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.
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