In preparation for well stimulation activities and the development of an enhanced geothermal system (EGS) in the Desert Peak geothermal field, a series of petrologic and rock mechanical tests were conducted on selected core samples to represent the proposed stimulation interval within Well 27-15. The stimulation interval (3000-3500 ft; 930-1085 m) consists of Tertiary rhyolite tuffs that overlie metamorphic basement rocks consisting of siliceous argillite and other fractured metasedimentary rocks. Petrographic and X-ray diffraction mineralogic analyses indicate that the rhyolitic rocks are variably devitrified, argillaceous, and siliceous; and host minor amounts of quartz-dolomite-calcite veins in siliceous units along the basement contact. Hydrothermal clays are dominated by expandable smectite-rich mixed-layer illite-smectite.

Deeper in the metamorphic basement, young fractures are only partially mineralized with dolomite, siderite, calcite, and barite. The latter represent lightly permeable open fractures identified by a suite of acoustic and electrical image logs and and pressure-temperature-spinner flowmeter surveys collected in this interval of the well. 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 fractures.

Rock mechanical testing was conducted on the core samples 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. The results of the laboratory tests were
used to construct Mohr-Coulomb failure envelopes for the proposed reservoir rocks. Pre- and post- test

measurements on the deformed core plugs indicate up to a 20-fold enhancement in permeability in the originally tight rhyolite units as a result of shearing. Assuming that failure occurs on the same structural features in the well as in the core, these laboratory studies directly test the shear dilation concept in these clay-rich rocks, and are being used in combination with borehole stress measurements and fracture logging to predict fluid pressures required for initiation of shear dilation and permeability development within the geothermal reservoir.

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