DAS Moment Tensor Inversion Analysis for Microseismic Events Observed During the 2023 Circulation Tests at Utah FORGE

Abstract

We analyzed moment tensor resolvability for microseismic events recorded with two distributed acoustic sensing (DAS) systems deployed at the Frontier Observatory for Research in Geothermal Energy or Utah FORGE. The seismicity was observed during circulation tests conducted in July 2023 to evaluate the hydraulic connectivity between two lateral wells, an injector well [16A(78)-32] and a production well [16B(78)-32]. Our analysis of condition numbers indicates that a full moment tensor inversion is not well constrained using only the borehole DAS arrays. Nevertheless, after running a bootstrap analysis, we observed that most of the sensitivity is concentrated in the strike and rake of the dislocation angles derived from the moment tensor solutions. The dip and the angle alpha (i.e., deviation from the pure double-couple case) show more consistency. Analysis of the best constrained moment tensor solutions shows microseismicity that activated in oblique-normal and strike-slip mode with predominantly opening angles around 30° and less. The opening component is consistent with stimulation from the hydraulic injection. We expect that microseismic source mechanisms can inform models of fracture distribution and connectivity important to characterize and to understand the performance of the Enhanced Geothermal System at FORGE. For more recent stimulation work at the site, the availability of a third DAS array in a deep vertical well and a dense surface array of geophones can likely be exploited to estimate fully constrained moment tensor solutions.