Implementing enhanced geothermal systems (EGS) will require improvements in understanding stimulation of crystalline rock to create appropriate flow pathways, and the ability to effectively simulate both the stimulation and the flow and transport processes in the resulting fracture network. The US Department of Energy (DOE) is addressing these and other challenges at multiple scales. The EGS Collab project, addressed here, is performing tests and modeling at the 10 m scale. The FORGE project is performing tests at the full reservoir scale. The EGS Collab team created an underground testbed at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, at a depth of approximately 1.5 km to examine hydraulic fracturing (Experiment 1). We are currently building a second testbed at SURF at a depth of about 1.25 km aimed at investigating shear stimulation (Experiment 2). In Experiment 1, we drilled eight boreholes in a well-characterized phyllite and installed geophysical sensors in six of them to create a well-instrumented testbed to allow careful monitoring of stimulation events and flow tests. Numerical simulation was used to answer key experimental design questions, to forecast fracture propagation trajectories and extents, and to analyze and evaluate results both in near-real-time and in detailed process studies. Stimulations performed in this testbed allowed quantification of processes occurring during stimulation and the examination of dynamic flow occurrences. Long-term ambient temperature and chilled water flow tests were performed in addition to many tracer tests to examine system behavior. Our second testbed, targeted at shear stimulation, is currently being built at the SURF Facility at a depth of about 1.25 km in amphibolite under a different set of stress and fracture conditions than Experiment 1.

1. Introduction

Enhanced (or engineered) geothermal systems (EGS) could help support the energy security of the United States. Estimates exceed 500 GWe for the western US, [Williams et al., 2008], and up to an order of magnitude larger [Augustine, 2016] for the whole US. Implementing EGS requires improving (1) the understanding and efficacy of stimulation techniques to allow optimal communication among multiple wells, (2) imaging and monitoring techniques for permeability enhancement and evolution, as well as associated microseismicity, (3) technologies for zonal isolation for multistage stimulations under elevated temperatures, (4) technologies to isolate zones for controlling fast flow paths and control early thermal breakthrough, and (5) scientifically-based long-term EGS reservoir sustainability and management techniques.

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