Title: |
Anisotropic Imaging of Created Fractures in EGS Collab Experiments Using CASSM Data |
Authors: |
Benxin CHI, Lianjie HUANG, Kai GAO, Jonathan AJO-FRANKLIN, Timothy KNEAFSEY and EGS Collab Team |
Key Words: |
Anisotropic, fracture imaging, seismic monitoring, elastic-waveform inversion |
Conference: |
Stanford Geothermal Workshop |
Year: |
2020 |
Session: |
EGS Collab |
Language: |
English |
Paper Number: |
Chi |
File Size: |
1294 KB |
View File: |
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A multi-level continuous active source seismic monitoring (ML-CASSM) system is used to monitor the fracture creation at the EGS Collab project testbeds located at the Sanford Underground Research Facility (SURF). Four fracture-parallel and two orthogonal wells with 24 hydrophones, 18 accelerometers and 17 dipole sources are used to acquire CASSM data during the stimulations in the first testbed. We recently obtained fracture images using elastic-waveform inversion and least-squares reverse-time migration in isotropic media. However, both core sample analyses and traveltime tomography of the campaign cross-borehole seismic data acquired from the site indicate that the host rock is anisotropic. To improve fracture imaging using CASSM data, we employ elastic-waveform inversion and least-squares reverse-time migration methods in anisotropic media instead of isotropic media. We first apply 3D anisotropic traveltime tomography and anisotropic elastic-waveform inversion to the CASSM data to obtain high-resolution elastic parameters within the stimulation region. To improve the robustness of elastic parameters inversion, we use the traveltime tomography results of the campaign cross-borehole seismic data as the initial models, and employ a multi-scale strategy during anisotropic elastic-waveform inversion. We analyze time-lapse CASSM data to detect waveform variations caused by the fracture creation during the stimulations. We finally apply 3D least-squares anisotropic elastic reverse-time migration to the time-lapse waveform differences to produce high-resolution fracture images. Our results of elastic parameters inversion indicate that the rock within the stimulation region is anisotropic and heterogeneous. Our least-squares reverse-time migration images reveal the shape and the location of the fractures created by hydraulic stimulations. Our results demonstrate that using ML-CASSM, anisotropic elastic-waveform inversion and least-squares anisotropic elastic reverse-time migration has great potential for imaging stimulation-created fractures.
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