Stress-strain response and seismic signature analysis of phyllite reservoir rocks from Blue Mountain geothermal field

Geothermal energy is the heat contained in rock and fluid in the earth's crust. In some areas the geothermal resource can be exploited by drilling wells to extract the heated fluids and piping the fluids to a power plant to generate renewable electricity. Geomechanical characterization of the rocks in geothermal reservoirs is necessary to (i) optimize a well stimulation program, (ii) reduce production decline, and (iii) build more accurate predictive models at reservoir scale. In this study, we present the results of a suite of experiments on mechanical characterization of phyllite rock specimens retrieved from DB-2 well (depth of 1260 m) at the Blue Mountain geothermal field in Nevada, United States. In order to investigate the effects of anisotropy on mechanical response of rock, one vertically-drilled and one horizontally-drilled sub-cores were extracted from the phyllite core. Multi-stage elastic, cyclic, creep, and multi-stage failure tests were performed to characterize the elastic, plastic, time-dependent, and failure properties as well as effects of anisotropy on the behavior of the phyllite specimens. The pressure-dependent static and dynamic moduli revealed that within the elastic region, the Young's modulus increased with the increment of differential stress during the loading stage. While, 10% to 50% difference between the elastic moduli of the two specimens confirmed the anisotropic nature of the material. Negligible creep response was observed in both specimens, with a more pronounced change in P- and S-waves velocities during hydrostatic stage for vertically-drilled sub-core. The failure envelope, developed using the multi -stage failure test on the vertically-drilled specimen, indicated good agreement with both Linearized Mohr Coulomb and Hoek-Brown failure criteria. On the other hand, significant changes were observed in the time frequency maps of the transmitted seismic waves during multi-stage elastic test (i.e. change in confining level), however, slight changes in the time-frequency map were observed during creep test (i.e. application of differential stress for a relatively long period).