There’s treasure buried deep beneath the viridescent foothills of Tuscany’s Apennine Mountains, where the stark metal trusses of the Venelle-2 drilling tower mark its location like an X on a map. This geothermal well reaches nearly two miles beneath the surface to a region where temperatures and pressures are so high that rock begins to bend. Here, conditions are ripe for supercritical geothermal fluids, mineral-rich water that exhibits characteristics of both a liquid and a gas. It’s not exactly gold, but if Venelle-2 could tap into a reservoir of supercritical fluids and use them to spin a turbine on the surface, it would be one of the most energy-dense forms of renewable power in the world.
But getting there isn’t so easy. Boring deep into the ground risks triggering an earthquake if a large chunk of rock slips out of place. This risk was amplified at the Venelle-2 well, which aimed to breach the K horizon, a poorly understood boundary between the hard rock near the surface and the more pliant rock below. What would happen when the drill punched through this layer into the supercritical fluids below was anyone’s guess.
And for now, the mystery remains. Drilling at Venelle-2 stopped just shy of the K horizon when temperatures at the bottom of the well overwhelmed the equipment. Sensors at the bottom of the well indicated temperatures had breached 1,000 degrees Fahrenheit and pressures 300 times greater than at the surface. Nevertheless, Venelle-2 is the hottest borehole ever created, and it demonstrated that it’s possible to drill at the extreme end of supercritical conditions. And this week, a paper published in the Journal of Geophysical Research showed that it could be done without producing any major seismic activity.
The authors say they hope their study will assuage fears that all geothermal drilling causes earthquakes. After all, the public usually hears about geothermal wells only when something goes wrong. But Venelle-2 shows that “there are also many positive cases of wells drilled for geothermal purposes,” says Riccardo Minetto, a researcher at the University of Geneva and coauthor of the study.
The Venelle-2 well is one of many boreholes that puncture the landscape of the Larderello-Travale geothermal field in central Italy, the same spot where the Earth’s heat was first used to generate electricity. That first experiment back in 1904 produced only enough power for five light bulbs, but today Larderello-Travale produces about 10 percent of the world’s geothermal electricity. In 2015, a consortium of European energy companies and research institutes launched the Descramble project to see if even more energy could be extracted from the geothermal field. The plan was to tap into reservoirs of supercritical fluids deep beneath the surface. If the energy-dense fluids could be extracted from a well, it would be another historic first for Larderello-Travale.
The Descramble team was not the first to go digging for supercritical fluids. Experiments in the US, Japan, Italy, and Mexico have all drilled into conditions that could produce supercritical fluids, which require temperatures above 700 degrees Fahrenheit and pressures 220 times greater than those at the surface. But only one project has actually found supercritical fluids. In 2017, researchers from the Iceland Deep Drilling Project, run by the Icelandic government and a consortium of national energy companies, reported they had reached supercritical fluids 3 miles beneath the surface. Three years later, they are still working on generating useful energy from the well.
The Descramble team started drilling Venelle-2 right around the time the Iceland Deep Drilling Project discovered supercritical fluids. They used hardened drilling technology to penetrate regions far hotter than any other geothermal well. But after six months of drilling, they were forced to stop just a few hundred feet shy of their goal. Temperatures at the bottom of the borehole were nearly 200 degrees hotter than what was encountered in the Icelandic well; too hot to safely continue.
Throughout the drilling process, an independent team of European geoscientists was monitoring a network of ultrasensitive seismometers placed around the Larderello-Travale geothermal field. The team recorded some seismic activity, but at normal levels for the region. Still, Minetto cautions against generalization. Supercritical geothermal wells are an emerging technology, and he says future attempts at drilling for supercritical fluids “might induce larger seismic events.”
Although Minetto acknowledged that no earthquakes have been linked to drilling for supercritical fluids, geothermal wells have caused major earthquakes in the past. Last year, South Korea experienced its second-largest earthquake in history and traced its origin to an experimental geothermal well. A few years earlier, an earthquake that rocked Basel, Switzerland, was also linked to a geothermal well. Some experts blame these seismic events on drilling into faults, which increases efficiency but also carries a much higher risk of triggering an earthquake. As to whether drilling for supercritical fluids carries more earthquake risk than drilling more conventional geothermal wells, Minetto says “there are still too many unknowns about supercritical fluids to give a proper answer.”
Even without an increased risk of earthquakes, supercritical geothermal wells have other drawbacks. Reservoirs of supercritical fluids appear to be somewhat rare, which limits their usefulness in transitioning the world to geothermal energy. And the fluids themselves wreak havoc on boreholes by destroying their liners and concrete plugs. “The fluids are very corrosive and dissolve a lot of stuff out of the rock that you need to deal with,” says Susan Petty, president of Hot Rock Energy Research Organization and cofounder of the geothermal company Alta Rock Energy. “It’s scary stuff.”
Instead, Petty advocates for building so-called “enhanced geothermal systems” that aren’t dependent on naturally-existing reservoirs of geothermal fluids. These types of wells drill deep into dry, hot rock and inject water from the surface. The water heats up to near-supercritical temperatures and is pumped back to the surface to spin turbine generators. It’s a technique borrowed from the oil and gas industry that promises to free geothermal energy from its dependence on natural hot-water reservoirs. If you drill deep enough, enhanced geothermal systems can be used almost anywhere.
The challenges of finding and reaching deep pockets of hot water and steam have limited geothermal electricity adoption around the globe. But if geothermal energy wasn’t limited to locations selected by nature, Petty calculates that it could provide an inexhaustible source of always-on, carbon-free electricity for the vast majority of the world.
But like supercritical wells, enhanced geothermal systems have been beset by technical challenges and fears of massive earthquakes. Both the Basel and Korean earthquakes involved enhanced geothermal wells. Whether this is a risk inherent to the technology or the choice of drilling location is an open question. Still, the enhanced geothermal concept has been slow to catch on. In the US, companies like Alta Rock Energy have struggled to attract funding for their capital-intensive projects, which receive a fraction of the federal subsidies allocated for wind and solar energy. As a new technology lacking much of a track record, enhanced geothermal systems also carry substantially more risk for investors.
“Geothermal suffers from a bit of a marketing problem,” says Jeffrey Bielicki, leader of the Energy Sustainability Research Laboratory at Ohio State University. “Even though it has a lot of beneficial characteristics, when people say ‘renewable energy’ they’re usually referring to wind and solar.”
Earlier this month, the US Department of Energy announced $25 million in research funding that will be deployed at Forge, its dedicated geothermal test site. It’s a start, but geothermal energy systems still have a long way to go before they hit a power grid near you.