New laboratory experiments simulating conditions beneath the Earth’s crust show it is possible to tap into energy from superhot rocks deep underground.
Sometimes referred to as the “Holy Grail” of geothermal energy, previous studies have concluded that the semi-solid state of these rocks may make this impossible. However, the research team behind these new experiments says their work is among the first to show a potentially viable approach.
In their published study, the research team from the Ecole Polytechnique Fédéral de Lausanne (EPFL) says geologists have been divided on whether this form of renewable energy was even accessible due to the material state of superhot, superdeep rocks.
“Rock under such high pressures and temperatures—more than 375oC, or 707 oF—is ductile, or gooey, as opposed to a smashable stone from your backyard,” explains a press release announcing the team’s findings. “As a result, some have argued that fractures can’t be created. And if they can, will they stay open?”
However, the team found that such rocks can form fractures that allow super-critical steam, a form of water vapor that can only exist under extreme conditions, to pass through the barrier and reach the surface. Accessing this team would provide a renewable source of energy from superhot rocks deep underground.
According to a 2021 report by the Clean Air Task Force cited by the study authors titled “Superhot Rock Geothermal, A Vision for Zero-Carbon Energy ‘Everywhere,” this supercritical steam “can penetrate fractures faster and more easily and can carry far more energy per well to the surface—roughly five to ten times the energy produced by today’s commercial geothermal wells.”
Tapping into Energy from superhot Rocks Deep Underground
Geoffrey Garrison, Vice President of Operations for Quaise Energy, who helped fund the work, says that the material state of this superdeep, superhot rock isn’t like the rock we are familiar with. Instead, the temperatures and pressures found this deep under the earth’s crust make it significantly more malleable, much like Silly Putty.
“If you pull it slowly, it stretches out and becomes elastic,” Garrison explains of the familiar children’s toy. However, the researcher says this situation changes if a force is applied more rapidly.
“If you pull a chunk of Silly Putty really quickly, it snaps,” Garrison explained. “And that is brittle behavior.”
This unusual behavior caused the team to wonder if the rock layers they were trying to access could work similarly. “If you stress the rock slowly enough under these extreme conditions, it may stretch and not fracture,” the researchers explained.
Unfortunately, testing how rock at these depths will react to various stresses is impractical. Instead, the team looked into a method for simulating these conditions in a laboratory setting. Fortunately, According to Associate Professor Marie Violay, head of the Laboratory of Experimental Rock Mechanics at EPFL, the team’s facility contained a specially designed simulator, allowing the team to test their theory without digging miles below the surface.
“The best part [of this research] was the development of a unique experimental machine capable of reproducing the pressure, temperature, and deformation conditions of deep supercritical reservoirs near the brittle-to-ductile transition,” the researcher explained. “Additionally, we were able to combine these experimental results with in situ X-ray images obtained from the ESRF (European Synchrotron Radiation Facility), offering a comprehensive view of the processes involved.”
As expected, the team found that superhot, superdeep rock could indeed fracture if enough pressure were applied rapidly. As the authors explain, “This work shows that rock will shatter under these conditions, but it needs to be stressed quickly to do so.”
Promising For Exploration of Deep Geothermal Reservoirs
In the study’s conclusion, the authors note that their experiments were limited to the lab and were not performed in an environment where superhot rocks turn water into supercritical steam. In fact, the team says very little actual data has been collected from this remote, harsh environment, especially regarding the idea of capturing energy from superhot rocks deep underground.
“There are very few in situ data available, and these are among the first experimental results that shed light on such extreme conditions,” Violay said.
Still, the team’s experiments showed that rocks in this type of environment could be permeable enough if stressed and cracked to allow for the collection of clean, renewable energy from superhot rocks deep underground.
“This work is exciting because it presents the first permeability measurements conducted during deformation at pressure and temperature conditions characteristic of deep supercritical geothermal reservoirs near the brittle-to-ductile transition in the crust,” Violay said. “We have shown that the brittle-to-ductile transition is not a cutoff for fluid circulation in the crust, which is promising for the exploitation of deep geothermal reservoirs.”
The study “Permeability partitioning through the brittle-to-ductile transition and its implications for supercritical geothermal reservoirs” was published in Nature Communications.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.