New results show potential for boost in geothermal energy

The quest for solutions to climate change includes new ways of generating energy. Geothermal energy, which is produced by harnessing the natural heat in the rocks and fluids beneath the crust, currently powers several parts of the planet, such as Iceland. It is clean energy producing 80% less carbon dioxide compared to natural gas plants, can be generated constantly and is unlimited.  

In conventional geothermal systems, heat from the crust warms water that is in underground reservoirs at temperatures of up to 350 C. This water is pumped to the surface, where it is used to generate electricity with turbines, to heat infrastructures and homes. The next frontier for geothermal energy is now to reach deeper and find supercritical water, at a temperature above 400 C, but many challenges lie on the way.

Rocks like chocolate

Today, geothermal energy is limited to the lower temperatures (<350C) of the upper brittle part of the crust. This is the zone that is closest to the surface, where rocks deform by cracking, allowing water to flow. Below this zone there is the Ductile zone, which is often 10 to 15 km underground. In this area, with higher pressure and temperature, rocks are ductile and have a different deformation mechanism that does not create voids for water to go through.

Gabriel Meyer, scientist at EPFL and corresponding author of the publication, explains it with an analogy: “The rocks behave a bit like chocolate: the ones in the brittle zone crack like a piece of chocolate would crumble after taking it out of the fridge, whilst the ones in the ductile zone behave like a piece of chocolate that you have left in the window sill and it has been softened up by the sun”.

In between these two zones there is the Brittle-to-Ductile transition zone, which is now being studied for geothermal energy. In places at the margins of tectonic plates or located on high geothermal gradient, such as Iceland or countries in the Ring of Fire, such as Japan, this zone can be found at shallower distances from the surface and can potentially be reached by drilling.

“We know that further underground the heat is higher and we could potentially find supercritical water and boost the geothermal plant energy output by a factor of 10, which is really huge, but this resource lies in semi-ductile rocks”, explains Meyer.

Dismantling preconceptions

Until today, researchers thought that the transition zone drew a lower limit for water circulation in the crust, since ductile rocks do not create voids when they deform.

“It is impossible to study these rocks in situ, as they are at too much depth, so no one really knew whether this is true or not. So we decided to study them in the lab and at the ESRF, where we recreated the conditions of pressure and temperature that take place in that zone”, says Meyer.

They scanned and analysed the fracture network produced in rocks deformed at conditions going from brittle to ductile, on the new beamline BM18, using X-ray tomography, and modelled fluid flow at the microscopic scale. “In our lab, we saw that our data were contradictory to current hypothesis, but we didn’t understand why. Then at the ESRF, with the help of Benoit Cordonnier, we understood how the permeability is being created in the transition zone, and where it is distributed in the rock", explains Meyer.

Benoit Cordonnier on the BM18 beamline. Credits: S. Candé.

“We found that there is a lot of permeability in this Brittle-to-Ductile transition and that it is distributed throughout the rocks, not just localised on main fractures and faults”, explains Cordonnier, who modelled the zone based on the results. This means that the water could flow in the rocks, just not in the same way as it does in the Brittle zone.

Whilst this research is very fundamental, it provides the scientific community with new insights. “Our paper really challenges the preconception of the transition zone like a cut-off area and the results are very encouraging”, says Meyer. “I am hopeful that in some years we will be able to use the Brittle-to-Ductile zone for geothermal energy, which will be key in our energy mix to stop using fossil fuels”, he concludes.

Reference:

Meyer, G.G.,  et al. Nat Commun 15, 7753 (2024). https://doi.org/10.1038/s41467-024-52092-0

Text by Montserrat Capellas Espuny

Top image: Image showing the transition from brittle to ductile zones, with localised fault deformation in the brittle zone, evolving into shear bands in the ductile zone. 3D scans of the experimental results from the brittle (top-left) and transition zones (bottom-right), conditions are represented. The image highlights the traditional geothermal energy versus the potential of deep geothermal energy. Two representations are provided: one with rock volume (grey) and permeable network (orange), and one showing only the permeable network.