Susan Petty wants to go hotter. If that means drilling near a volcano, she’s in. Drilling deeper than any geothermal well to date? Fine. Petty’s company, Seattle-based AltaRock Energy, is gearing up to do both those things at its site near the Newberry volcano, about 32 kilometers south of Bend, Oregon.
Petty has worked in the geothermal industry for nearly four decades, doing everything from assessing well performance to optimizing power plants. She participated in the influential 2006 MIT study, commissioned by the Department of Energy, that concluded that in principle geothermal could satisfy all the US’s energy needs.
In conventional geothermal energy, heat is extracted from naturally occurring hot water. The method of enhanced—or engineered—geothermal systems (EGS) involves fracturing hot rocks to form a permeable heat reservoir. Water is flowed through the fractured rocks, and the absorbed heat is extracted to use either directly or after conversion to electricity. Petty says EGS has the potential to make geothermal, a source of renewable energy, possible almost anywhere. (See also the story in Physics Today, September 2018, page 22.)
Running the country completely on geothermal may be unrealistic, she admits. But she is committed to advancing extraction and storage technologies so that geothermal energy can play a much larger role in satisfying our energy demands.
PT: How did you get into science?
PETTY: I was always a math nerd. In 1970, during my sophomore year at Princeton, I went to an exhibit by the geology department of rocks that the Apollo mission had brought back from the Moon. I got chatting with people from the geology department, who said, “With your background, you should think about geology.” So I took Geology 101 instead of some other physics class. It was so exciting. Plate tectonics was just starting to be a thing. I switched my major from math to geology.
Everyone in geology was fantastic. But as a woman you were discouraged from going into geology or engineering.
PT: How so?
PETTY: For example, in those years there were no women’s bathrooms in the engineering building. So we—I think there were four of us—commandeered the men’s bathroom on the fourth floor of the engineering building. We put a big, hand-painted sign over the sign that said “Men.” The men kept using it anyway. So we put a little bowl with a live turtle in the urinal. The men got the message.
PT: Did you go straight to your master’s after Princeton?
PETTY: No. First I took a job doing groundwater surveys for the state of Delaware. I was involved with the early groundwater contamination cleanup sites. After a couple of years, I decided to get a master’s degree. I went to the University of Hawaii, which had a joint program between geology and civil engineering.
PT: Where did you go from there?
PETTY: I was really excited when I saw a job ad for a geothermal reservoir engineer, which is basically a hot-groundwater hydrologist. I took that job at the Idaho National Engineering Laboratory.
I worked on the Raft River geothermal project in Idaho. It was the first geothermal project to run a binary power plant, where you run hot water through heat exchangers to boil a working fluid that vaporizes and turns a turbine.
At the time, the Department of Energy had a technical assistance program. If someone came to them and wanted help, they’d put together a team of us—at the time there were no consultants or anybody you could hire, because geothermal was new in the US. I would go and do the reservoir and drilling portion. I got to work on projects all over the western US.
PT: Can you give an example?
PETTY: The L’eggs pantyhose factory in Las Cruces, New Mexico. They had to soften the nylon at just the right temperature and then cool it down again to lock the stitches in place. All of that used a lot of hot water. At the time, natural gas was really expensive, so they wanted to have their own geothermal well. We helped them site the well, we supervised the drilling, and we tested it.
PT: What did you do next?
PETTY: In 1982 my boss, one of our consultants, a drilling engineer, and I started a company. We did geothermal and other kinds of high-temperature well testing. It was great for a while, but then the prices of natural gas and oil started to drop and geothermal couldn’t compete, so there weren’t a lot of new projects. We wound up the business, and I went into consulting.
PT: How deep can wells get?
PETTY: The deepest wells in the world are drilled to about 10, 11, maybe 12 kilometers deep. They are mostly drilled in deep sedimentary basins for natural gas. We don’t drill that deep for geothermal. The deepest geothermal wells I know of are 3.5 kilometers deep. A geothermal well costs more than an oil and gas well of the same depth, because we have to flow much more fluid, and for that we need a bigger-diameter well.
PT: What role does fracking play in geothermal?
PETTY: Some of the methods used in oil and gas don’t work for EGS, mainly because we develop geothermal in rocks that are very different rocks from the layered sediments that hold oil and gas. The temperatures are hotter in geothermal. In oil and gas they use viscosifiers to make the fluid carry proppants, basically sand, into the cracks to keep them propped open. We tried doing the same thing, but the viscosifiers would just gum up with the heat. If the rock was really hot, it would turn into this black, sticky, gooey stuff you could never get out. It was terrible.
We tested all sorts of proppants and viscosifiers, and then we realized you could keep the fractures open by cooling them. That was a real breakthrough. If you just flowed water through the cracks you had made with fracturing, you could keep them open. That’s still how it’s done today.
PT: How did you form AltaRock Energy?
PETTY: After we finished the MIT study, the panel members were going around the US talking about what we could do to make EGS more cost effective. Someone from a venture capital firm approached me to see if I wanted to start a company. I put together a business plan, and a month and a half later I had a term sheet and had started AltaRock.
PT: How has it been?
PETTY: It was extremely scary. I needed to take my ideas and put them into practice. And I was no spring chicken. I was 56 when I started AltaRock in 2007. Venture capital firms are keen on patents. I had one patent. They wanted to monetize quickly. I had no idea how to commercialize EGS. It was quite a challenge. The problem is that the only way to make money with geothermal is to generate electricity with it, so you have to build a power plant. But that is not venture capital territory.
So instead we do incremental improvements to geothermal projects that are in financial distress. We exercise our technology on those projects, and earn an interest in them in exchange for spending the money to fix them up. We took a plant in Nevada from not being able to pay its debts to a positive cash flow. We are still trying to get another two to a point where they produce.
PT: What’s next for AltaRock?
PETTY: We think we have some new and exciting technology. High temperatures make EGS more efficient and economical. One superhot well can produce 25–50 megawatts. For comparison, the Haynesville shale gas wells in Louisiana produce 8–10 megawatts electric equivalent, a conventional geothermal well produces 5 megawatts, and a typical EGS produces 7 megawatts. And the generation costs would be 4.5 to 5 cents per kilowatt-hour, which is competitive with wind or solar or coal.
Right now you could probably supply 10 GW of electric power in the western US with superhot EGS by going to the sides of volcanoes and other places where shallow areas are hot.
PT: How deep do you need to go?
PETTY: We took over two wells on the side of the Newberry volcano. The deepest well is about 3 kilometers deep, with a temperature of 340 °C. We want to get to 500 °C, which should be at about 4.5 to 5 kilometers in depth. When we do that, we are hoping to create fractures and to produce very high temperature fluids. We are guessing that if we drill to 500 °C, we might be able to produce 450 °C at the surface. We would use surface equipment made with technologies borrowed from nuclear plants.
Deepening the well, doing the fracturing, and then doing a bunch of science that goes with it so you understand what you did will cost about $14 million. We are applying for grants from the government and foundations.
PT: What are the science questions?
PETTY: When you get to these high temperatures, you are getting into a realm where the rock is transitioning from hard and brittle to ductile, and we don’t know a lot about those rocks’ behavior. We hope to test theories about how the tectonic stresses operate, how we can best do the fracturing, and how we can operate reservoirs at high temperatures. We partner with universities, Pacific Northwest National Laboratory, and the Norwegian state oil company.
PT: Why have wind and solar gotten more attention than geothermal, and how do they work together?
PETTY: The capital cost for wind and solar has come down. That hasn’t happened with geothermal because it is based on resources rather than equipment.
As far as the balance, we are finding that out now. In Europe they are going to town on geothermal, thanks to incentives like feed-in tariffs. If we can learn to store geothermal energy, it could be the filler to take solar and wind from 50% of energy at peak generation to being 50% renewables all the time. Geothermal could go from about 7% to perhaps 25% of California’s energy. I think that should be the goal for the whole US—from less than 1% geothermal to 25% or even 30%.
