Title:

Automatic Moment Tensor Analyses, In-Situ Stress Estimation and Temporal Stress Changes at the Geysers EGS Demonstration Project

Authors:

Douglas S. DREGER, O. Sierra BOYD, Roland GRITTO

Key Words:

EGS development, fracture characterization, moment tensor analysis, stress state and temporal changes

Conference:

Stanford Geothermal Workshop

Year:

2017

Session:

Enhanced Geothermal Systems

Language:

English

Paper Number:

Dreger

File Size:

1951 KB

View File:

Abstract:

We investigate seismicity in the vicinity of the EGS development at The Geysers Prati-32 injection well. The goals of our study include the ability to estimate the activated fracture area and volume, to estimate stress drop and stress drop changes as well as fluid saturation and temporal changes in fluid distribution. In this study, we have developed an automated moment tensor analysis technique applicable to earthquakes M≥1.0 to obtain sufficient earthquake source parameters for subsequent in-situ stress estimation and temporal changes during the injection phases. The result of our effort has been the compilation of a 168-event waveform-based seismic moment tensor catalog for events ranging in moment magnitude from 0.7 to 3.7. The moment tensor catalog was subsequently used to invert for the stress tensor, and to investigate possible temporal changes resulting from the fluid injection. The results demonstrate the quality of the seismic moment tensor catalog through the relatively small uncertainties in the recovered stress tensors. We find that there is 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. The magnitude of these rotations is consistent with other nearby empirical observations (Martinez-Garzon et al., 2013) and thermo-hydromechanical simulation results (Jeanne et al., 2015). We find that there is a systematic reduction in the stress shape factor, R, as injected volume increases, indicating an evolution toward a more tensile stress state.


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