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Circulation Experiments at Utah Forge: Post-Shut-in Fracture Growth Revealed by Limited Near-Surface Monitoring

The Utah Frontier Observatory for Research in Geothermal Energy (FORGE) is a large-scale experimental facility de-risking Enhanced Geothermal Systems (EGS). Full-scale testing allows the study and development of new tools, methodologies, and strategies to improve the economic viability of EGS and mitigate induced seismic activity. After the completion of an injection-production well doublet at a depth of 2.5 km within basement rock, four circulation experiments were conducted in July 2023. These experiments showed that the two wells were successfully connected via the fracture network created during the 2022 stimulation treatments, which were pumped to evaluate different injection methods required to form a heat exchange system.

We analyze the microseismic activity induced during the circulation experiments using only five (near-)surface stations of the regional permanent seismic network and state-of-the-art full-waveform-based methods for detection and location. The microseismicity rates and magnitudes (<M0.45) induced during the circulation experiments are similar to those observed during the 2022 stimulations. This was unexpected when considering fluid flow through pre-stimulated rock volumes. During the circulation experiments, flow data indicated that highly productive reservoir volumes were both aseismic and seismically active. The seismic activity occurred predominantly after shut-in of the last two circulation stages. It is attributed to the complex dynamics of opening, further propagating, and closing of hydraulic fractures during circulation in the absence of major conductive features that allow for substantial flow into the production well. The complexity of the fracture network created within a granitoid rock, such as that at FORGE, can result in poorly stimulated volumes within the zones that are 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.

Session: Seismic Monitoring, Modelling and Management Needed for Geothermal Energy and Geologic Carbon Storage - II

Type: Oral

Room: Tikahtnu Ballroom A

Date: 5/2/2024

Presentation Time: 11:30 AM (local time)

Presenting Author: Peter Niemz

Student Presenter: No

Additional Authors