An introduction to the next clean energy frontier: Superhot rock and the opportunities for responsible development
This blog is part of a series exploring and explaining the science behind next-generation geothermal energy, with a special focus on superhot rock geothermal, through a curated tour of influential technical and academic papers. This edition highlights key features of the World Resources Institute’s 2024 issue brief Next-generation geothermal: Considerations and opportunities for responsible development.
With its relatively low emissions and land-use impacts, next-generation geothermal has the opportunity to be a widespread and reliable source of clean electricity with minimal impacts on the environment.
However, understanding the benefits and potential impacts of next-generation geothermal is critical to ensuring its responsible development, especially as the technology scales toward higher-temperature applications like superhot rock geothermal. Fortunately, we can learn a great deal from today’s lower-temperature projects.
In 2024, the World Resources Institute published an issue brief, Next-generation geothermal: Considerations and opportunities for responsible development (WRI issue brief), which explores the environmental and public health impacts of next-generation geothermal and outlines what we currently know about its benefits and potential risks.
Here are our key takeaways from that work, and what they mean for the future of next-generation geothermal.
The risk of groundwater contamination is low.
In contrast with oil and gas development, there haven’t been any known cases of groundwater contamination associated with geothermal development – including for next-generation geothermal projects. The WRI issue brief notes that this is likely due to a few reasons:
- Next-generation geothermal development typically occurs at a greater depth and in denser rock formations than oil and gas development, often separating the heat extraction zone from ground water aquifers and sources of drinking water by 1,000’s of feet (Figure 1);
- In geothermal wells, all sections that pass through groundwater aquifers are cased and cemented – protecting local groundwater from the water that is circulated within the geothermal system;
- Closed-loop geothermal systems don’t typically use hydraulic fracturing – instead, their operations use a network of pipes that fully contain the fluid below ground; and
- Hydraulic fracturing fluids for enhanced geothermal systems (EGS), which are used to create, or enhance, the reservoir permeability, use fewer (or sometimes no) chemical additives.
To help provide a clearer and more transparent understanding for the chemistry and composition of the fluid used for hydraulic fracturing in EGS development, the FracFocus database recently updated their system to allow for submissions from non-oil and gas wells, including from geothermal wells. This update occurred after the release of the WRI issue brief and will help to improve public awareness of the chemicals used in the development of EGS projects and any potential impacts on the environment.
Induced seismicity is a risk that can be managed.
Microseismicity (i.e., small earthquakes that aren’t felt by humans) are a beneficial part of reservoir development in EGS, since they can help developers identify ideal locations for production wells. However, induced seismicity that is felt in populated regions can cause issues for EGS projects. These induced earthquakes are typically caused by changes in the rock’s pore pressure associated with fluid injection and withdrawal from the reservoir or from changes in temperature associated with the injection of fluids that are cooler than the surrounding rock.
In 2017, induced seismicity at the Pohang project in South Korea caused structural damage and injured 90 people. Although the Pohang event is the only known incident that caused harm to people, a 2006 earthquake associated with EGS operations in Basel, Switzerland caused mostly non-structural damage to local buildings. Both projects were shut down because of the induced seismic events.
As is detailed in Pollack et al. (2021), induced seismicity resulting from injection tests and acid treatments in St. Gallen, Switzerland was able to retain support from the public, possibly due to public engagement prior to initiation of the project. Although the project failed due to technical issues, the induced seismicity was a secondary reason for the project failure.
Importantly, the size of earthquakes can be minimized using protocols, such as the 2012 U.S. Department of Energy (DOE) Protocol for Addressing Induced Seismicity Associated with Enhanced Geothermal Systems (Induced Seismicity Protocol), which encourages developers to implement an outreach and communication program, review the local potential for ground shaking, establish and implement a local seismic monitoring plan, quantify the seismic hazards and risks, and develop a risk-based mitigation plan.
To date, no community has felt induced seismicity associated with a DOE funded project that required the use of the DOE’s Induced Seismicity Protocol. Requiring the use of this protocol, or a similar protocol in other regions, could help to minimize the risk of induced seismicity in future EGS development.
Water use impacts can be minimized using non-freshwater sources.
Water use for next-generation geothermal systems can be divided into three categories: 1) startup, which includes drilling, reservoir stimulation, and circulation testing, 2) operational (underground), and 3) operational (aboveground), with startup water consumption (#1 above) having a relatively small impact on the total water consumed over the life of the project for EGS.
The amount of water required for next-generation geothermal projects can vary depending on the permeability of the bedrock, the production temperature of the fluid, the type of geothermal system (i.e., enhanced geothermal versus closed-loop geothermal), and the type of power plant used (e.g., binary versus flash).
When comparing similar systems (e.g., flash to flash or closed-loop to closed-loop), next-generation geothermal systems operating at higher temperature, like superhot rock geothermal, consume less water per megawatt-hour due to the higher energy density of the fluid. Additionally, harder bedrock with fewer cracks is expected to consume less water. As temperature and pressure increase, the structure of rocks changes, allowing less water to pass through. As superhot rock geothermal systems will inherently be operated in higher temperature, and typically higher pressure, settings, they are expected to retain more water. The amount of water required to operate individual next-generation geothermal projects is difficult to quantify without consistent reporting on water usage by early project developers who are leading the way in the development of these important energy systems.
Not all water consumption has the same impact on water scarce regions. The U.S. DOE’s 2019 GeoVision study found that ~90% of next generation geothermal in the U.S. can be developed using non-freshwater sources, such as brackish or municipal wastewater. Early projects have touted successes with the use of non-potable water, such as Fervo’s use of degraded water, and Calpine’s use of recycled wastewater at The Geyser’s.
Consistent reporting that distinguishes between the amount of water withdrawn versus consumed, the type of water used (e.g., brackish versus potable), the type of bedrock, and the amount of water used for drilling, reservoir stimulation, circulation testing, and operation is essential in minimizing potential project pushback in water scarce regions and sparking innovation in ways to decrease water needs and encourage the use of non-freshwater sources.
Geothermal is good for clean air and public health.
Geothermal facilities release minimal conventional air pollutants (e.g., particulate matter, sulfur dioxide, and nitrogen oxides) that are harmful to public health. The U.S. DOE’s 2019 GeoVision study estimates that the reduction in air pollution associated with expanding geothermal development could result in estimated monetary health benefits between $6 billion and $23 billion.
The WRI issue brief notes that conventional dry steam and flash geothermal facilities emit fewer noncondensable gases that are commonly associated with negative impacts on public health and climate change (e.g., carbon dioxide and hydrogen sulfide) than fossil sources. Next-generation geothermal facilities may emit even fewer noncondensable gases, but the quantity of noncondensable gases emitted will depend on the geologic setting. Furthermore, a 2024 National Renewable Energy Lab (NREL) fact sheet notes that, on a lifecycle basis, geothermal has extremely low greenhouse gas emissions – similar to solar and wind – with the WRI issue brief pointing out that binary plants emit essentially zero greenhouse gas emissions from their operations.
What’s next?
Maintaining the environmental benefits of next-generation geothermal as it continues to progress to new regions and to hotter temperatures is imperative to the technology’s success. Lessons learned and thorough reporting and data sharing from lower temperature geothermal systems will be important to ensuring the success of higher temperature projects. For example, the WRI issue brief proposes a review led by the National Academies or a national lab to examine emerging issues and unique risks of next-generation geothermal. Sharing of information on these risks, as low as they may be, and engaging with impacted communities early and often about the potential impacts of each project, are essential to continued project successes.
Stay tuned for more from CATF as we continue to push forward bold ideas on how to scale superhot rock geothermal as an essential source of reliable and clean electricity.
This blog is part of a series exploring and explaining the science behind next-generation geothermal energy, with a special focus on superhot rock geothermal:
- Superhot rock geothermal and a vision for firm, global clean energy
- Superhot rock geothermal and pathways to commercial liftoff
- Superhot rock and the future of geothermal
- Superhot rock and the challenges in developing enhanced geothermal systems
Through a curated tour of influential technical and academic papers, the series aims to provide a fresh perspective from a geoscientist entering the geothermal industry. The goal is to share my learning journey and encourage collaboration around these groundbreaking solutions, which are critical to achieving a clean energy future. Whether you’re new to geothermal or looking to deepen your knowledge, I hope this series offers valuable insights into this fast-evolving field.
