Ultra-deep fracking for limitless geothermal power is possible: EPFL
David Szondy

The prospect of virtually unlimited clean geothermal power is now substantially brighter. EPFL’s Laboratory of Experimental Rock Mechanics (LEMR) has shown that the semi-plastic, gooey rock at supercritical depths can still be fractured to let water through.

Along with nuclear power in the form of fission or fusion – and one or two other cutting-edge sources, geothermal power holds the genuine promise of making the concept of general energy shortages as outdated as worrying about saber-toothed tigers. By tapping into the enormous heat of the Earth's interior, it's theoretically possible to extract enough clean power to meet all humanity's energy needs for millions of years to come, solving the biggest challenge of climate change more or less overnight.

The problem is that all that lovely energy is trapped miles beneath the Earth's crust, and the costs of reaching it are astronomical. As a result, today's geothermal power is a niche source that's only available in a few scattered volcanic regions where the heat's much closer to the surface – typically a long way from where the energy is needed.

But pretty much everywhere on the planet, there's a much more powerful supercritical geothermal resource waiting to be tapped, if you could just drill down far enough to get to the really hot rocks found way below the surface. We're still only talking about a fraction of the distance through the Earth's crust, but it's hot enough down there to heat water to temperatures over 400 °C (752 °F). 

At these temperatures, water goes "supercritical" and starts acting like something halfway between a liquid and a gas, flowing as easily as a gas but retaining the density of a liquid. This phase can be used to extract a lot of energy. In practical terms, if you can get water up to supercritical temperatures, it can run a geothermal power plant with 10 times the output of a conventional one using lower-temperature water.

The bad news is that drilling to such depths – sometimes beyond the world-record 12 km (7.5 mile) depth of the Kola borehole – is currently beyond the cutting edge of engineering, although there are some very promising projects that could solve this issue in relatively short order.

The good news is that if we could master drilling that deep, we'd be able to set up geothermal plants pretty much anywhere on the planet – for example, on the abandoned sites of coal-fired power plants that have been shut down. They've already got the grid connections and plenty of steam turbine equipment, why not turn climate swords to ploughshares?

There are many issues yet to solve – one of them being that geothermal requires maximal contact between rock surfaces and the fluid they're heating, and one of the best ways to vastly increase that contact area is to fracture the rock in a process remarkably similar to the one used in oil and gas fracking. Fervo Energy has aptly demonstrated how much of a difference this approach can make to a geothermal plant.

But since nobody's ever drilled down that far, science hasn't been able to say whether the rock down there can crack and let water through. Observations taken close to the 10 km (6.2 mile) mark have shown rock starting to behave very differently to how it acts nearer the surface.

Instead of being hard and brittle, it gets soft and plastic and gooey – suggesting that it might not be possible to fracture rock and run water through it at supercritical temperatures. 

At least, this was the picture until an EPFL team led by Gabriel Meyer did some laboratory tests using a new gas-based triaxial apparatus, high-resolution synchrotron 3D imagery, and finite element modeling.

"When you get near the 10-kilometer (6.2-mile) mark, the rock no longer fractures but instead deforms uniformly, like soft caramel, and its behavior becomes complex," said 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."

What Meyer and his team did was reproduce the pressure and conditions found in the Earth's crust to observe how it changes during what is called the brittle-to-ductile transition (BDT). These laboratory tests are particularly important because it's nearly impossible to make such observations in the real world. Instead, the test rig recreated the temperature and pressure conditions in the rock sample, which was scanned with a synchrotron to create 3D images that were fed into a computer simulation.

They found that the rock acts less like putty than Silly Putty – the popular toy that acts like both a liquid and a solid. If you handle Silly Putty, you can easily mold it into whatever shape you like and if you set it down it will very slowly flow like a liquid. But the clever bit is that you can take this soft, flowing putty and if you hit it with a hammer, it shatters like glass.

According to the new EPFL study, the rock capping the supercritical zone acts in a similar way. Though it's ductile, it can be fractured so that water can flow through it. Meaning that with some sophisticated deep fracking technology, it might be feasible to build some very serious geothermal plants.