The EGS (Enhanced Geothermal Systems) Collab project is performing stimulation experiments in highly monitored and well-characterized intermediate-scale (~10-20 m) field test beds. The characterization and monitoring system includes an array of electrodes grouted in place within six monitoring wells, with 16 electrodes per well. The electrodes enable characterization of the 3D low-frequency electrical properties of the host rock, and changes in those properties during stimulation and tracer testing using both static and time-lapse Electrical Resistivity Tomography. In this talk we present: 1) ERT array design and installation, 2) baseline (pre-stimulation) imaging results, 3) time-lapse data collected during stimulation operations, and 4) real-time imaging during flow testing. We will also discuss aspects and lessons learned for conducting ERT imaging operations in the presence of high fluid pressure gradients during stimulation and flow operations. Results to date reveal a highly heterogeneous rock fabric, ranging over four orders of magnitude in bulk electrical conductivity. Baseline imaging results reveal the test bed is located within a folded and dipping system of alternating high and low conductivity layers, and are consistent with borehole logs and a discrete fracture map generated from detailed inspection of oriented cores. The raw ERT data show remarkable sensitivity to flow and pressure fluctuations during testing. Time-lapse imaging shows flow initiated in a stimulated fracture enters and is predominantly governed by the natural fracture system. Ultimately the ERT imaging data sets provide crtical information for validating and verifying model predictions in preparation for research at DOE’s Frontier Observatory for Research in Geothermal Energy (FORGE).

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