Distributed fiber optic acoustic sensors (DAS) installed in boreholes have provided a new and data-rich perspective on fracturing processes and microseismicity created by stimulation. The use of fiber optic sensors is increasing in the oil and gas industry but less so at geothermal sites. These sensors measure strain (or strain rate) with high spatial resolution (~ 1 m) along the fiber and can survive extreme conditions. Here, we explore the information that these sensors may reveal in an Enhanced Geothermal System (EGS) system, which includes both low-frequency signals associated with fracture opening and high-frequency microseismic signals. As a test case, we use parameters from the FORGE site in Milford, Utah, which is expected to create a reservoir at a depth of roughly 2 km in a crystalline granite formation with temperatures of more than 175°C. The subsurface signals are simulated in two ways: 1) a massively parallel multi-physics code that is capable of modeling hydraulic stimulation of a reservoir with a pre-existing discrete fracture network, and 2) a parallelized seismic wave propagation code for high-frequency seismic signals created by microseismic activity. The objective is to understand how fracture geometry could be constrained with fiber optic sensors located both in the main borehole and in nearby monitoring boreholes, and how well microseismic events could be characterized in terms of locations and moment tensors.

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