Editors’ Vox is a blog from AGU’s Publications Department.
The temperature of the Earth’s crust increases with depth, often reaching hundreds of degrees Celsius at just a few kilometers below the surface. Geothermal operations aim to harness this abundant energy source by directing a steady flow of fluid through hot rocks.
Traditional geothermal operations target porous or fractured reservoirs to achieve economical fluid flow levels. On the other hand, enhanced geothermal systems (EGS) stimulate rocks to enhance permeability, allowing access to heat that would otherwise be locked in the subsurface. EGS have the potential to scale-up access to this source of abundant, green energy; however, they also have the potential to cause earthquakes.
A new article in Reviews of Geophysics explores the seismic risks of EGS and provides “good practice” guidelines. We asked the authors to describe the seismic risks of EGS operations, their proposed framework for managing risk and what needs further development.
How do enhanced geothermal systems (EGS) work?
An EGS operation starts by injecting highly pressurized fluids underground to create or enhance fractures in the rock, forming a human-made reservoir. This stimulation process can be repeated for multiple stages along the length of a horizontal well to maximize the size of the reservoir created.
Next, cold water is circulated through these fractures at lower pressure, where it absorbs heat from the surrounding rocks. The heated fluid is then collected back at the surface, where it is used to generate electricity or to provide heat directly. Heat can be extracted via this fluid circulation process for decades.
Why can EGS operations cause earthquakes and what happens when these earthquakes occur?
EGS operations require the creation of artificial fluid pathways to allow heat extraction. To achieve this, permeability needs to be increased by hydraulic stimulation, where fluids are pumped into the rock under high pressure. These fractures produce small earthquakes, or microseismic events, which are a normal and essential part of creating an efficient heat exchanger.
If stimulated fractures interact with pre-existing fault systems, the increase in pore fluid pressure can reactivate these faults and cause larger earthquakes.
However, if stimulated fractures interact with pre-existing fault systems, the increase in pore fluid pressure can reactivate these faults and cause larger earthquakes. This triggering mechanism is analogous to earthquakes induced by hydraulic fracturing for shale gas.
These larger earthquakes can pose risks to nearby communities and geothermal operators, making careful management of this process very important. For example, these induced earthquakes can grow large enough to be felt, damaging, or even harmful. Consequently, some jurisdictions have abandoned geothermal development due to concerns of these earthquakes.
How can EGS operations manage the risks of induced seismicity?
How an EGS operation manages the risk of induced seismicity depends on the operational phase: starting from exploration and planning, to stimulation and circulation, and finally post-operation. Earthquakes are predominantly induced during the stimulation and circulation phases, but sometimes are observed during drilling. Trailing seismicity (aftershock-like earthquakes continuing after operations stop) can occur during the post-operation phase.
During the exploration and planning phases, due diligence in characterizing the subsurface is important to pre-screen for reactivatable faults that are consequential for hazard/risk assessments. This subsurface characterization and risk assessment will be important for proactive measures, like selecting an adequate site and tailoring stimulations.
This next step starts with the design of a risk-based traffic light protocol (TLP), which prescribes when an operation needs to change (yellow-light) or stop (red-light). Red-lights need to ensure intolerable amounts of risk are not encountered, by using risk-based principles that account for trailing seismicity. The red- and yellow-light thresholds thus provide performance requirements for effective monitoring. This provides the core elements for reactive measures, where real-time monitoring informs how operations proceed via the TLP.
The last important piece is effective communication throughout the project. To ensure that all relevant stakeholders are aware of the risks and the efficacy of the management strategy – so that well-informed opinions/decisions can be made.
What are the main modules of the Induced Seismicity Risk Management Framework (ISRMF)?
The modules of the ISRMF comprehensively reflect all the steps required for risk management throughout the EGS project lifecycle. We introduced six main modules for the ISRMF: seismic risk pre-screening (Module 1), data collection and research (Module 2), communication and outreach (Module 3), comprehensive seismic hazard and risk assessment (Module 4), technical mitigation measures (Module 5), and financial mitigation measures (Module 6).
Seismic risk pre-screening (Module 1) provides a first-order estimation of risks, and (if required) is superseded by a comprehensive seismic hazard and risk assessment (Module 4). Technical mitigation measures (Module 5) include the design of the TLP, which subsequently informs reactive mitigation measures and seismic monitoring. Communication and outreach (Module 3) are important for public education, fostering trust, and managing risk perceptions.
Who might find the ISRMF useful?
We expect any stakeholders who are concerned about induced seismicity to find the ISRMF useful.
The ISRMF provides guidelines to managing the risks of induced seismicity. Thus, we expect any stakeholders who are concerned about induced seismicity to find the ISRMF useful. For example: companies in the EGS sector, regulators, governments, the impacted public, and researchers.
Specifically, we recommend that EGS operators take responsibility for developing and implementing an ISRMF, while regulators ensure the framework is properly planned and executed. Regulators should also appoint an independent expert group to validate the implementations. Furthermore, the ISRMF could serve as a reference for the public, enabling them to monitor whether seismic risks are properly managed and overseen by the operator and regulator. Researchers will be able to use these guidelines as an indicator of the state-of-the-art to continue improving upon.
While our ISRMF focuses on EGS, other subsurface projects (such as CO2 sequestration) will also find this framework useful.
What are the future directions for managing induced seismicity risk?
While good practice guidelines for risk management exist, open-ended questions still remain. Some are conceptually straightforward, albeit limited by data availability. For example, there is no consensus on the robustness or efficacy of mitigation measures, the actions that operators take during the yellow-light to avoid red-lights. In addition, validating techniques are needed to pre-screen regions that are seismically susceptible before operations commence.
On the other hand, some risk management deficiencies point to more fundamental shortcomings. For example, it is poorly understood how exactly the largest earthquake magnitudes will grow during injection or how intensely trailing seismicity will continue after shut-in. These considerations have serious implications for effective risk management. Open and transparent sharing of stimulation and earthquake data will be critical to resolving these questions.
—Ryan Schultz (Ryan.Schultz@sed.ethz.ch, 
0000-0002-1796-9622), ETH Zürich, Switzerland; Wen Zhou (0000-0001-8026-4724), Delft University of Technology, The Netherlands; Federica Lanza (0000-0002-8168-6766), ETH Zürich, Switzerland; and Iason Grigoratos (0000-0002-1637-2007), ETH Zürich, Switzerland
Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.
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