Characterization of EGS Fracture Network Lifecycles Page: 4 of 126
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Foulger Consulting
1 Executive summary
Geothermal energy is relatively clean, and is an important non-hydrocarbon source of energy. It
can potentially reduce our dependence on fossil fuels and contribute to reduction in carbon
emissions. High-temperature geothermal areas can be used for electricity generation if they
contain permeable reservoirs of hot water or steam that can be extracted. The biggest challenge
to achieving the full potential of the nation's resources of this kind is maintaining and creating
the fracture networks required for the circulation, heating, and extraction of hot fluids. The
fundamental objective of the present research was to understand how fracture networks are
created in hydraulic borehole injection experiments, and how they subsequently evolve.
When high-pressure fluids are injected into boreholes in geothermal areas, they flow into hot
rock at depth inducing thermal cracking and activating critically stressed pre-existing faults. This
causes earthquake activity which, if monitored, can provide information on the locations of the
cracks formed, their time-development and the type of cracking underway, e.g., whether shear
movement on faults occurred or whether cracks opened up. Ultimately it may be possible to
monitor the critical earthquake parameters in near-real-time so the information can be used to
guide the hydraulic injection while it is in progress, e.g., how to adjust factors such as injectate
pressure, volume and temperature.
In order to achieve this, it is necessary to mature analysis techniques and software that were, at
the start of this project, in an embryonic developmental state. Task 1 of the present project was to
develop state-of-the-art techniques and software for calculating highly accurate earthquake
locations, earthquake source mechanisms (moment tensors) and temporal changes in reservoir
structure. Task 2 was to apply the new techniques to hydrofracturing (Enhanced Geothermal
Systems, or "EGS") experiments performed at the Coso geothermal field, in order to enhance
productivity there. Task 3 was to interpret the results jointly with other geological information in
order to provide a consistent physical model.
All of the original goals of the project have been achieved. An existing program for calculating
accurate relative earthquake locations has been enhanced by a technique to improve the accuracy
of earthquake arrival-time measurements using waveform cross-correlation. Error analysis has
been added to pre-existing moment tensor software. New seismic tomography software has been
written to calculate changes in structure that could be due, for example, to reservoir depletion.
Data processing procedures have been streamlined and web tools developed for rapid
dissemination of the results, e.g., to on-site operations staff.
Application of the new analysis tools to the Coso geothermal field has demonstrated the effective
use of the techniques and provided important case histories to guide the style of future
applications. Changes in reservoir structure with time are imaged throughout the upper 3 km,
identifying the areas where large volumes of fluid are being extracted. EGS hydrofracturing
experiments in two wells stimulated a nearby fault to the south that ruptured from south to north.
The position of this fault could be precisely mapped and its existence was confirmed by surface
mapping and data from a borehole televiewer log. No earthquakes occurred far north of the
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Foulger, Gillian R. Characterization of EGS Fracture Network Lifecycles, report, March 31, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc895319/m1/4/: accessed March 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.