| Title | Seismic Analysis of Spatio-Temporal Fracture Generation at the Geysers EGS Demonstration Project |
|---|---|
| Authors | Douglas S. DREGER, O. Sierra BOYD, Roland GRITTO |
| Year | 2018 |
| Conference | Stanford Geothermal Workshop |
| Keywords | EGS development, fracture characterization, moment tensor analysis, stress state and temporal changes, fluid saturation and temporal changes |
| Abstract | We analyzed seismicity near the EGS development at The Geysers, CA Prati-32 injection well to evaluate the development and the physical attributes of a subsurface fracture network. The goals of our study include the ability to estimate the orientation and the activated fracture area, to estimate stress orientation and stress orientation changes as well as fluid saturation and spatio-temporal changes in fluid saturation. We compiled a 168-event waveform-based seismic moment tensor catalog for events 0.7≤Mw≤3.9 based on a semi-automated moment tensor analysis technique to be used for in-situ stress estimation during the injection phases. We found an approximate 15-degree counterclockwise rotation of the least compressive stress σ3, and a rotation of the maximum compressive stress σ1 toward the vertical as the injected volume of water increased. We developed a rupture-area magnitude scaling relationship of The Geysers earthquakes obtained from finite-source inversion for fault slip that reveals a very high correlation to published results. Based on the rupture area-magnitude relationship we mapped Mw from the LBNL earthquake catalog to estimate the activated fracture area and its location within the injection volume of Prati-32. Analysis of the Vp/Vs-ratio based on the double-difference Wadati technique revealed the development of an injection-derived steam plume during the early injection phase, particularly in the northern parts of the Prati-32 study area. |