Permeability in the Super Deep

Iceland’s got it made. Thanks to the thirty-odd volcanoes scattered about the island, geothermal energy provides the country with 25 percent of its electricity and heats 90 percent of homes there—making up 66 percent of its primary energy. The rest of us may not be so lucky, but we also sit atop heat ripe for geothermal power.  

It’s just a lot harder to get to.  

Go six to 10 miles below the surface and the rock is hot enough—more than 750 °F—to turn water supercritical. “Unlocking deep geothermal energy would mean free energy, seven days a week, 24 hours a day, and with a high efficiency,” said Benoît Cordonnier, a beamline scientist at The European Synchrotron.   

But to achieve that energy utopia there are some difficulties. Aside from the obvious problem of how to drill effectively and cheaply to that depth, no one really knows exactly what we’ll find there. The chief question, if you want to send water down a miles-long hole to absorb heat at the bottom, is whether or not the rock there is permeable enough.  

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The rock at that depth is in what geologists call the brittle-ductile transition zone. It’s not like the hard pebbles and boulders we find at the surface, but nor is it like molten lava. “It can heal with time, but it’s not a completely viscous material,” Cordonnier said. “Imagine a chocolate that you leave on the window still during summer—it’s still solid, but you can see that it will not behave in the same ways.” Such a material still flows and closes on itself. “So everybody believed that these cracks would close back quite fast, and you would not have any economic interest for this,” he explained. 

To find out, Cordonnier, in collaboration with researchers from the Swiss Federal Technology Institute of Lausanne’s Laboratory of Experimental Rock Mechanics, decided to recreate the conditions of rocks at such depths, here on the surface. They loaded granite into a piece of precision machinery called a TARGET press that can apply as much as 500 megapascals of pressure, deform rock samples, and heat them past 2,000 °F—though they didn’t need either extreme to imitate rock at the depths of interest.  

After putting the granite through its paces, they discovered that, at the kinds of pressures and temperatures found several miles down, rock is indeed permeable. “What this experiment has shown is that first you have permeability that is much bigger than anticipated,” Cordonnier said. “Instead of creating a localized crack, you create a myriad swarm of little cracks that are just joining together and allowing a permeability that is not negligible.”  

As the press compressed the rock, cracks began to form in the classic way: first at a 30-degree angle in the direction of stress from the piston, then evolving from there. But as the pressure became more extreme, the fissures became less localized and more homogeneously distributed.  

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Cordonnier and his colleagues examined the fracture networks created in the rock after removing the pressure and scanning the sample. In addition to the visual evidence of permeability, they also injected water and fluid into the rock and measured the pressure on the other side, confirming its permeability. 

Such a homogeneous fracture network is perfect for sending in water to harvest heat—as long as the tiny ducts stay there long enough. “The main question that remains is how long this this network can stand for,” Cordonnier said. Under a more or less static pressure, will such networks remain the same for days, months, or years?  

To get at the answer, the team plans to run similar experiments, but scanning the samples in situ. They’ll also try other types of rock that transition at shallower depths and are looking to develop a new press that allows them to more easily scan in real time, all in the hopes of proving it’s worth the cost to drill miles into the earth to reach hot rock.  

“Deep geothermal energy remains a bit of a dream, because the costs are enormous,” Cordonnier said. “We just want to show the feasibility, to see if it makes sense, because this would be more available than solar. It would be similar to fusion: quite available and free.” 

Michael Abrams is a technology writer in Westfield, N.J.