Lawrence Berkeley National Laboratory

Abstract Mapping fracture networks in Enhanced Geothermal Systems (EGS) is essential for optimizing reservoir performance, yet complex fracture evolution during stimulation remains difficult to resolve. This study examines the evolution of microseismicity and fracture networks during stage 3 of the 2022 EGS stimulation at the Utah Frontier Observatory for Research in Geothermal Energy site. We map the fracture network represented by 20 clusters of seismic events identified by waveform similarities with cross‐correlation. We characterize their geometric properties such as strike, dip, length, and width, and analyze the time evolution of activated fractures. The results reveal a systematic fracture evolution: early activation of pre‐existing natural fractures, complex network development during peak injection, and continued activation of less favorably oriented fractures post‐injection. Magnitude calibration using the Principal Component Analysis of cross‐correlated waveforms improves relative amplitude measurements, refining estimations of the Gutenberg‐Richter b‐values with spatial variations in b‐values suggesting stress re‐distribution across the stimulated area. Analysis of the stress state of selected fractures further shows that fractures requiring higher excess pore pressure primarily activate at the end of injection and post‐injection, highlighting stress transfer due to pore pressure as a dominant triggering mechanism. These findings provide insights into fracture propagation, stress evolution, and seismic hazard assessment in EGS reservoirs. Plain Language Summary Enhanced geothermal systems require injecting fluids underground to create fracture networks that allow heat extraction, but understanding how these fractures develop remains challenging. In this study, we analyzed thousands of small earthquakes (microseismic events) recorded during stage 3 of the 2022 reservoir stimulation at the Utah FORGE site. By grouping microseismic events with similar signals, we identified 20 distinct clusters that trace out the fracture network. We measured the size, orientation, and time activation of these fractures to understand how the network evolved. Our results show that natural fractures were activated first, followed by a more complex network growth during peak injection, and later, less favorably oriented fractures after injection ended. We also improved earthquake magnitude estimates and found that stress conditions vary across different parts of the created reservoir. Importantly, many fractures required extra fluid pressure to slip, especially toward the end of injection, showing that pore pressure and stress transfer drive ongoing activity. These findings improve understanding of how fractures evolve in EGS reservoirs and can help manage both energy production and seismic hazard. Key Points Activation of a complex network of natural pre‐existing and hydraulic fractures during and after stimulation Higher ‐values near the wellbore, lower values at the edges of the seismicity cloud, and spatially variable values in between Fractures that are less optimally oriented in the stress field require a higher excess pore pressure for activation