The EGS Collab – Discoveries and Lessons from an Underground Experiment Series

Abstract

The EGS Collab project performed densely-monitored rock stimulation and flow tests at the 10-m scale in the Sanford Underground Research Facility (SURF) in Lead, South Dakota to inform challenges in implementing enhanced geothermal systems (EGS). This project, supported by the US Department of Energy, gathered data and observations from the field tests to understand processes and to build confidence in numerical modeling of the processes. The project was organized into three sets of field experiments. Experiment 1 examined hydraulic fracturing at a depth of approximately 1.5 km in a well-characterized phyllite. Geophysical monitoring instrumentation installed in six of eight sub-horizontal boreholes was used to monitor stimulation events and flow tests. The other two boreholes were used to perform and carefully measure water injection and production. More than a dozen stimulations and nearly one year of flow tests were performed. The stimulation and dynamic flow tests allowed for the collection and analysis of detailed observations of processes. Flow tests of ambient temperature and chilled water injections were performed with intermittent tracer tests to examine system behavior. We achieved adaptive control of the tests using close monitoring of rapidly disseminated data and near-real-time simulation. Numerical simulation was critical in answering key experimental design questions, forecasting fracture behavior, and analyzing results. We were successful in performing many simulations in near-real-time in conjunction with the field experiments, with more detailed simulations performed later. Experiment 2 examined hydraulic shearing in an amphibolite test bed at SURF at a depth of about 1.25 km with stress and fracture conditions that are different from Experiment 1. Approximately five fracture set orientations were encountered and the testbed was designed accordingly to maximize the likelihood of shear stimulation. The testbed, designed for primarily remote operation, consisted of nine boreholes, in addition to two earlier-drilled characterization boreholes. Four boreholes contained grouted-in geophysical instruments, and the other five open boreholes were adaptively used for injection, production, and geophysical monitoring. Experiment 3 was performed in the same test bed as Experiment 2 and consisted of investigating various stimulation strategies, ultimately resulting in connecting the injection and production boreholes with hydraulic fractures. After creating a flow-through system, ambient-temperature and chilled water flow tests were performed with intermittent tracer tests to help understand flow and heat transfer in the system. Numerical simulations were used to design each of the experiments and in turn were validated using the comprehensive monitoring datasets that were collected during each of the field experiments.