Quick look
Injecting carbon-rich bio-oil into the deep shafts of abandoned crude oil wells could be an economically feasible system of removing carbon dioxide from the air for long-term storage.
AMES, Iowa - Filling abandoned oil and gas wells with bio-oil made from plant-based leftovers like corn stalks and forest debris could help remove carbon dioxide from the atmosphere, returning carbon underground in deep shafts once used to extract it.
The emerging practice, the focus of a recent study by an Iowa State University research team led by mechanical engineering professor Mark Mba-Wright, has a two-birds-one-stone appeal. Unwanted organic matter collected from forests and fields helps sequester carbon in long-term storage while also reducing the emissions and safety risks posed by the hundreds of thousands of orphaned U.S. oil wells.
"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," Mba-Wright said.
Based on research by Mba-Wright's team, a network of 200 mobile bio-oil production facilities could be an economically and technically feasible expansion of the technology, which is already in limited commercial use. The study, recently published in Energy Conversion and Management, estimated that the proposed system could sequester carbon dioxide for about $152 per ton, making it competitive with other methods of carbon dioxide removal but with far less upfront investment.
"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.
Projecting costs
The core of the proposed system is fast pyrolysis, a process of transforming dried particles of biological material into liquid bio-oil by exposing them in an oxygen-free environment to a few seconds of high heat - temperatures that can exceed 1,000 degrees Fahrenheit. The organic matter holds carbon it pulled from the air via photosynthesis when alive.
The solid byproduct, biochar, can be sold to farmers as a soil amendment. The gas byproduct is captured to be reused as a fuel to help generate the intense heat pyrolysis requires. But the main aim of fast pyrolysis is producing bio-oil, the dense and carbon-rich fluid that forms as the vapor released in the process condenses.
Various uses for bio-oil have been identified and more are being studied. But injecting it into empty fossil fuel wells would maximize bio-oil's carbon capture potential and take advantage of existing underground well shafts that otherwise cost about $1 million to cap. Filling a crude oil well, at an average width of about 1.6 feet and depth of nearly 2.6 miles, takes more than 216,000 gallons of liquid. The 2021 bipartisan infrastructure law allocated $4.7 billion to seal about 120,000 abandoned wells, but estimates cited by the study suggest there are 300,000 to 800,000 undocumented orphaned wells in the U.S.
Under the proposed system, each mobile fast pyrolysis unit would process about 10 tons of feedstock a day. In the Midwest, the main biomass source studied was corn stover, the portion of the plant left in the field after harvest. In the West, it would be wood debris removed from forests to lessen wildfire threats. Researchers also studied switchgrass and oriented strand board as feedstock for the mobile pyrolizers, which would deposit bio-oil in centralized terminals for transport to well sites.
Units would cost about $1.3 million to build, and the bio-oil would need to sell for at least $175 per ton, the study estimated. Carbon removal costs vary by feedstock, with the rate for wood-based materials estimated at about $100 per ton. Abatement costs fall even lower when calculated to account for the carbon in biochar and a learning rate - a factor that estimates how capital and operating costs would reduce over time.
"The more units they build, the better they would get at building them," Mba-Wright said.
Not an 'either/or'
The study was funded in part by Charm Industrial, a San Francisco-based startup which has already struck several carbon-removal deals with large corporations to use vacant wells to permanently store bio-oils. Companies increasingly seek carbon-removal credits to meet commitments to reduce emissions, said Peter Reinhardt, CEO and co-founder of Charm Industrial.
"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," Reinhardt said.
Charm approached Iowa State seeking an independent and detailed assessment of the system's potential, Mba-Wright said.
"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," he said. "There are a lot of steps involved in getting this to work at scale."
A key takeaway is that the system stacks up well against the dominant method of removing atmospheric carbon, a technology called direct air capture that extracts carbon dioxide from air. Direct air capture systems have similar per-ton abatement costs, but they're far more expensive to build and have few other associated benefits, Mba-Wright said.
"What we're trying to show here is that carbon removal doesn't need to be either/or. There are a lot of opportunities," he said.
The techno-economic analysis by Mba-Wright and his colleagues will help companies make reliable investments in their net zero portfolios, and it highlights a path to new revenue streams in rural areas where the biomass is collected, Reinhardt said.
"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," he said. "As the carbon-removal sector grows, Charm is grateful to work with farm and forest communities to grow this opportunity."
