In this study, we present the results of microseismic obtained from the stimulation and crossflow activities conducted on two horizontal geothermal wells. Our monitoring approach involved the installation of seismic sensors in both surface and shallow borehole locations, enabling real-time monitoring of seismic activity. Notably, sensors placed on the surface of hard rock formation outcrops exhibited superior signal-to-noise ratios compared to those in shallow boreholes, attributed to the presence of unconsolidated alluvial sands in the shallow subsurface across most of the site. Furthermore, we measured downhole microseismic events using fiber optic cables permanently integrated into four wells. This multi-well fiber optic microseismic acquisition successfully detected approximately 50,000 microseismic events over a three-month monitoring period, a significant increase in detectability compared to the roughly 1,500 detections from the surface and shallow borehole network. However, a challenge in the fiber optic cable acquisition was fiber loss during stimulation due to erosion in poorly cemented areas. The magnitude of events ranged from -2 to 1.5, with the majority falling below 0.5, confidently detected via the fiber optic system. The largest seismic event detected by the surface network registered at magnitude M=1.8 Ml. Importantly, all seismic events remained below the green category threshold established by our Traffic Light System. As a result, no operational changes were required or warranted through all phases of the project to-date, including drilling, stimulation, and well testing. Microseismic events during crossflow remained confined within the stimulated rock volume, exhibiting a direct correlation with reservoir pressure measured by a downhole gauge. This strongly suggests confined crossflow dynamics. Analyzing azimuth data for individual stage events reaffirmed the NE-SW local stress orientation, consistent with the local stress field orientations derived from borehole image log data. In the vertical dimension, microseismic events were symmetrically distributed approximately 300 feet above and below the lateral during the stimulation of the first well. This alignment closely correlated with low-frequency strain rate data from fiber optic observations, indicating fractures extending about 300-500 feet shallower than the stimulation well. Our findings underscore the successful implementation of fiber-based multi-well distributed acoustic sensing (DAS) microseismic monitoring in geothermal fields. Fiber-based microseismic event locations offer insights into crucial fracture geometry parameters, such as fracture orientation, length, and height, as well as microseismic diffusion rates. The absence of reliable three-component borehole tools for operations above 200 °C further underscores the importance of fiber-optic-based microseismic measurements. Enhancements in event resolution and accuracy remain feasible with the incorporation of three-component borehole measurements.

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