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

Date: 1/1/2011


In the Bradys Hot Springs geothermal field of northwestern Nevada, Well 15-12 has been selected as a candidate well for mechanical and chemical stimulation as part of an enhanced geothermal system (EGS) demonstration project (U.S. Department of Energy project with Ormat Nevada and GeothermEx). Well 15-12 has recently been drilled and logged, and is an attractive candidate for EGS work with downhole temperatures of 400°F (204°C) in metamorphic basement rocks. This non-commercial well is close to the Bradys Fault and has similar formations as production wells in the northern part of the field, but injection tests show an injectivity index of less than 1 gpm/psi, and no significant circulation losses in the basement from the casing shoe at 4,245 ft to the total depth (5,096 ft). Although no core is available from 15-12, companion core samples are available from the target stimulation interval owing to the presence of a fully cored well (BCH-3) drilled on the 15-12 pad. Preliminary tasks of the EGS program are to characterize the rocks transected by the candidate well with an emphasis on texture and basic lithotypes, alteration and clay mineralogy, the natural fracture and vein system, and geomechanical character of various rock units. To this end, X-ray diffraction (XRD) mineralogical analyses and petrologic analyses of well cuttings from 15-12 and corresponding whole core samples from BCH-3 cores were performed. Core samples from BCH-3 were selected for determination of basic core properties (density, porosity, and permeability) and mechanical testing to determine rock strengths and other elastic properties. These measurements will be used in the design of hydraulic and chemical stimulation activities in Well 15-12. Core analyses on the BCH-3 core samples indicate generally high ambient porosities (8-10%) but low permeabilities (<0.01 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 variety 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 compressive 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 basement, 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 crystalrich 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, sliding 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 metamorphic 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 pressures. 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 preexisting features.

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