A team of EPFL scientists has provided insight into the mechanisms at work in geothermal reservoirs located deep underground, known as supercritical reservoirs. Through a combination of computer simulations and lab experiments, they showed that rocks located between 5 and 8 kilometers deep in the Earth's crust are also permeable to fluids.

Geology is a field with many unanswered questions. And the deeper you dig into the Earth's crust, the further you venture into unknown territory, as the rock becomes increasingly hidden and hard to reach. The deepest hole in the world is on the Kola Peninsula in Russia, and extends 12 kilometers—yet that's less than 25% of the average depth of the continental crust.

And even though geologists have been able to dig that far, it's nearly impossible to take measurements at such depths. That's why many scientists are working to replicate the conditions inside their research labs.

This is the method that Gabriel Meyer, a postdoc at EPFL's Laboratory of Experimental Rock Mechanics (LEMR), chose to use for his research. He's looking specifically at the changes that take place in rocks under supercritical conditions.

"Scientists have observed a transition in the mechanical behavior of rocks at extreme depths," he says. "I wanted to be able to understand what's going on, since we can't actually watch the process out in the field."

His task has now become easier thanks to a new instrument developed at LEMR that can replicate the pressure and temperature conditions at such depths. It took Prof. Marie Violay, the head of LEMR, and her research group nearly six years to create this piece of equipment. The first discoveries enabled by the instrument have just been published in Nature Communications.

Water at more than 400°C

Geothermal reservoirs with supercritical water—or water at temperatures above 400°C—are the next frontier of geothermal energy. Engineers have been exploring the potential of this natural resource in different parts of the world for the past 15 years or so, since it could boost energy production by a factor of 10 relative to conventional geothermal plants (which source heat closer to the surface).

Supercritical reservoirs are located approximately 10 kilometers underground, and the trick is to reach them. The first conclusive tests of supercritical geothermal technology were carried out entirely in volcanic regions, where supercritical temperatures can be found at depths of 5 kilometers.

"When you get near the 10-kilometer mark, the rock no longer fractures but instead deforms uniformly, like soft caramel, and its behavior becomes complex. Deformation occurs at the level of the crystalline structures in the grain. I wanted to find out whether water could circulate within rock that has transitioned into this unusual ductile form," says Gabriel Meyer, postdoc at EPFL's Laboratory of Experimental Rock Mechanics.

Supercritical water is neither a gas nor a liquid, but is in a state that allows large amounts of energy to be extracted. The mechanical properties of rock also change under such conditions. Whereas rock close to the surface can be brittle and contain a number of microfractures, it becomes ductile at high depths.

"When you get near the 10-kilometer mark, the rock no longer fractures but instead deforms uniformly, like soft caramel, and its behavior becomes complex," says Meyer. "Deformation occurs at the level of the crystalline structures in the grain. I wanted to find out whether water could circulate within rock that has transitioned into this unusual ductile form."

3D imagery

To measure rock permeability, Meyer and his colleagues transformed granite samples from brittle to ductile by exposing them to the same temperature and pressure conditions that are found deep inside the Earth's crust. The LEMR instrument works by exerting pressure on a rock sample and deforming it with a piston.

Both the temperature and pressure gradually increase, simulating the conditions between a few hundred meters and several kilometers underground. Then, the research team used a synchrotron to produce 3D scans of the deformed samples, enabling them to view the permeability.

"Geologists long thought that the brittle-to-ductile transition point was the lower bound for water circulation in the Earth's crust," says Meyer. "But we showed that water can also circulate in ductile rock. This is a highly promising discovery that opens up further avenues of research in our field."