Monitoring fluid flows without seismicity associated with continuous water injection into geothermal reservoirs

Accurate monitoring of subsurface fluid behavior, particularly temporal variations in fluid migration, is critical for resource development activities, including geothermal energy, shale oil and gas, and carbon dioxide capture and storage (CCS). In Enhanced Geothermal Systems (EGS), comprehending fluid behavior is essential for optimizing injection strategies, improving production efficiency, and ensuring safety during fluid injection operations. This study employs time-lapse seismic travel-time and attenuation tomography, along with hypocenter migration monitoring, to delineate fluid pathways, including aseismic zones. We conducted a case study at the Okuaizu geothermal field, Japan, where artificial recharge tests were conducted from 2015 to 2024 using a dedicated local microseismic monitoring network. By estimating both seismic wave velocity and attenuation structures, we identified macroscopic fluid behavior not associated with seismicity, as well as detailed fluid pathways inferred from hypocenter distributions. The recharged water induced seismicity near the recharge well, and also migrated to broader areas without significant seismic activity. An integrated understanding of fluid flows, both with and without seismicity, could contribute to improvement in resource developments and storage strategies by providing deeper insights into subsurface fluid dynamics.