| Title | Rock Mechanical Testing in Support of Well Stimulation Activities at the Desert Peak Geothermal Field, Nevada |
|---|---|
| Authors | Lutz, Susan Juch; Hickman, Stephen; Davatzes, Nicholas; Zemach, Ezra; Drakos, Peter; Robertson-Tait, Ann |
| Year | 2010 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Rock mechanics; Laboratory tests; Confining conditions; Frictional strength; Failure envelopes; Mohr-Coulomb; Shear dilation; Enhanced geothermal system |
| Abstract | 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 representative of the planned stimulation interval within Well 27-15. The interval consists of Tertiary rhyolite tuffs that overlie metamorphic basement rocks consisting of fractured metasedimentary rocks. 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 pre-existing fractures. Rock mechanical testing was conducted on 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. Comparison of the test results indicate overall moderate rock strengths; with unconfined compression stress estimates of 117–186 MPa for the more siliceous lithologies, and about 82–110 MPa for argillaceous and devitrified rhyolites. Quasi-static values for Young’s modulus and Poisson’s ratio ranged from 6538 MPa (in argillaceous rhyolites) to 41,700 MPa (in siliceous metamudstones), and 0.08 to 0.24, respectively. Residual compressive strength measurements were used to evaluate the propensity for frictional failure along natural fractures seen in the stimulation interval within Well 27-15. In the test samples, residual friction angles from constructed Mohr circles determined coefficients of sliding friction (?) in the range of 0.66 to 0.96. Pre- and post- test measurements on the core plugs indicate up to a 20-fold permeability enhancement in originally tight rhyolite units as a result of shear failure. 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. |