Lawrence Berkeley National Laboratory

The EGS Collab project performed well-monitored rock stimulation and flow tests at the 10-m scale in an underground research laboratory to inform challenges in implementing enhanced geothermal system (EGS) technology. This project, supported by the US Department of Energy, gathered data and observations from the field tests and compared these to simulation results to understand processes and to build confidence in numerical modeling of the processes. The project consisted of 3 Experiments, each comprising test and testbed design, many individual tests, numerical simulation, and analysis. The Experiments were performed in two deep underground testbeds at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. Field experiments are now complete, significant data sets have been collected and analyzed, and some analysis continues. Experiments using underground test facilities have many advantages in that they allow: • Three-dimensional characterization of the stimulated volume by complementary geophysical methods surrounding the experiment • Using techniques that are currently not applicable under geothermal condition to provide processes insight • Comprehensive tracer testing and detailed characterization of complex fluid movements • Understanding the geometry of the stimulated network at the meso-scale and its implications for effective fracture surface area, rock block size, and heat exchange. Underground testing has its own set of complications, however, which affect the ability to perform tests as desired and affect the experiment results. Included here are the inability to flow at desired injection rates due to stress gradients caused by drift cooling, and the need to strongly limit induced seismicity because the distance to people and equipment. Experiment 1 examined hydraulic fracturing at a depth of 1.5 km in a well-characterized phyllite. Eight subhorizontal boreholes were used in this Experiment. Geophysical monitoring instrumentation was deployed in six boreholes 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 in the testbed were performed. Detailed observations of processes occurring during stimulation and dynamic flow tests were collected and analyzed. Flow tests using ambient-temperature and chilled water 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 was intended to examine hydraulic shearing of natural fractures at a depth of 1.25 km in amphibolite. The stresses, rock type, and fracture conditions are different than in Experiment 1. The testbed consists of 9 boreholes, in addition to 2 exploratory characterization boreholes. Four boreholes drilled as two fans of 2 monitoring holes contained grouted-in monitoring sensors. The remaining five open boreholes drilled in a five-spot pattern were adaptively used for injection, production, and monitoring. Approximately five fracture set orientations were encountered in the testbed along with a low-stress rhyolite sill at 35 m below the access drift in exploratory well TV4100. The testbed was designed to optimize the potential for shear stimulation while also avoiding the low-stress rhyolite. Experiment 2 focused on stimulating a fracture in the most likely orientation to shear, however shear stimulation did not occur probably due to cementation from natural secondary mineralization. Other fracture sets encountered were also cemented and had orientations less likely to shear. Experiment 3 consisted of several stimulations in the same testbed as Experiment 2 allowing different stimulation approaches including ramped-rate injection, rapid injection, and oscillating-pressure injection. Ultimately these methods created hydraulic fractures, one of which was used for a medium-duration cold water injection test. The major findings of the EGS Collab Project include: 1. Significant shear stimulation did not occur during our stimulation attempts. Shear stimulation may occur but under a limited set of conditions not encountered. 2. Our stimulations resulted in hydraulic fractures that required hydraulic propping. Pumping at pressures exceeding the minimum principal stress may not be feasible in an enhanced geothermal system. 3. The systems we generated were complex hydraulic fracture/natural fracture systems, and these systems changed over time in response to applied pressures and flowrates and to unknown stimuli. 4. The project attempted alternative stimulation methods, which did not provide significant flow improvement. 5. Thermal breakthrough was not achieved as designed, most likely because flow to production boreholes was not adequate. 6. The combination of geophysical tools used provided excellent understanding of many important processes. 7. Microearthquakes (MEQs) didn’t necessarily identify flow paths. 8. Engineering tools bounding expected seismicity are needed. This report provides a summary of tests and analyses performed for EGS Collab Experiment 2 (Shear Stimulation in Testbed 2) and Experiment 3 (Alternative Stimulation methods). Much of the EGS Collab work has been published in journals and conference papers, presented in conferences, included in written reports, and submitted in data sets to the Geothermal Data Repository (GDR). The entirety of these written works is included as an appendix to this report, and this report serves as a summary and framework pointing to these published papers, presentations, and reports.