| Title | Computational Investigation of Hydro-Mechanical Effects on Transmissivity Evolution During the Initial Injection Phase at the Desert Peak EGS Project, NV |
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| Authors | Stefano BENATO, Donald M. REEVES, Rishi PARASHAR, Nicholas DAVATZES, Steve HICKMAN, Derek ELSWORTH, Paul SPIELMAN, Joshua TARON |
| Year | 2013 |
| Conference | Stanford Geothermal Workshop |
| Keywords | EGS, reservoir stimulation, THMC, simulation |
| Abstract | A low flow rate, low pressure shear-stimulation injection phase of an Engineered Geothermal System (EGS) experiment at Desert Peak produced improved injection rate under constant wellhead conditions consistent with hydraulically-induced mechanical shear failure (Modes II and III) within the rock mass. The observed pressure response is computationally simulated and utilized for calibration of a numerical solution. The latter will be part of a future study that will simulate more complex stimulation phases that account for the roles of tensile dilation (Mode I), thermal and chemical processes. The present study offers a new perspective on the complex hydro-mechanical interactions between injected fluid, the existing natural fracture/fault network and the stress field. We use statistical fracture analysis and hydro-mechanical modeling to define and demonstrate a conceptual framework for the Desert Peak EGS experiment. Discrete network simulations, based on site-specific fracture attributes, are used to derive equivalent permeability tensors of the background fracture networks for comparison with preferred fluid migration directions observed in both hydraulic and tracer tests. FLAC3D, a hydro-mechanical simulator, is used to investigate changes in stress and displacement according to a Mohr-Coulomb frictional model subjected to perturbations in pore pressure. Conditions for shear failure due to pressure and stress field alterations exist in areas of the model consistent with the location of micro-seismicity monitored during the EGS experiment. The calibrated FLAC3D hydro-mechanical model will eventually be coupled to the TOUGHREACT numerical framework to investigate the near-field evolution of reservoir transmissivity associated with thermal, hydraulic, mechanical and chemical (THMC) processes. This approach is valuable in guiding EGS experimental design from beginning to end as it incorporates all of the key mechanisms responsible for both temporary and permanent changes in reservoir permeability that occur during stimulation and subsequent long-term production. |