Deep heat

At 6000°C, the Earth’s core is hotter than parts of the sun. Tapping into this residual heat could yield an unlimited and renewable resource for energy production, Anthony King reports.

Drill 500m beneath your feet and you will encounter temperatures around 25°C. Continue boring downwards and by 3km the mercury rises to 89°C before hitting 139°C around a depth of 5km. Temperatures above 160°C get over the economic threshold for steam turbine generation. A British Geological Survey report in 2020 estimated the heat at 4.5km depth could provide 2280MW capacity for generating electrical power (BGS Open Report, OR/20/049). Go deeper and there’s even more.

‘The deeper you go, the hotter it is, the more energy can be produced,’ says Silviu Livescu, a petroleum engineer at the University of Texas at Austin, US. ‘If we harvested the heat within 10 miles of the surface, we would produce more energy by an order of magnitude than what is currently produced by all the hydrocarbon resources around the world.’ One key advantage is its reliability, which means that unlike wind or solar it can ‘play a role as a generator of baseload power,’ a recent Boston Consulting Group report notes (bcg.com/publications/2022/the-advantages-of-geothermal-energy).

Geothermal energy for power production exists already in parts of the world where heat is close to the surface, as in Iceland, Italy and Hawaii. Such conventional hydrothermal systems rely on heat transfer from the mantle to shallow porous rocks in regions of tectonic activity. Italy was the first to generate electricity in this way in 1904.

Geothermal treasure

Deep geothermal means the heat below 500m, usually more, with some companies targeting depths beyond 5km or even 10km; standard drilling technologies rarely go beyond 3km. Nonetheless, some believe that oil and gas companies should be looking to tap geothermal energy for power. ‘Oil and gas flirted with geothermal in the 1970s and 80s, somewhat driven by the energy politics of the time,’ says geologist and geothermal researcher Philip Ball at the non-profit Clean Air Task Force (CATF) in Boston, US. ‘But there were a few very early failures and it seeded doubt in people’s minds.’ Cheap oil and gas in the 1980s put one Italian project near Naples on ice, for example.

The UK has two engineered geothermal systems (EGS). This involves drilling one or more wells, injecting water and using hot fluid or steam at the surface to turn the turbine. The United Downs Deep Geothermal Power Project was launched in 2009 by Geothermal Engineering. A production well was completed to 5275m in 2019, reaching hot granite rocks beneath Cornwall. At a depth of 5km, temperatures go above 180°C, requiring specialist equipment. ‘We are working towards construction of the power plant to begin in 2023, with the plant up and running in 2024,’ notes Business Development Manager Hazel Farndale at Geothermal Engineering. ‘The planned output is around 3MW of baseload electricity and 10MW of renewable heat energy.’ The company recently received planning permission at two other Cornish sites, Penhallow and Manhay, she adds.

Diagram of the drilling at United Downs
GEL

Also drilling into Cornwall granite is Eden Geothermal, which has completed a well to a depth of 4881m and is now installing a coaxial cable into the well so it can be used to heat the biomes, greenhouses and offices of the Eden Project visitor attraction and botanic gardens. It took 150 days and 28 drill bits to complete the well, notes Augusta Grand, CEO of Eden Geothermal.

Geothermal drilling at the Eden Project in summer 2021, using a 450t hook load capacity Bentec Eurorig.
Toby Smith

Drill challenge

Worldwide, however, there are around 50 drilling attempts to about 350°C, compared with tens of thousands of oil and gas wells. While at Keele University, UK, in 2020, Ball reported that oil and gas companies showed little interest in geothermal technologies, preferring to invest in solar, wind and biomass to gas (J. Energy Resources Tech., 2020, 143, 010904). ‘It astounds me that it isn’t been taken up more,’ says Ball. This situation is changing.

The longest operating EGS power plant is in North Alsace, France, where several wells bore down into the Paleozoic granite reservoir at 5km depth. A pilot power plant began there in 2009, with commercial electricity production starting in 2016. Meanwhile, a US DoE sponsored field laboratory called the Utah FORGE (Frontier Observatory for Research in Geothermal Energy) aims to perfect technologies to sustain fluid flow and energy transfer from a geothermal reservoir. ‘This is the first active demonstration in the world that is researching about engineering such a geothermal system,’ says Ball. He describes it as crucial for advancing EGS technologies to temperatures around 225°C or more.

The challenge is to drill economically beyond 5km, which exhausts many expensive and heavy drill bits. ‘Usually, oil and gas drill down to [50 to 80°C], so even going to around [150°C] is a big stretch,’ says Livescu. ‘We are talking about much more expensive materials.’ Oil and gas wells can include rubber and plastic materials that need to be redesigned for deeper depths. Also, oil and gas usually drill through sedimentary basins, whereas geothermal often targets very hot metamorphic rock such as granite or basalt, which typically are non-porous. ‘Beyond 10km really stretches our technological abilities right now,’ says Ball.

The involvement of oil and gas companies could reduce the cost of geothermal drilling.

Advocates in Texas such as Livescu say there is profitable heat, not just hydrocarbons, beneath the ground, while a recent report, The Future of Geothermal in Texas (doi: 10.26153/tsw/44084), set out the reasonings as to why the state should consider this resource. However, not all locations will be equally suitable, Ball warns. ‘There’s a lot of engineers who will tell you all we need to do is frack or drill to depth, get the water to flow and produce electricity,’ says Ball. ‘But as a geologist I can say that the rock type is really important.’

The deeper you go, the hotter it is, the more energy can be produced. If we harvested the heat within 10 miles of the surface, we would produce more energy by an order of magnitude than what is currently produced by all the hydrocarbon resources around the world.
Silviu Livescu a petroleum engineer at the University of Texas at Austin

Direct capture

Houston-headquartered Fervo Energy plans to drill multiple wells in a single location and use fibreoptic cables to monitor the geothermal wells. The company has announced it will design the first geothermal plant that will power direct air capture (DAC) of CO₂ for burial. One study estimates that full integration of DAC plants with all existing geothermal power plants in the US could capture almost 13m t/year of CO2 (GRC Transactions; geothermal-library.org/index.php?mode=pubs&action=view&record=1034141). In 2021, Fervo announced it would partner with Google to provide geothermal power to data centres throughout Nevada.

Also in Houston, Sage Geosystems aims to make geothermal power plants affordable everywhere using off-the-shelf oilfield equipment and technology. The company’s technologies are claimed to ‘work in almost any rock formation where the temperature at the bottom of the well is between 100 and 250°C’. Using CO₂ turbines and supercritical CO₂ as a working fluid is reported to double the efficiency of converting heat to electricity, compared with a traditional geothermal steam turbine.

Ball says supercritical CO₂ can make it possible to run a ~3MW plant at 100°C versus ~1MW with a water-based system. ‘A number of entities like Southwest Research Institute, Siemens and Toshiba are developing supercritical CO₂ turbines that can be applied in geothermal settings,’ he adds. GreenFire Energy, headquartered in San Francisco, California, US, has compared water, supercritical CO₂ and organic hydrocarbons in closed-loop geothermal systems and announced ‘the world’s first field-scale demonstration of a closed loop geothermal energy production’ in Coso, California, US, in 2020.

Oil companies are starting to pay attention. Occidental Petroleum and partners recently received $9m from US DoE to drill high-temperature geothermal wells. Criterion Energy Partners last year leased acreage along the Texas Gulf Coast, close to a DoE-funded geothermal energy demo project. In March 2022, petrochem firm Repsol received two geothermal exploration permits for Tenerife, Spain. Another company in Europe is GA Drilling in Bratislava, Slovakia, which develops drilling compatible with oil and gas rigs and boasts that 80% of components are manufactured in-house. The firm developed a pulse plasma drilling head that blasts rock with powerful jets of water, reducing time and costs.

Separately, the company is now testing an advanced anchor bit – a sort of robot with arms for stabilising the drill as it goes down into a well. This is now being trialled at a drilling facility just outside Houston, Texas, US. ‘Energy companies are becoming obsessed with geothermal,’ according to Bryant Jones, a policy expert at Geothermal Rising, a non-profit association in the US. ‘The last two years have seen over two dozen geothermal start-ups founded in the US with many of the founders coming from oil and gas.’

Fusion maser

Oil and gas is a traditionally slow-moving and risk averse business. But some radical approaches to drilling have emerged in recent years. ‘Everybody was focusing on using mechanical drilling,’ recalls Paul Woskov, a physicist at the Massachusetts Institute of Technology, US. ‘But there’s only so far you can push a grinding mechanical process into the earth.’ Woskov is an expert in plasma and fusion science – which tries to combine hydrogen atoms to generate helium and energy, the same process that powers our sun.

Woskov was familiar with gyrotrons, powerful devices that generate electromagnetic waves with outputs of up to 2MW and used to heat plasmas to 100m°C in fusion experiments. Without plasma present, a gyrotron could easily destroy the tough chamber walls of nuclear fusion chambers. So Woskov decided to point a gyrotron downward and test it on hard rocks. He soon succeeded in blasting a 50mm wide hole in a block of granite and basalt, with a fraction of the power available to gyrotrons. The devices convert electrical energy to a high-power beam with 50% efficiency and could affordably heat basalt ‘to vaporisation temperatures for less than one-hundredth of the cost to drill a mechanical well to 10km,’ says Woskov. He estimates it could take just weeks with his setup to drill to this depth.

Advances in fusion science mean remote monitoring technologies that can take measurements at 3000°C are already available. Gyrotrons could melt and turn basalt or granite into a vitrified material, creating a strong glass wall borehole that could eliminate the cost of casing and transmit heat efficiently to the surface.

In 2019, Woskov cofounded Quaise Energy on the back of a decade of research at MIT Plasma and Fusion Center. The firm plans to set up a full-scale hybrid drilling rig that combines rotary drilling and millimeter wave drilling in 2024, and then launch the first ‘super-hot enhanced geothermal system of up to 100MW of thermal energy from a handful of wells’. The ambition is to convert an existing fossil-fired power plant by 2028, this time powered by supercritical geothermal steam. A company has already obtained rights to abandoned coal-fired power plants in the US for this purpose. ‘Unlike for fusion, three-quarters of our plants are already built,’ says Woskov.

Quaise Energy’s ambition is to drill down to 20km and 500°C and Woskov envisages a drilling field with 100 or more wells, interconnected horizontally to function like an old-fashioned steam radiator. Such a power plant could run for decades and drive a 100MW power plant. ‘The volume of rock you need to mine decreases as you go deeper because there’s more energy per unit of rock,’ says Woskov. ‘If we can poke a hole deep enough, we could convert fossil fuel plants to clean geothermal energy almost anywhere on earth.’

Policy pump

Compared with wind and solar energy, geothermal energy has been slow to take off. In 2000, around 50TWh of geothermal power was generated globally, which increased to 92TWh by 2019, according to the International Energy Agency.

In the recent Texas report, technologist Jamie Beard argues that the Texan oil and gas industry is well placed to join a green drilling boom for geothermal energy. Beard is the founder of Project InnerSpace, a non-profit devoted to the development of geothermal energy.

An all-hands-on-deck scenario could see 1.4m geothermal wells drilled between now and 2030, delivering 77% of global projected electricity demand. The Inflation Reduction Act passed into law by US President Biden has opened to door to potentially billions of dollars in tax credits for deep geothermal energy.

The BGS report warns the UK is falling behind European countries such as France, the Netherlands and Germany in tapping its geothermal potential. It advises government to encourage and guide financing from the private sector for development. Feed-in tariffs have been introduced in Germany, Switzerland, Austria, Spain and Greece, for example. The report also notes a lack of awareness of the potential of deep geothermal energy in national and local government, as well as UK investment firms.

‘Geothermal is a policy-free zone,’ Grand complains. ‘We are waiting for a [UK government] white paper to come out later this year, with specific support for geothermal as in the Netherlands, Germany and France.’ Eden Geothermal has put a second well on hold at the moment, partly due to the lack of a government support mechanism. ‘The UK needs to develop a policy that systematically helps us develop geothermal,’ says Ball.

‘In the last five years, the innovation coming out of the geothermal domain has been quite remarkable. Once it is put into practice, I think the costs can be driven down quite substantially,’ Ball enthuses. The recent BGC report estimated that, even with just 1 to 3km well depths, the geothermal market could exceed $100bn. With ambitions of net zero by 2050, it is an industry that deserves serious attention from politicians and policymakers.