| Title | Borehole seismic study in a geothermal site at Utah FORGE, USA |
|---|---|
| Authors | Yingping Li, David Li, Lianjie Huang, Yingcai Zheng, Joseph Moore |
| Year | 2023 |
| Conference | World Geothermal Congress |
| Keywords | Enhanced Geothermal Systems, Fracture Stimulations, Micro-seismic, Distributed Acoustic Sensing, Geophones |
| Abstract | Geothermal energy is a clean, renewable resource that can be harvested for heating and electricity. EGS (Enhanced Geothermal Systems) technologies enhance and create geothermal resources by a variety of stimulation methods. FORGE stands for Frontier Observatory for Research in Geothermal Energy. The Utah FORGE geothermal field laboratory is one of such FORGE sites (Moore et al, 2020). Between 4/19-5/3, 2019, during the 2C phase of the Utah FORGE project, a fiber-optic DAS (Distributed Acoustic Sensing) system and twelve three-component (3C) geophones were installed in observation well 78-32 to monitor micro-seismic events induced by three-stage stimulation cycles in treatment well 58-32 (Pankow et al, 2020). The durable nature of optical fibers in a harsh, high temperature and pressure environment, have allowed DAS systems to become favored down-hole tools for geoscientists to explore and characterize geothermal resources and monitor geothermal reservoirs (Li et al, 2021). The continuous recording of geophone and DAS waveforms in the monitoring well provides a unique opportunity to get underground media properties. We apply a shear-wave splitting analysis method based on eigenvector rotation to the induced micro-earthquakes in stage 1 of stimulation cycles recorded using a 3C geophone array with a sensor spacing of 30 m deployed in a borehole. We find that the faster S1-wave is along the radial direction at about an azimuth of E12.7oS, while the slower S2-wave is in the tangential direction perpendicular to that of S1-wave. This radial direction is consistent with major-semi axis orientation of induced micro-earthquake distribution. The averaged S-wave splitting rate (SSR) is 0.83%, indicating the average S1-wave velocity is faster than that of S2-wave by 0.83%. We will apply this method to seismic events occurring in stages 2 and 3 of the stimulation cycles to examine any variations of SSR in a time-lapse manner. A 1-km long optic fiber was installed in well 78-32 to record DAS data. The DAS recording system has a channel spacing of 1 meter, gauge length of 10 m, and time sampling rate of 0.5 ms. The co-located DAS system and geophone array allow us to calibrate and compare the DAS waveforms with those recorded by geophones. We compare DAS recordings with the 3C rotated geophone waveforms to examine the similarities and differences of signatures of P and S waves. Time- and frequency-domain cross-correlations will be used to pick first P and S wave arrivals for both DAS and geophone data, and a relative event location method will result in relative positions of the micro-seismic events. We expect the relocation results will better delineate the geometry of fracture zones. The relocations of seismic events should help us to identify co-located seismic event pairs, and we can apply an empirical green function method to retrieve relative source time functions for source rupture properties of larger events. We anticipate that results from anisotropic analysis, relative locations and source time functions will lead to better understanding of geological formation properties and seismic source properties of induced earthquakes by fracture stimulations in the EGS site. |