The geothermal model provides key insight into extraction

CAMBRIDGE, Mass. — Geothermal energy harvested from super-hot rocks miles beneath our feet could play a key role in the energy transition, but first we need to develop ways to not only access those rocks, but also extract heat from them. Now a computer model sheds light on the latter issue by describing for the first time what happens when rock at these depths and temperatures is exposed to fluids that can ultimately transfer the rock’s heat to the surface.

Essentially, the model shows the formation of microscopic cracks creating a dense “permeability cloud” throughout the affected rock. This is in contrast to the much larger and fewer macroscopic cracks caused by currently used engineered geothermal systems (EGS), which operate closer to the surface and at much lower temperatures.

Simulations using the model, as we wrote about in the latest issue of the magazine Geothermal energy“confirm that a superhot system can deliver five to 10 times more energy than is typically produced from EGS systems today for up to two decades,” says Trenton Cladouhos, vice president of geothermal resource development at Quaise Energy, which funded the work.

Cladouhos broadly described the model and importance of superhot rock geothermal systems on May 21 during the North American Geothermal Transition Summit. His talk was titled “Superhot Rock EGS: Methods, Challenges and Paths Forward.”

Authors Geothermal energy the authors are Samuel Scott from the Institute of Geosciences of the University of Iceland, Alina Yapparova from the Institute of Geochemistry and Petrology at ETH Zurich, Philipp Weis from the German Research Center GFZ in Potsdam, and Matthew Houde, co-founder of Quaise.

Super hot rock energy

Cladouhos’ remarks focused on the challenges of harvesting heat deep underground, where superhot rocks have temperatures exceeding 707 degrees Fahrenheit (375 degrees Fahrenheit).^aboutC). Water permeating through these areas would become supercritical. This steam-like phase carries 3-4 times more energy than regular hot water, and when connected through pipes to turbines on the surface, it turns 2-3 times more efficiently into electricity.

According to “The Future of Geothermal Energy”, recovering just 2% of the thermal energy stored in hot rock located 3 to 10 km (2 to 6 miles) beneath the continental United States is equivalent to 2,000 times the US’s annual primary energy consumption. A 2006 study by MIT on the potential of geothermal energy in the United States.

One of the key problems in accessing this energy is simply getting to it. Drill bits used in the oil and gas industry are not designed to withstand the extreme temperatures and pressures miles away where a major geothermal energy deposit lies. So Quaise is working on an entirely new way of drilling using millimeter wave energy (which is a cousin of the microwaves many of us cook in) that can literally melt and vaporize rock.

However, drilling into very hot rock is only the first challenge. Extracting heat is a puzzle that is at least as difficult as getting there, Cladouhos says.

Scientists around the world are working on engineered geothermal systems, essentially underground radiators or heat exchangers, to accomplish just this. Companies such as Eavor and Fervo Energy are developing and using various approaches, but none have been demonstrated at temperatures above about 200°C. ^aboutC

“If we really want geothermal energy to be a game-changer, we need to operate at very high temperatures, above 375°C.^aboutC,” says Cladouhos.

But little is known about what happens when super-hot rock at great depths is exposed to cold water pumped at high pressure.

New understanding

Currently, there are three general concepts for harvesting geothermal energy closer to the surface or from a depth of about two miles. These include closed-loop systems, based on a series of horizontal underground pipes connecting two wells. Water pumped into one well flows through these pipes, extracts energy from the rock, and then flows back to the surface through the second well. This is the approach Eavor takes. Another concept involves connecting two horizontal wells with a system of hundreds of artificial fractures. This is the approach that Fervo Energy takes.

Cladouhos model described at the Geothermal Transition Summit and a week later at the Clean Air Task Force workshop Bridging the Gaps: Advances in Superhot Rock Energy in Iceland – represents a new concept in the use of geothermal energy, focusing on what can happen when cold water is injected in very hot and very deep conditions.

Enter microcracks. “The idea is that wells are connected through a large ‘cloud’ of permeability, rather than specific, much larger fractures,” says Cladouhos. “So it’s more of a distributed connection than a localized one.”

The model is based on our knowledge of formation changes under these extreme conditions. Think of the vast outcrops of orange rocks from which copper and gold ore are mined. This is also proven by tests carried out in Japan, which showed the formation of microcracks under similar conditions in the laboratory. The latter work was reported last year in A Geothermal energy paper.

What’s next

Cladouhos notes that the model, which Scott and colleagues are continually refining, “will aid in future tests of the superhot rock in the field.” Quaise aims to do this in the next year or two at a place like Newberry Volcano in central Oregon, where super-hot conditions can be achieved at shallower depths.

He concludes: “This is a model. We don’t know if the permeability caused by microfractures is enough to connect two wells in the real world. Now we need to test this and other ideas about how superhot rocks crack in the field. Ultimately, a hybrid approach involving planar fractures, natural fractures and microcracks may be needed.”