Oil and gas are not the only things in the ground that can power our lives. Heat in the form of geothermal energy is rapidly taking its place alongside other sources of renewable energy, buoyed by the lessons learned from decades of drilling for oil.
“Geothermal is this fantastic resource,” said Susan Hamm, director at the Geothermal Technologies Office in the US Department of Energy. “It’s an always-on renewable energy resource that harnesses the Earth’s natural heat. It improves domestic energy security and flexibility.” These benefits and challenges are the basis of the GeoVision report created by the Energy Department.
Hamm, along with Tim Latimer, the cofounder and chief executive officer of Fervo Energy, a company working on geothermal energy, and Aparna Raman, president of reservoir performance at Schlumberger, spoke during the Unconventional Resources Technology Conference.
Hamm and Latimer laid out the benefits of geothermal sources of energy beyond its renewability. “It provides dispatchable base-load power,” Hamm said. “And this dispatchable nature is really key. You can turn it on, you can turn it off. You can turn it up, you can turn it down.”
“It not only produces clean power,” Latimer added, “it does it around the clock, 24/7. It’s a fantastic complement to wind and solar resources to decarbonize the hardest parts of the electric grid.”
Hamm mentioned that geothermal complements wind and solar by being widely available, pointing out that geothermal sources are available across the United States (Fig. 1). She also touted geothermal’s reliability, saying that, while solar can provide energy 20% of the time and wind 45% of the time, geothermal sources can provide energy more than 90% of the time.
“In order to have a geothermal resource, you need to have three things,” Hamm said. “You need to have heat at depth, which we’ve already shown is everywhere just in differing amounts. You need to have a fluid in the subsurface, and you need to have pathways for that fluid to move around to get the heat so you can get it back out.” An enhanced geothermal system (EGS) is a system wherein one or more of those aspects is created where it did not previously exist. “You either fracture the subsurface or you add water or you do both in order to be able to recover that stranded heat.”
Fig. 1—US geothermal resources.
Latimer, who began his career as a drilling engineer, turned toward geothermal sources after working in the Eagle Ford shale, where high temperatures were a problem to be solved. In solving the high-temperature problem, he said cues were taken often from the geothermal industry.
He said that a goal for geothermal energy is to produce 100 GW by 2050. “That’s around a $300 billion market opportunity,” he said, adding, “That would offset as much as 500 megatons of carbon dioxide emissions per year.” Latimer said that the 100 GW goal “is around 15 to 20% of all US electricity. And that’s a really important number because that’s the part of the grid right now that we know that wind and solar resources are going to be insufficient to accomplish. So, all of the efforts around decarbonization require technology like geothermal to be effective. And it’s a huge prize worth going for.”
While geothermal energy has been used for decades, it hasn’t seen the success that solar and wind have, and Latimer said the reason is scaling. “Unless you live in places like Iceland or Northern California or Kenya that have fantastic near-surface resources, we haven’t been able to expand the footprint. So, even though we have a vision to get to dozens of gigawatts in the US, right now, we only stand at about 3. Moving beyond that requires us to solve some of these technology challenges.”
Geothermal energy is produced through EGS by injecting water into a heat reservoir, where it moves through the rock and heats up. This heated water or steam is then produced through production wells, where it can be captured at the surface and used to produce energy. While this system may appear to be similar to shale petroleum production, significant differences produce some pitfalls. “The rocks are hotter, and the rocks are harder, so that presents a lot of challenges,” Latimer said.
One of these challenges is flow. “Flow rates in these systems need to be really high,” he said. “80 liters/second is something like 50,000 B/D, so you need to have a really highly productive system.” Another challenge is hitting the right hot spot with the well and maintaining the heat without cooling the reservoir too much.
“The vast majority of geothermal has been developed with vertical systems using openhole completions,” Latimer said, “just really reliant on the drill-and-pray technique of intersecting these natural systems.”
The solution, Latimer said, is horizontal, multistage well design—horizontal to hit the sweet spot and multistage to keep it hot. And that’s where the experience of the oil and gas industry comes into play.
“Many of our present technologies and work flows can be leveraged for geothermal projects,” Raman said, pointing out that Schlumberger recently added a New Energy division, “which actually charts the course for taking technologies that might be applicable into this space.”
The oil and gas industry’s experience can benefit the geothermal industry in some surprising ways. For instance, in order for an EGS to be successful, water must be able to travel easily between the injection and production wells. This is a situation about which the petroleum industry has given a lot of thought.
“One of the most talked about issues in unconventionals these days is fracture-driven interaction, or what we call ‘frac hits,’” Raman said. “The industry has invested very heavily to understand this parent/child well interaction, and this is a problem in unconventionals. What is beautiful here is the unconventionals problem is an EGS solution.”
“Rather than trying to avoid frac hits … that’s actually the design objective of geothermal,” Latimer said. “We need the fluid from the injection well to reach the production wells.”
The lessons learned by the petroleum industry are going to be vital for the success of the geothermal industry in the US. “Directional drilling, distributed fiber-optic sensing, physics-based simulation, and high-temperature flow control all are key things to make geothermal systems work,” Latimer said, adding that he hopes to bring these advances, “the things that unlocked the North American shale boom,” to geothermal.
