Mapping Fracture Zones with Nodal Geophone Patches: Insights from Induced Microseismicity During the 2024 Stimulations at Utah FORGE

Abstract The Utah Frontier Observatory for Research in Geothermal Energy (FORGE) is a field-scale enhanced geothermal system (EGS) laboratory for studying new techniques and strategies to overcome problems that have halted or hindered the broad application of EGS, including induced seismic activity and the cost-efficient monitoring of EGS reservoirs. During the latest stimulation experiments in April 2024, we deployed a network of geophone patches to improve the (near-) surface monitoring network. These patches consist of nine closely spaced nodal geophones. By stacking the waveforms within the patches, we obtain signal-to-noise ratios similar or superior to permanent seismic stations installed in 30 m deep boreholes or at the surface. That demonstrates that a patch setup is a valuable low-cost option that temporarily improves the monitoring of induced or natural seismic activity. Using only the (near-) surface networks, we map the growth of two main fracture zones in great detail during the seven-stage stimulation of the injection well and the subsequent stimulation of the production well. We provide the first revised reference catalog for the April 2024 simulations at Utah FORGE with high-quality relative relocations calibrated via previous high-resolution absolute downhole locations. The microseismic data imply that the fracture zones constitute at least two isolated features that connect the two wells, forming efficient pathways for fluid to circulate between the injector and the producer.