Old oil wells are getting a new job - storing carbon

For over a century, oil and gas companies have been pulling carbon out of the ground in the form of fossil fuels. Now, researchers are flipping that idea on its head – by putting carbon back underground. And not just anywhere, but into the very same wells that once helped release it.

This new approach doesn’t rely on fancy tech or sci-fi ideas. Instead, it uses leftovers from farms and forests – like corn stalks and tree trimmings – to create a thick liquid called bio-oil.

Packed with carbon pulled from the air by plants, bio-oil can then be pumped into abandoned oil wells for permanent storage.

Old oil wells meet new solutions

Across the U.S., there are hundreds of thousands of old oil and gas wells sitting empty. Many of these wells weren’t properly sealed and now pose environmental and safety hazards.

Plugging the wells costs about $1 million each. At the same time, tons of plant waste is discarded every year, ranging from corn stover in the Midwest to forest debris in the West. This is where researchers at Iowa State University saw a match.

“On the one hand, you have these underutilized waste products. On the other hand, you have abandoned oil wells that need to be plugged. It’s an abundant resource meeting an urgent demand,” said Professor Mark Mba-Wright, who led the study.

Oil wells for carbon capture

The process at the center of the research is called fast pyrolysis. It sounds complicated, but the idea is simple: you heat dried plant bits to extremely high temperatures (over 1,000°F) in the absence of oxygen.

The result is three products – bio-oil, biochar, and gas. The bio-oil captures most of the carbon. The gas helps fuel the system. The biochar can be sold to farmers to improve soil.

“One of the innovations here is that you can do carbon capture with units the size of a skid loader or a combine. You can start small,” Mba-Wright said.

Bio-oil machines for the job

The team modeled a network of 200 mobile pyrolysis units that could be moved around rural areas to process plant waste on-site.

Each machine would handle about 10 tons of biomass daily, producing bio-oil that could be stored in terminals before being transported to empty well sites for injection.

Each unit would cost around $1.3 million to build. According to the study, the system could sequester carbon for about $152 per ton, which makes it cost-competitive with other carbon capture methods. For wood-based materials, the cost could drop closer to $100 per ton.

Filling a single crude oil well requires more than 216,000 gallons of liquid – plenty of space to lock away carbon for good.

Undocumented oil wells

The researchers estimate that there could be as many as 800,000 undocumented orphaned wells in the U.S., far beyond the 120,000 wells targeted by a $4.7 billion federal cleanup effort passed in 2021.

“The more units they build, the better they would get at building them,” Mba-Wright said. This is thanks to what’s called a “learning rate,” which means that building more machines over time helps lower both production and operating costs.

This isn’t just theory. A San Francisco startup, Charm Industrial, has already started using this method and signed deals with large companies to help them meet climate goals. These companies want carbon-removal credits, and bio-oil injection is emerging as one of the most promising ways to earn them.

“We hear it time and again: after taking a close look among their options, leading carbon-removal buyers find that bio-oil sequestration represents one of the highest-quality and most cost-effective approaches,” said Peter Reinhardt, CEO and co-founder of Charm Industrial.

Opportunities for carbon removal

Charm funded part of the Iowa State study to better understand the technology’s full potential.

“While they were confident about the technology itself, they were looking for some validation of how much carbon could be sequestered and how economical the process could be,” Mba-Wright said. “There are a lot of steps involved in getting this to work at scale.”

One important detail is that this method stacks up well against another popular carbon-removal option – direct air capture. That system also removes carbon from the air, but it’s more expensive to build and doesn’t offer the side benefits of using agricultural waste or creating new revenue for rural communities.

“What we’re trying to show here is that carbon removal doesn’t need to be either/or. There are a lot of opportunities,” Mba-Wright said.

A win for rural economies

Besides helping the climate, this system could give new life to the rural economy. Farmers could sell their leftover crop waste. Forest managers could get paid to clear wildfire-prone debris.

Furthermore, small towns could host bio-oil terminals and benefit from new jobs and investments.

“Iowa State’s experts showed that bio-oil sequestration using corn stover can deliver a high-value, durable carbon removal product that outcompetes other technologies, while providing new markets for crop residues and delivering new economic value to the rural economy,” Reinhardt said.

“As the carbon-removal sector grows, Charm is grateful to work with farm and forest communities to grow this opportunity.”

The full study was published in the journal Energy Conversion and Management.

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