The EGS Collab Project, sponsored by the U.S. DOE Geothermal Technologies Office, is performing simulations and stimulations in a deep underground laboratory to increase the understanding needed to efficiently implement enhanced geothermal systems (EGS). In Experiment 1, we created an underground test bed at the Sanford Underground Research Facility (SURF) in Lead, SD at a depth of approximately 1.5 km to examine hydraulic fracturing in crystalline rock. Our host rock at this location – phyllite – was well-characterized using numerous field-based geophysical and geological techniques, and laboratory testing. We densely instrumented the test bed, consisting of an injection well, a production well, and six monitoring wells, to allow careful monitoring of stimulation events, and performed long-term flow tests. We performed long-term ambient temperature water injection tests, and as an analog to EGS we performed chilled-water injection tests. System changes resulting from these water injections were monitored using geophysical techniques, flow and pressure measurements, tracer tests, and changes in microbiology. Numerical simulations have been key in providing guidance to experiment design questions, to forecast fracture propagation trajectories and extents, and to interpret processes from the measurements. Hydraulic stimulations were effective at connecting our injection and production wells via hydraulic and natural fractures. Several monitoring techniques showed residual displacements and changes in the fracture network over the course of the tests. Injection pressures required to maintain a constant injection rate rose over the duration of the tests; however, sharp pressure declines resulting from even momentary flow disruptions resulted in less pressure required to flow water at the same rate. Thermoelastic effects were clearly observed when temperature changes in the injected water occurred. The testbed for Experiment 2 is currently being prepared at a depth of about 1.25 km at SURF, and is aimed at investigating shear stimulation. The second testbed is in amphibolite and is subjected to different stress conditions than those in Experiment 1. We have drilled a horizontal and a vertical borehole to aid in testbed understanding, and have performed a number of measurements to characterize the test bed. These include borehole logging, 18 stress tests including 10 using the SIMFIP tool, and extensive fracture mapping of the drift walls and the boreholes. Numerical simulation has been used to forecast fracture propagation trajectories considering the uncertainty in the stress orientations and magnitudes.

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