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Enhanced geothermal systems (EGS) play a crucial role in the shift from fossil to renewable energy. The Utah Frontier Observatory for Research in Geothermal Energy (FORGE) is a large-scale experimental EGS that aims to overcome setbacks faced by EGS projects, such as economic viability and concerns about induced seismic activity, by studying new tools and strategies. Four circulation experiments were conducted after the completion of the well doublet at a depth of 2.5 km in granitoid rock. The circulation experiments showed that the injection well and the production well were successfully connected via the fracture network that was created during the 2022 stimulation treatments pumped in the injection well. Still, the flow during the circulations was relatively low. We analyze the microseismic activity induced during the circulation experiments using only five (near-)surface stations of the permanent seismic network. The circulation-induced microseismic rates and magnitudes (<M0.45) are similar to those observed for the stimulations one year before, which is unexpected for fluid flow through pre-stimulated rock volumes. We image the complex seismic response of the reservoir during the circulation by using full-waveform-based analyses for event detection and relative relocations. We apply the same workflow to the stimulations in 2022 and the circulation in 2023 and calibrate the event locations of the stimulations with high-quality downhole locations. This approach allows obtaining reliable event locations for the circulation experiments, overcoming the reduced resolution of the permanent seismic network compared to downhole monitoring systems. Consequently, we can differentiate between aseismic volumes and seismically highly productive volumes. Our study shows that due to the complexity of the fracture network within the granitoid rock, poorly stimulated volumes may exist within zones mapped by large-scale microseismic clouds. Hence, the presence of microseismic activity may not guarantee the presence of stimulated fractures that can enable efficient fluid flow, and the lack of microseismic activity does not imply the lack of stimulated fractures. The microseismic activity was induced predominately after the shut-in, which we attribute to the complex dynamics of opening, propagating, and closing hydraulic fractures in the absence of major conductive features that allow for substantial flow into the production well.
Niemz et al. (Thu,) studied this question.