Engineers repurpose the fracking techniques used in gas and oil drilling to expand a cleaner, greener underground power source.
By Thomas K. Grose
Mukul Sharma’s expertise earned him an unusual sobriquet: the Frack King. A pioneer in the research and development of horizontal oil-and-gas drilling and hydraulic fracturing, or fracking, he helped open up vast natural gas and oil reserves locked in underground shale formations. The resulting boom made the United States a net energy exporter and sped the demise of coal-fired power plants. It also drew the ire of environmentalists, who blamed the extraction method for causing air and water pollution and earthquakes. Sharma, a professor of petroleum and geosystems engineering at the University of Texas–Austin, offers no apology for his fracking work. But he is now redirecting the science and engineering behind it to boost the availability of a green, clean, and renewable power source: geothermal energy.
Geothermal plants churn out baseload electric power generated from the intense heat below the earth’s crust. The deeper one goes, the hotter it gets. The ability to exploit this heat is currently tightly constrained by subterranean geography. But the same technologies that extract gas and oil from rock and shale can also remove obstacles to these underground heat sources. “We have learned a tremendous amount about fracturing technologies in the last fifteen years, and a lot of that learning can go into employing geothermal anywhere,” says Sharma, who was elected to the National Academy of Engineering in 2018 for his research on production of underground carbon energy. He is now cofounder of a start-up, Geothermix, created to develop and market the adaptation of fracking to produce geothermal energy.
An Engineered Aquifer
The leading technique in what could bring the next domestic energy boom is called enhanced geothermal systems (EGS). Water is shot under very high pressure through wells drilled into hot, dry rocks more than a mile below ground to fracture them and create a human-engineered aquifer, or reservoir. There, the injected water heats to temperatures of about 200ºC before it’s pushed back to the surface through an extraction well. The boiling-hot water produces steam or heats another “working fluid” that produces vapor. The steam or vapor turns a turbine and produces electricity.
Geothermal technologies are basically the same as those used in fracking for oil and gas, notes Jefferson William Tester, a professor of sustainable energy systems at Cornell University’s School of Chemical and Biomolecular Engineering. “They just require refinement, and we’re smart enough as engineers to do this.” He says geothermal should be a big part of the US energy portfolio, “and I think it will be. I’m optimistic.”
The US Department of Energy agrees. “Tapping into geothermal energy—a clean and reliable energy source underneath our feet that is available in all corners of this country—is a key part of our plan to expand and diversify America’s clean energy market,” Energy Secretary Jennifer Granholm said in September. In February DOE announced $46 million for seventeen EGS-related research projects at universities, including UT–Austin; national laboratories; and a few start-ups, all dispersed through the Frontier Observatory for Research in Geothermal Energy (FORGE) program based at the University of Utah. Another $3.5 million was announced in April to develop geothermal-specific machine-learning software. September brought $12 million more to make EGS more efficient and commercially attractive. Between 2015 and the end of 2020, straddling the Obama and Trump administrations, DOE’s Geothermal Technologies Office spent more than $470 million on research and development.
Investor and industry interest in geothermal energy is growing as energy companies and utilities face political pressure to reduce greenhouse gas emissions. Geothermal’s cost is comparatively low. Unlike wind and solar, which are intermittent, geothermal offers an always-on source of energy, reducing the need for such costly storage technologies as batteries. Citing the research firm PitchBook, Axios early this year reported that globally, geothermal investments exceeded $675 million in the first half of 2020, up sharply from prior years.
Immense Potential
Civilization has long harnessed warmth from the primordial furnace deep underground. Ancient Greeks and Romans tapped water rising from geothermal hot rocks to heat buildings and baths. Icelanders, who have bathed in hot springs for centuries, began piping geothermal water into homes early in the last century. (That’s a practice called direct heat, which some US researchers consider another worthwhile alternative to using fossil fuels in some areas.) Geothermal-based electricity emerged first from Larderello, Italy, in 1904.
The United States didn’t produce geothermal electricity until 1960, at a plant in Northern California, but is now the world’s leader in this type of power, with plants scattered across mainly Western states. US output doesn’t amount to much, however: 0.4 percent of net electricity generation. Geothermal electricity plants are limited to areas with known high-quality hydrothermal reservoirs—usually locales where above-ground manifestations, such as geysers and steam vents, make them obvious.
Production from existing sites represents a tiny fraction of what the nation could generate with much deeper drilling and better scouting of locations, DOE contends. The department’s 2019 GeoVision report projects a 26-fold increase by 2050—to sixty gigawatts, or to 8.5 percent of the country’s total electricity output. “But we think that forecast is too conservative,” says Bob Metcalfe, an engineering professor of innovation at UT–Austin. He is principal investigator at the Geothermal Entrepreneurship Organization (GEO), an incubator for new geothermal companies.
The US Geological Survey says undiscovered hydrothermal sources could yield thirty gigawatts—half the 2050 output projected by the GeoVision report—and those would be sites accessible by conventional means, requiring permeable rock and an existing water supply. EGS would get around those obstacles and add many more drilling areas. According to GEO, use of geothermal energy could become widespread within a decade.
The environmental hazards posed by hydraulic fracturing and horizontal drilling for gas and oil largely disappear when the same technology is applied to geothermal sources, EGS proponents say. There is little or no release of greenhouse gases. And unlike fracturing for oil and gas deposits, the process used for geothermal energy does not require injection of toxic chemicals. “Working fluids” to drive turbines are nontoxic, and if they contain carbon dioxide, it will be inside pipes.
As for earthquakes, any type of underground work can generate some seismic movement. But Jennifer Miskimins, a professor of petroleum engineering at the Colorado School of Mines, rates the risks of a quake from underground EGS reservoirs as quite slim. She and other experts say disturbances that can occur with oil and gas fracturing mostly result from pressure exerted by underground disposal wells, which geothermal systems do not need. Also, the amount of water required by the reservoirs is relatively small, and it is continually circulating, which keeps pressure fairly consistent. Risks can be further mitigated, according to Sharma, by good well monitoring and proper location scouting to avoid areas with known faults. Overall, Tester says, if operators follow best practices to measure, monitor, and manage a reservoir’s hydraulics and pay attention to in situ stresses, “the risks of inducing damaging earthquakes are extremely low.” Still, Metcalfe admits that while most environmental groups GEO staff have spoken with support EGS, “for some, any mention of fracking and they go bananas. It’s much more of a PR problem than anything else.”
Proving It Works
Sharma became interested in developing EGS back in the mid-1980s but dropped the idea when it became clear that it couldn’t be done economically with the technology of the time. Using only vertical wells with just a couple of fractures didn’t allow the injected water to make much contact with the rock. By employing horizontal drilling, with dozens or even hundreds of fractures, “the contact area goes up by several orders of magnitude,” Sharma says, yielding many times the amount of heat. “Now we have to prove it’s actually going to work,” he adds. “We have the drilling capacity now to do this. It’s well within our wheelhouse.”
That’s not to say there are no hurdles to overcome. For example, when drilling for gas and oil, engineers work to ensure the wells don’t connect. But for geothermal, “we want the wells to communicate as much as possible. That’s a challenge while doing thirty to forty fractures,” Sharma says. So diagnostic technologies need to be employed that can determine how many fractures are connected, and how well. “A lot of the diagramming is transferable from the oil and gas industry,” Miskimins says. For example, fossil fuel wells use fiber optic tracers embedded in their casings to monitor elements like seismic activity, thermal readings, and well connections. Chemical tracers are used to monitor mud circulation, casing leaks, and reservoir conditions.
Finding the right locations is a key focus of researchers. One source is the US Geological Survey’s heat maps, which can guide operators to hot spots. Satellite imaging is increasingly used to interpret underground environments. And data analytics, building on data from past oil, gas, and water explorations, can also provide clues. GEO staff are talking to Silicon Audio, a start-up founded by Neal Hall, an associate professor of electrical and computer engineering at UT–Austin, about branching into geothermal exploration. The firm uses high–dynamic range optical microphones for seismic monitoring.
While hot rock can be found anywhere, the deeper you drill, the more it costs. “The thing is finding economical sites, where increased temperatures are available relatively close to the ground,” Miskimins says. Early EGS operations will aim for easily accessible rock; Sharma speaks of drilling down 6,000 to 8,000 feet, or up to 1.5 miles, to reach rocks with temperatures of around 200ºC. Future EGS plants will likely go deeper, perhaps six miles, where rocks can hit 350ºC or more. Those temperatures produce so-called supercritical water that contains four to ten times more energy per unit of mass than water at 200ºC. “The potential for making power with supercritical water becomes much, much better,” says Cornell’s Tester. “There’s a big improvement in efficiency, and if you can lower the drilling costs, you can do it anywhere.” Going that deep into hard, hot rock places more stress on drilling equipment. Miskimins says this problem is “very solvable” but will require that the existing tools be upgraded: “You have to make sure they can withstand the heat.”
Vaporized Rock
Existing fracturing technology offers the clearest pathway to geothermal power, but some researchers are pursuing entirely new approaches. For instance, Paul Woskov, an MIT research engineer specializing in nuclear fusion, has devised a method to blast through rock using high-frequency radio waves produced by a gyrotron, a powerful machine used in fusion experiments. The waves vaporize rock at temperatures of 3,000ºC to 3,500ºC. The technology has been licensed to Quaise, an affiliate of the Seattle geothermal company AltaRock, which last year got a $3.9 million grant from the Advanced Research Projects Agency–Energy to test it at the Oak Ridge National Laboratory. “It’s a major effort to see if we can advance this to deeper holes,” Woskov says. The first phase of the tests began in October, and preparations for future phases are “well along,” the company has reported.
Another potential technology, advanced geothermal systems (AGS), is a closed loop in which all the heated liquid stays inside sealed pipes and wells. Key parts of the system are still at the laboratory stage, Metcalfe says. Meanwhile, Eavor Technologies of Calgary, Alberta, has a concept it calls the Eavor-Loop, comprising two vertical wells connected by many multilateral horizontal wellbores—essentially creating a radiator. Tester says he thinks Eavor-Loop can work, but cautions, “You need a lot of surface area and a lot of distance between [the two wells].” Those are big engineering and cost hurdles, he says. A start-up that GEO is shepherding, Sage Geosystems of Houston, offers several types of technology, all designed to pull heat from sedimentary rocks. One of these combines fracking—to create an aquifer atop hot rocks—and closed-loop pipes. Another employs carbon capture, utilization, and storage (CCUS) technology. The captured CO₂ is injected into hot underground formations and then cycled to the surface in closed-loop wells to generate power.
Sage Geosystems, Eavor Technologies, and HyperSciences, a drilling technology company in Spokane, Washington, are among the start-ups attracting investments from venture capitalists and oil and gas companies, Metcalfe says.
Sharma’s start-up, Geothermix, is in touch with potential investors and aims to start drilling and producing power at a demonstration site in either Texas or New Mexico. The electricity it yields should cost five to ten cents per kilowatt hour, making it competitive with solar, wind, nuclear, and gas. Sharma is confident that EGS can be both economical and profitable: “I wouldn’t be doing it otherwise.” Producing hundreds of kilowatts initially, the firm plans eventually to reach twenty-five megawatts. “We intend to start small and scale up from there,” says the Frack King. That’s how a new energy industry is born.
Thomas K. Grose is Prism’s chief correspondent.
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