at 1 MPa confining pressure for crystal-supported rhyolite, and 37 MPa at 1 MPa confining pressure for sericitized metamorphic rocks at the top of the base- ment, up to 304 MPa at 20 MPa confining pressure for pebbly meta-volcaniclastic rocks. At temperatures of 200°C, quasi-static values for Young's moduli range from 48 to 55 GPa (in crystal- rich rhyolite tuffs) up to 60-76 GPa (in the metamorphic rocks); Poisson's ratios range from 0.10 to 0.29. The results of the laboratory tests were used to construct Mohr-Coulomb failure envelopes for the proposed reservoir rocks, and to evaluate the propensity for frictional failure along natural fractures in the open-hole interval of Well 15-12. At 200°C, slid- ing friction angles for the residual effective compressive strength range from 29.1° to 43.7°. Corresponding coefficients of sliding friction values range from 0.56 in chlorite-altered meta-basalt, up to 0.98 in clast-supported meta-volcaniclastic rocks. The meta- morphic rocks in the open-hole interval of 15-12 have abundant veins, clayey shear planes, and other planar features that may be amenable to shear stimulation under higher wellhead fluid pres- sures. Hydraulic stimulation of the well is intended to enhance formation permeability through self-propping shear failure along the most optimally oriented and critically stressed of these pre- existing features.