An introduction to the next clean energy frontier: Superhot rock geothermal and permeability partitioning through the brittle-to-ductile transition
This blog is part of a series exploring and explaining the science behind next-generation geothermal, 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 2024 article by Meyer et al., Permeability partitioning through the brittle-to-ductile transition and its implications for supercritical geothermal reservoirs. The full blog series, in addition to their reference reports, can be found at the Superhot Rock Resource Library.
Next-generation geothermal systems have the potential to provide cost competitive, clean, baseload power anywhere it is needed. This potential increases as geothermal systems reach higher production temperatures, significantly improving their cost-competitiveness with other energy solutions and making high-enthalpy geothermal systems a particularly attractive addition to clean energy portfolios. Superhot rock geothermal (SHR), for example, has the potential to generate up to 10x the energy output for a single well when compared to lower temperature conventional geothermal systems.
However, there are technical gaps that must be bridged before SHR can be available anywhere it is needed. One of these gaps is in the reservoir creation and heat extraction for SHR systems, which raises questions about whether a sustainable reservoir, which is where the water is heated and then extracted in geothermal systems, can be created in a SHR setting.
Some of these questions are beginning to be answered. Although not yet reaching SHR temperatures, Mazama Energy recently announced the creation of an enhanced geothermal system (EGS) with a bottomhole temperature of 331°C at their site in Oregon– the hottest EGS reservoir to date. And in a 2024 Nature Communications article, Permeability partitioning through the brittle-to-ductile transition and its implications for supercritical geothermal reservoirs, Meyer et al. analyze the permeability creation in rocks (i.e., how interconnected the cracks in the rock are – potentially allowing for fluid flow across a reservoir) created under a range of high-temperature and high-pressure conditions. Their findings, which I highlight below, provide important insight into reservoir formation in SHR settings.
Why might superhot rock geothermal reservoirs be more difficult to create?
SHR reservoirs might be more difficult to create and maintain because they would exist beyond the brittle-to-ductile transition (BDT) in the Earth’s crust, a transition zone between the more shallow and cooler crust that breaks in a brittle manner and the deeper and hotter crust that behaves in a more ductile, plastic manner.1 Why operate in this difficult region? Beyond the benefit of higher production temperatures, and therefore cost competitiveness, an additional motivator is that reservoir creation beyond the BDT has the potential for reduced seismicity, due to the absence of large faults beyond the BDT.
Numerous EGS reservoirs have been created in the brittle portion of the Earth’s crust with Fervo’s Cape Station in the U.S. and Soultz-sous-Forêts in France being two examples. However, there is very little evidence so far of EGS reservoir creation beyond the brittle portion of the BDT. This begs the question of whether and how a SHR reservoir can be created in rocks that behave in a ductile manner and how that reservoir will behave over the life of the project.
Permeability creation in high enthalpy systems is possible
To better understand the potential for creating permeability beyond the BDT, Meyer et al. developed a laboratory experiment that placed granite rock samples at elevated temperature and pressure and assessed the change in permeability of the rocks after being exposed to each temperature and pressure setting. These granite samples are representative of many deep geothermal reservoirs and were also chosen due to their homogeneity and low porosity. Future research could conduct similar studies with other types of rocks in addition to the specific type of granite used in this study. Although the pressures weren’t changed between samples in this study, Meyer et al. exposed the samples to temperatures of 200°C, 400°C, 600°C, and 800 °C. For reference, SHR projects aim for temperatures above 400°C. Unexpectedly, the results show that the permeability of the rock samples at 200°C and 400°C didn’t change substantially, but the permeability of the 600°C experiment increased five-fold and the permeability of the 800°C increased thirty-fold. Meyer et al. determined that this was due to how strain (i.e., the deformation or change in shape of the rock resulting from an applied force) is distributed across the rock samples. In the lower temperature samples, much of the deformation is focused in a smaller region, creating fewer larger cracks. However, in the higher temperature rock samples, the deformation is densely spread across the rock sample (Figure 1).
Figure 1. Representative illustration of the difference between the rock deformation in the lower temperature, brittle rock samples (left) and the higher temperature, semi-ductile rock samples (right). Modified from Meyer et al. (2024).
In other words, Meyer et al. found that, as depth and temperature increase, a greater portion of the strain is taken up by a larger portion of the rock sample instead of a smaller and more localized region.
Meyer et al. also found evidence suggesting that heat transfer between the surrounding rock and the subsurface working fluid in high enthalpy systems could take place in more regions of the reservoir beyond just a few targeted fractures. This would allow the subsurface working fluid to come into contact with more surface area within the reservoir and gather heat from a larger portion of the rock, allowing the system to work more efficiently and potentially slowing the temperature drawdown of the reservoir.
Furthermore, the research by Meyer et al. suggests that, with the presence of a dense array of small fractures instead of a smaller number of larger fractures, the risk of creating shortcuts between injection and production wells may be minimized, reducing the risk of the subsurface working fluid not gathering sufficient heat before it flows up through the production well.
What’s next?
The research by Meyer et al., in addition to the recent success demonstrated by Mazama Energy at their Oregon site, suggests that there is significant promise in the ability to create SHR EGS reservoirs. Meyers et al. caution that how their findings will translate to the field scale remains uncertain. Additionally, they caution that their research didn’t demonstrate how the increase in permeability would evolve over time, which is a critical aspect of both creating and sustaining a SHR EGS over the lifetime of the project. This shows the need for further research on this topic, such as long-term field-scale demonstrations that can provide answers to this question.
Answers to these types of unknowns will be important for future development of SHR EGS systems. The ability to develop geothermal reservoirs beyond the BDT and sustain those reservoirs long-term is essential to the success of high enthalpy geothermal systems and the ability to deploy geothermal technology anywhere it is needed – at a cost competitive price.
Stay tuned for more from CATF as we continue to push forward bold ideas on how to scale superhot rock geothermal as an essential form of clean firm electricity anywhere it is needed.
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, 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.
