The EGS Collab Hydrofracture Experiment: Seismic Velocity and Elastic Moduli Characterization

An Enhanced Geothermal System (EGS) allows for the generation of electricity using the Earth\u27s heat by improving (\u27enhancing\u27) the fracture permeability of rock and flowing fluid through the optimized medium. The complex behavior of EGS fracture systems and heat flow processes are being studied at various scales to determine the practical capabilities of EGS technology. The EGS collaborative (Collab) project is focused on experimentation of intermediate-scale (i.e., 10\u27s of meters) EGS reservoir generation processes and model validation at crystalline rock sites. A key phase of the project involves seismic characterization of a rock mass intended to be representative of EGS reservoir rock. A suite of boreholes was drilled from inside a mine drift on the 4850-foot (~1.5 km) level of the Sanford Underground Research Facility (SURF) in Lead, South Dakota. The boreholes, comprised of one stimulation (injection) well, one production (extraction) well, and six monitoring wells, were each nominally drilled approximately 200 feet (~60 meters) deep into the surrounding crystalline rock formation near the location of a previous experiment at this site (kISMET). Active source seismic data were collected using an electrical sparker source and an electro-mechanical impulse source to generate compressional (P-) wave and shear (S-) wave energy, respectively, at varying depths in the stimulation well. Seismic receivers were deployed in the sub-parallel production well, in addition to receivers installed in the monitoring wells, to detect P- and S-wave arrivals. Over the summer, I picked all the P-wave arrivals and helped generate initial tomographic models. The 3D P- and S-wave velocity models associated with these are presented here with a discussion of the elastic parameters they imply. The rock is found to be more complicated and heterogeneous than expected. Velocity and calculated elastic moduli values are reasonable for crystalline rock. These elastic parameters are used for modeling and monitoring seismic hypocenters that are associated with fracture propagation during EGS stimulation activities