Technological advances in the geothermal energy sector are making some geoscientists hopeful about the potential to deliver its energy resources almost anywhere on the planet.
For decades, places such as California, Hawaii and Iceland have taken advantage of their locations near tectonic settings and in-situ reservoirs, tapping into the Earth’s heat where molten rock is closest to the surface and where the Earth’s crust is thin. The heat is then used for electricity generation or direct-use purposes, such as space heating and industrial processes.
For example, the Geysers powerplant in California – the world’s largest producing geothermal system – generates approximately 835 megawatts of baseload energy, according to the U.S. Geological Survey.
Until recently, this type of conventional geothermal energy did not attract widespread attention because of its limited availability – notably confined to the Pacific Ring of Fire and other plate boundaries.
However, the growing need for clean, resilient, baseload and/or load-following energy coupled with recent advancements in oil and gas technology have prompted geoscientists to push the boundaries of geothermal resources. Looking beyond the natural settings of the resource, geoscientists are working to bypass geographic and geologic limits so that geothermal energy can be used around the world.
Geothermal Rising (formerly the Geothermal Resources Council) reports that the produced baseload and flexible power generation capabilities of geothermal energy can help stabilize the grid with reliable, continuous, sustainable energy, capacity and ancillary services.
“There are a lot of emerging ideas right now and that’s really driving a spike in geothermal,” said AAPG Member Ken Wisian, associate director of the Bureau of Economic Geology at the University of Texas at Austin. Rather than focusing solely on existing in-situ fluids, geoscientists are looking to engineer circulation systems that “hoover” the Earth’s heat up to the surface.
If such technology proves viable, geothermal energy could be expanded to both densely-populated and remote areas of need.
“Currently, you have to go where nature concentrates the resource,” Wisian explained. “That has the potential to change, which could change the whole energy grid structure. It’s an excellent use of existing industry skills. If you’re in the oil and gas industry right now, probably 90 percent of your knowledge or better is immediately applicable to geothermal.”
Sometimes called “advanced geothermal systems” or “geothermal anywhere,” these non-conventional approaches are essentially closed-loop (or semi-closed) systems that circulate a working fluid, often through a sealed downhole heat exchanger, to absorb and transport heat without physically interacting with the environment.
Closed-loop geothermal systems, or CLG, can operate in a broad range of temperatures and rock compositions – from relatively low-temperature sedimentary zones to hot, dry-rock formations. This range of operating parameters not only increases the number of viable geothermal projects, but also allows the use of high-temperature resources that dramatically increase power output, according to Geothermal Rising. Furthermore, CLG can produce power from previously unproductive geothermal wells.
Currently, geothermal energy makes up less than 1 percent of the grid in the United States, despite the country being the world’s largest producer in terms of megawatts. That number could substantially jump if advanced geothermal systems can be commercialized. That possibility “is the largest driver of interest,” Wisian said. “It is a strong idea that has attracted startups and a lot of capital. Venture capitalists are flooding the market.”
Maps – the most current and comprehensive – published by the Southern Methodist University’s Geothermal Laboratory show prospective areas for harnessing conventional geothermal energy based on reservoir temperatures and reasonable drilling depths, and the majority of locations are in tectonically active regions west of the Rocky Mountains.
“With new technology, as it comes on board and is proven, you open up most of the U.S. to being viable for geothermal energy production,” Wisian said. “If you can develop the technology to tap geothermal anywhere, you’re going up orders of magnitude on available geothermal energy, and the same kind of mapping applies everywhere in the world. So, you eliminate that key restriction on current geothermal, which is having to go where nature concentrates the resource, and instead you create your own artificial system.”
Geoscientists also are exploring enhanced geothermal systems, which optimize production from conventional geothermal systems that don’t produce sufficient volumes of well fluids, yet still contain heat. EGS stimulates the well or creates a separate flow system on the periphery of an existing geothermal reservoir, typically through hydraulic fracturing, to allow sufficient geofluid flow rates through the permeable rock layers. Closed-loop geothermal schemes using non-aqueous working fluids are also being explored, said AAPG Member Justin Birdwell, president of the AAPG Energy Minerals Division and research environmental engineer and geochemist at the USGS.
While EGS has not been demonstrated in a large-scale commercial implementation, there are several pilot-scale projects in progress around the world that are generating interest, Birdwell said.
“EGS could be utilized in a lot of other areas where traditional hydrothermal resources are not present,” he said.
EGS has the potential to expand the United States’ geothermal resources by 40 percent and ultimately comprise 10 percent of the nation’s energy resources, Birdwell added.
“Having that for baseload is critically important to having a truly, fully renewable or sustainable energy system in the U.S. and elsewhere,” he said.
EGS also could open additional resources in sedimentary basins where oil and gas are present and data from the subsurface, including temperature profiles, are available.
While geothermal energy is not currently a pervasive energy source, its benefits are plentiful. It is a low- or carbon-neutral energy source that is both baseload and load-following. It is secure and resilient, with a practically infinite supply from the Earth’s crust.
Operations and maintenance costs are low, and – if new technology proves itself out – it can be generated in locations of need with no requirement for external fuel supply, Wisian said.
While conventional geothermal wells require finding and maintaining a large volume of flow through the correct perennial zone for production, advanced geothermal systems generally have a lower risk because they do not depend on finding and maintaining porosity or permeability.
“It’s simply a matter of how deep you drill and which rocks you penetrate, as you are engineering your own circulation system,” Wisian said. “A conventional geothermal system has definite geographic boundaries. These newer systems potentially can be drilled anywhere.”
Once an engineered system is completed, if additional power is needed at a later time, a simple solution might be drilling one or more step-out wells.
“When you’re drilling in or around a city, as that city expands, you can add to the grid a few megawatts at a time, in an organic way, as opposed to hundreds of megawatts with a conventional power plant,” Wisian said.
Lastly, he reminded that geothermal energy is becoming a desirable option from a tax-credit and environmental, social and governance point of view.
Geothermal’s prospects appear exciting following decades of very slow growth in the United States. Having never received the robust research and development funding as the oil and gas industry has, the geothermal industry has consequently lagged in technological advances. However, in the last decade, as directional drilling and other technology that has reduced drilling costs have come online, new ideas for the economic harnessing of geothermal energy are being brought to life by start-up companies.
Also, as advances in technology that convert heat into electricity occur, prospects for geothermal energy become even better. For the last several decades, most new conventional geothermal power plants have been binary systems, typically using organic heat transfer fluids to generate electricity. There now is interest in developing CLG systems that use working fluids, such as supercritical CO₂, rather than naturally-produced fluids, such as brines or other hydrothermal water sources, Birdwell said. These would allow for the construction of massive subsurface heat exchangers to extract heat from reservoirs below 150 degrees C, as opposed to the traditional lower limit of 200 degrees.
As heat transfer fluids improve, production at even lower temperatures, such as 120 degrees, might be possible.
“That lower limit is being pushed all the time,” Wisian added.
“Assuming you sustainably extract heat at a useable rate from those kinds of systems, they are very exciting in theory, and there is a lot of money going into R&D right now,” Birdwell said.
Interest also is growing in repurposing oil and gas wells.
“I’m seeing interest from pretty much every major oil and gas company and some medium-sized companies, too,” Wisian said. “That’s on the edge of technology right now. It takes very particular circumstances to make a repurposed oil well geothermally viable, but the technology is improving rapidly, so that’s a ‘hold-on-to-your-thought’ for a year or two, and we’ll see.”
Wisian noted that the federal government’s Geothermal Technology Office has issued several calls for “Wells of Opportunity” proposals that would repurpose existing oil and gas wells to geothermal.
AAPG Member Malcolm Ross, “black swan detector” at the Rothwell Group, said he is seeing an interest in using geothermal energy to power oil and gas operations to lower CO₂ footprints of existing operations.
“The opportunity lies with co-production, where geothermal-style energy production systems can be added to hot, high-water-cut wells to produce locally consumed electricity. Furthermore, CO₂ produced in the local operations can be dissolved in waste brine being re-injected,” he explained. “And that does not even touch the potentially large economic benefit of producing strategic metals, like lithium, which is sometimes found in the brine already being produced. The key to this is that it depends only on extending existing infrastructure, rather than trying to convert an existing structure or build something new.”
Downsides and Challenges
Even as a promising new era of geothermal energy emerges, there are challenges to overcome.
As it stands, there are large upfront costs for new geothermal installations due to drilling. Yet costs are expected to decline as technology is developed and proven. Likewise, the cost of geothermal power per kilowatt-hour will be higher in first-generation plants compared to natural gas, but these also are expected to decline with scale, Wisian said.
Newer geothermal technology is either fairly new or unknown, with old- or first-learned knowledge clouding the emerging sector, Wisian said, noting that a lack of knowledge must be addressed both in the industry and the government. The newly “exploding” potential for geothermal energy must be better communicated to give the resource leverage, he said.
Another challenge lies in how a geothermal project will succeed or fail – and the risk associated with that.
“A geothermal system goes from resource to power generation in one spot,” Wisian explained. “The entire project largely succeeds and fails as one unit.”
Advanced geothermal systems also require more contact volume, whether through drilling a complex pattern or an engineered fracture system to expose fluids to more surface area. Creating that volume can be expensive unless done often and at a large scale.
Generally speaking, a geothermal plant must produce 10 megawatts or more to be considered economical, Ross said. Once up and running, though, a well-managed geothermal system can last 50 years, as seen in the California Geysers plant, and even longer than 100 years, as seen in the Larderello plant in Tuscany, Italy – the oldest geothermal plant in the world.
As geoscientists look to the future of geothermal energy, some are dreaming big.
Ross, who is a member of the Geothermal Committee for the EMD, would like to see CLG systems developed below abandoned coal plants.
“I would also like to see closed-loop and geothermal in general used to correct the energy poverty imbalance. I think that in remote places, islands, and especially in high latitudes like Alaska, Siberia – where there is no grid service and a small amount of heat – geothermal will go a long way to relieving energy poverty,” he said.
Projects taking advantage of the East African Rift System in Kenya are rapidly working to nearly double their geothermal capacity to 1.6 gigawatts by 2030, as reported by Bloomberg in July 2021.
“We have seen in Kenya, for example, that systems that take advantage of moderate resources can do wonderful things for Africa and other developing nations,” Ross said. “I’m out there trying to move geothermal forward for everyone, everywhere.”