IN-SITU STRESS AND FRACTURE CHARACTERIZATION FOR PLANNING OF AN EGS STIMULATION IN THE DESERT PEAK GEOTHERMAL FIELD, NEVADA

An integrated study of natural fracture geometry, fluid flow and stress was conducted in Desert Peak well 27-15 in preparation for development of an Enhanced Geothermal System (EGS) through hydraulic stimulation. This stimulation will be carried out at depths of ~3000 to 3500 ft in units comprised of silicified rhyolite tuffs and metamorphosed mudstones at ambient temperatures of ~180 to 195° C. Our previous analyses of borehole image logs from this well showed that the current minimum horizontal principal stress, Shmin, is oriented 114 ± 17o and that numerous fractures in the planned stimulation interval are optimally oriented for normal faulting. As an extension of this earlier work, a hydraulic fracturing stress measurement was conducted at the top of the intended stimulation interval and indicates that the magnitude of Shmin is 1995 ± 60 psi, which is ~0.61 of the calculated vertical (overburden) stress at this depth. This Shmin magnitude is somewhat higher than expected for frictional failure on optimally oriented normal faults under current reservoir pressures given typical laboratory measurements of sliding friction (Byerlee’s Law). However, Coulomb failure calculations using coefficients of friction derived from laboratory tests on representative core samples from a nearby well (Lutz et al., 2010) indicate that shear failure could be induced on well-oriented preexisting fractures in well 27-15 once fluid pressures are increased by several hundred psi above the ambient formation fluid pressure. This geomechanical model will be tested during hydraulic stimulation of well 27-15, which is intended to enhance formation permeability through selfpropping shear failure. If this stimulation is successful, then preferential activation of normal faults should generate a zone of enhanced permeability propagating to the SSW, in the direction of nearby geothermal injection and production wells, and to the NNE, into an unexploited portion of the field.

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