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Title: Simulations of CO2 injection into fractures and faults for improving their geophysical characterization at EGS sites

Abstract

Here, we propose the use of CO2 in push-pull well tests to improve geophysical identification and characterization of fractures and faults at enhanced geothermal system (EGS) sites. Using TOUGH2/ECO2N, we carried out numerical experiments of push-pull injection-production cycling of CO2 into idealized vertical fractures and faults to produce pressure-saturation-temperature conditions that can be analyzed for their geophysical response. Our results show that there is a strong difference between injection and production mainly because of CO2 buoyancy. While the CO2-plume grows laterally and upward during injection, not all CO2 is recovered during the subsequent production phase. Even under the best conditions for recovery, at least 10% of the volume of the pores still remains filled with CO2. To improve EGS characterization, comparisons can be made of active seismic methods carried out before and after (time lapse mode) CO2 injection into the fracture or fault. Here we find that across the CO2 saturation range, C11 (the normal stiffness in the horizontal direction perpendicular to the fracture plane) varies between maximum and minimum values by about 15%. It reaches a maximum at around 6% gas saturation, decreasing exponentially to a minimum at higher saturations. Finally, our results suggest that CO2 injection can bemore » effectively used to infiltrate fault and fracture zones reaching about optimal saturation values in order to enhance seismic imaging at EGS sites.« less

Authors:
 [1];  [1];  [2];  [1];  [1];  [1];  [3]
+ Show Author Affiliations
  1. Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
  2. University of Louisiana, Lafayette, LA (United States)
  3. Schlumberger-Doll Research, Cambridge, MA (United States)
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Fossil Energy (FE); USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Geothermal Technologies Office; Environmental Design Research Association (EDRA)
OSTI Identifier:
1476533
Alternate Identifier(s):
OSTI ID: 1550069
Grant/Contract Number:  
AC02-05CH11231
Resource Type:
Accepted Manuscript
Journal Name:
Geothermics
Additional Journal Information:
Journal Volume: 69; Journal Issue: C; Journal ID: ISSN 0375-6505
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; CO2 injection; faults characterization; faults imaging; S=seismic imaging; EGS

Citation Formats

Borgia, Andrea, Oldenburg, Curtis M., Zhang, Rui, Pan, Lehua, Daley, Thomas M., Finsterle, Stefan, and Ramakrishnan, T. S. Simulations of CO2 injection into fractures and faults for improving their geophysical characterization at EGS sites. United States: N. p., 2017. Web. doi:10.1016/j.geothermics.2017.05.002.
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Borgia, Andrea, Oldenburg, Curtis M., Zhang, Rui, Pan, Lehua, Daley, Thomas M., Finsterle, Stefan, & Ramakrishnan, T. S. Simulations of CO2 injection into fractures and faults for improving their geophysical characterization at EGS sites. United States. https://doi.org/10.1016/j.geothermics.2017.05.002
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Borgia, Andrea, Oldenburg, Curtis M., Zhang, Rui, Pan, Lehua, Daley, Thomas M., Finsterle, Stefan, and Ramakrishnan, T. S. Thu . "Simulations of CO2 injection into fractures and faults for improving their geophysical characterization at EGS sites". United States. https://doi.org/10.1016/j.geothermics.2017.05.002. https://www.osti.gov/servlets/purl/1476533.
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@article{osti_1476533,
title = {Simulations of CO2 injection into fractures and faults for improving their geophysical characterization at EGS sites},
author = {Borgia, Andrea and Oldenburg, Curtis M. and Zhang, Rui and Pan, Lehua and Daley, Thomas M. and Finsterle, Stefan and Ramakrishnan, T. S.},
abstractNote = {Here, we propose the use of CO2 in push-pull well tests to improve geophysical identification and characterization of fractures and faults at enhanced geothermal system (EGS) sites. Using TOUGH2/ECO2N, we carried out numerical experiments of push-pull injection-production cycling of CO2 into idealized vertical fractures and faults to produce pressure-saturation-temperature conditions that can be analyzed for their geophysical response. Our results show that there is a strong difference between injection and production mainly because of CO2 buoyancy. While the CO2-plume grows laterally and upward during injection, not all CO2 is recovered during the subsequent production phase. Even under the best conditions for recovery, at least 10% of the volume of the pores still remains filled with CO2. To improve EGS characterization, comparisons can be made of active seismic methods carried out before and after (time lapse mode) CO2 injection into the fracture or fault. Here we find that across the CO2 saturation range, C11 (the normal stiffness in the horizontal direction perpendicular to the fracture plane) varies between maximum and minimum values by about 15%. It reaches a maximum at around 6% gas saturation, decreasing exponentially to a minimum at higher saturations. Finally, our results suggest that CO2 injection can be effectively used to infiltrate fault and fracture zones reaching about optimal saturation values in order to enhance seismic imaging at EGS sites.},
doi = {10.1016/j.geothermics.2017.05.002},
journal = {Geothermics},
number = C,
volume = 69,
place = {United States},
year = {Thu May 18 00:00:00 EDT 2017},
month = {Thu May 18 00:00:00 EDT 2017}
}
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Publisher's Version of Record
https://doi.org/10.1016/j.geothermics.2017.05.002
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A Closed-form Equation for Predicting the Hydraulic Conductivity of Unsaturated Soils1
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