The Pore Space Beneath Our Feet: What We Mean When We Say Carbon Storage
By Sandra Steingraber, Senior Scientist, SEHN
The grainy rocks that underlie many parts of the United States—including the place where I grew up in central Illinois—are currently being sized up to serve as a mass carceral system for molecules of carbon dioxide (CO2) apprehended from the stacks of coal-burning power plants, gas-fired power plants, ethanol distilleries, and various other industries.
This is the storage part of carbon capture and storage (CCS), a process by which solvents (or membranes) are used to capture CO2 from industrial exhaust, pressurize it into a liquid, and then transport it via pipeline to places with rocks deemed capable of functioning as subterranean CO2 prison houses for all eternity.
Another type of carbon capture, still in the research and development stage, uses chemical processes to snag CO2 molecules right out of the atmosphere and then bury them. This is called direct air capture. Either way, the official term for the storage component of the process is “geological sequestration.” (An alternative experimental process that seeks to turn the captured CO2 into stone is called mineralization. See sidebar story for a taxonomy of the carbon capture landscape.)
Promoted as a solution to climate change, carbon capture has received major support by the Biden Administration—and withering criticism from both scientists and frontline communities. Heretofore, most of these objections have focused on the above-ground components—the parts that you can see, hear, and smell.
The carbon-capturing equipment, for example, is highly visible if only because it’s typically powered by massive gas turbines. The turbines’ own emissions are not captured. Nor are any toxic co-pollutants such as benzene, soot, and smog-making nitrogen oxides, which still pour freely out of the stacks. Hence, the energy-intensive act of capturing carbon makes local air pollution worse, raising health risks in the communities where it is deployed. And because power plants and other heavy industries targeted for carbon capture are disproportionately located in low-income neighborhoods and communities of color, the act of capturing carbon is an environmental justice issue.
In short, point-source carbon capture is loud, smelly, and polluting—and extends the lifespan of polluting industries that might otherwise be shut down and replaced by, say, solar arrays. Unsurprisingly, people who live in the surrounding community and whose health is being imperiled have opinions.
(It also doesn’t work very well. A 2019 study found that equipping a coal plant with carbon-capturing technology would, over a 20-year period, reduce its CO2 emissions by only 10 percent. A 2020 review of the evidence found that carbon-capturing technology suffers from systemic failures and, as is currently practiced, releases into the atmosphere more CO2 than it removes.)
A second highly visible component of CCS is the network of pipelines that transport the liquefied CO2 to their stony dungeons. Widespread development of CCS at commercial scale requires massive pipeline construction, with one industry-funded study proposing 66,000 miles of CO2 pipelines, including more than 13,000 miles of interstate lines. These pipelines may leak or rupture in ways that cause asphyxiation hazards for nearby residents due to the ability of CO2 to displace oxygen.
Unsurprisingly, people who live near proposed pipeline routes also have opinions. For example, the disclosure that at least two of the three CO2 pipelines planned for Iowa are seeking to use eminent domain to seize property from landowners who refuse to sign easements has been met with public outcry. Hundreds of Iowans have signaled that that they won’t sign, and many landowners and farmers have joined a coalition with Indigenous organizations and environmental groups to oppose the pipelines outright. On November 9, 2022, protesters yelling “carbon capture pipelines are not safe!” disrupted the proceedings of the National Carbon Capture Conference in Des Moines.
The Vague Territory of Pore Space
After the capture and the journey through the pipelines comes the out-of-sight-out-of-mind part of the process. The language here very quickly becomes vague. The liquid CO2 is injected into “suitable geological formations” where it “remains in the pore spaces of the rock for a long time.” [Emphasis to vagueness added by me.]
Exactly, you may ask, what is pore space? The short answer is simple. Certain rocks, like sandstone, are made of grains. Pore space refers to the microscopic area between the grains. You can think of “suitable geological formations” for carbon storage as stony sponges with many very tiny holes able to absorb and hold liquids. Except there is a catch: the spaces between the pores in these deep rock layers are already occupied by brine. Hence, as the liquid CO2 is pumped into the rock, the salty liquid inside the pores has to move elsewhere and/or the pressure within the pores goes up. (To control these problems, some underground carbon storage operations extract the brine and bring it up to the surface where it becomes a waste product that itself has to be stored somewhere and “managed.”)
Like oil on water, liquid CO2 is less dense than brine so will rise, via buoyancy, when injected into a brine-filled rock until it bumps up against whatever layer of rock lies above. This lid—typically made of shale—is called the cap rock and, for geological carbon sequestration to work, it needs to be impermeable and without fractures.
Once it floats up underneath the cap rock, the captured CO2 can migrate horizontally a substantial distance. When does it stop? It’s not clear. In some cases, “carbon dioxide stops moving within decades to centuries” whereas in other cases it “will remain potentially mobile for thousands of years.”
Thus, when assessing whether the porous rocks below our feet can hold our carbon emissions until the end of time, one task is to determine whether or not the overlying cap rock, extending in all directions, has any pre-existing cracks and another is to predict whether or not the act of injecting the carbon through that same rock layer will create cracks. The ability to control pressures within the pore space and avoid fracturing the lid during injection depends on very specific geo-mechanical conditions and the tools to continuously appraise them.
The world’s largest carbon capture and storage project—a $3.2 billion operation attached to Chevron’s $54 billion Gorgon LNG plant in Western Australia—has been plagued with pore space problems. As of November 2022, according to Chevron’s annual report to the state government, it was only operating at one-third capacity after six years of trying. Unanticipated high pressures within the sandstone layer chosen for storage slowed the rate of CO2 injection and required the construction of wells to remove the water inside the rock to make room for the CO2.
But then the wells clogged with sand, and the captured CO2 was vented into the air, and this headline appeared in the Sydney Morning Herald: “Gas giant’s $3.2 b effort to bury carbon pollution is failing.”
Earthquakes
Increasing pore space pressures by injecting liquid into it raises questions about seismicity. We know with certainty that injection of fracking waste into porous geological formations increases pore pressure in ways that can trigger stressed fault lines to slip. The result can be earthquakes. The injection of liquefied CO2 likely poses similar risks. Researchers have documented microseismic events at multiple carbon storage projects, including the Illinois Basin-Decatur Project in central Illinois.
And how do we predict whether a site with the kind of rock judged suitable for holding CO2 for all eternity is at high or low risk for earthquakes? According to a consensus study report from the National Academies of Sciences, Medicine, and Engineering, we don’t: “Information and knowledge are not sufficient to distinguish low-risk from high-risk sites. While many sites have experienced a larger number of microseismic events because of CO2 injection, whether these indicate the potential for larger events is uncertain. Research is needed.” [Emphasis to vagueness added, again, by me.]
Capacity
One way to keep CO2 out of the atmosphere is to leave fossil fuels in the ground, electrify everything, and make the electricity from wind, water, and solar sources. This path to decarbonization would obviate the need for coal-burning power plants, fracking operations, and most of the ethanol plants scattered across the midwestern landscape. (More than 98 percent of gasoline sold in the United States contains up to 10 percent ethanol. Without internal combustion engines in vehicles, demand for ethanol drops off greatly.)
The other path—the carbon capture and storage path—requires not only the ability to find places in the subterranean landscape that are made up of grainy rocks topped by uncracked stone lids but also the ability to predict how much CO2 the rocks can collectively hold without leaking or triggering earthquakes.
The capacity of a carbon sequestration site is equal to the total volume of all microscopic spaces between all the grains, and, of course, this is a tricky thing to measure. The Prairie Research Institute (PRI), for example, is currently attempting to establish the feasibility of a commercial-scale CO2 storage complex in Macon County, Illinois—more specifically in a thick layer of rock 3,000 feet underneath Macon County (and parts of Christian County) called the Mt. Simon sandstone. It’s a pilot project designed to figure out if 50 million metric tons or more of CO2 captured from nearby smokestacks can fit inside this rock and stay there—and, more broadly, if the whole larger geological formation, which underlies most of the state plus parts of Indiana and Kentucky, might be put to work storing tens of billions of tons of CO2.
Everything about this plan depends upon how to accurately model the collective pore space of a rock soaked with saltwater located more than a half mile below the sunlit cornfields.
Questions about pore space
Last month, I spoke at a stakeholder meeting related to PRI’s study results, soon to be submitted to the Governor and the Illinois General Assembly, which commissioned the report. What caught my eye was the authors’ offhand admission that the predicted capacity of the Macon County storage site is estimated at “plus or minus 30 percent.”
Really? The size of the bunker thought suitable to serve as the forever home for coal plant emissions might be one third bigger or smaller than we thought? What happens if there is less pore space than we thought? What happens if the capacity of the rock to store carbon is exceeded? Will the cap rock crack? Will the CO2 escape? And what about earthquakes?
Consider that robust modeling of both pressure build-up and CO2 plume migration are requirements for any carbon storage design, according to the National Academies report. More specifically, it’s design work that requires “multi-scale, multi-physics, computationally intensive modeling with supercomputers and experimental tools… combined with advanced algorithms for solving large sets of non-linear equations.”
Seems like a lot of math. Seems like you wouldn’t want to be off by a third.
Other questions: Who owns the pore space? Who can grant the right to sequester CO2 in the tiny spaces between grains of rock? What if the people who live above the pores object?
Also, underground carbon storage facilities are essentially prisons with roofs and floors but no walls.* What if the sequestered CO2 migrates sideways into pore spaces outside the injection zone? Who holds the right of refusal for these pore spaces?
According to the National Academies report, “once trapped below the seal, the CO2 is expected to remain sequestered permanently unless it encounters a permeable fault or fracture in the seal or a leaky wellbore.”
Who decides unless is okay?
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This essay expands upon expert testimony delivered by Sandra Steingraber on October 5, 2022 to representatives of the University of Illinois’ Prairie State Research Center in Champaign at a “listening session.” Last year the Center was charged by the Illinois General Assembly to prepare a report on carbon capture and storage as a climate mitigation strategy to be submitted to the Assembly by the end of December. Based on its presentation in October, the Center, which has received millions of dollars for CCS technology, appears poised to recommend it.
* Confined formations that are bounded on all sides by impermeable rock do exist although they make less-than-ideal storage because of the possibility of pressure build-up and the need to remedy that problem by extracting the salty fluid inside them, which then requires storage of its own—adding more expense and additional environmental harms.
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The Taxonomy of Carbon Capture and Storage
by Sandra Steingraber and Peter Montague
Industrial CO2 storage projects use one of three methods.
The first type is saline storage, in which porous, saltwater-filled rocks a half mile or more under the earth’s surface are repurposed for use as CO2 storage lockers. Saline storage is also called geological sequestration and is the focus of the accompanying essay. No commercial-scale saline storage operations have proceeded past demonstration projects.
The second type uses depleted oil fields as the warehouse. This is the oldest form of CO2 storage, and it’s already up and running. As originally intended, its purpose was not to keep CO2 out of atmosphere but to use it to push any remaining oil still stubbornly clinging to the rocks up to the surface in a process known as enhanced oil recovery (see Peter Montague’s essay in this edition of the Networker).
To date, about 75 percent of all captured CO2 has been used to recover more oil for commercial use. Of course, that oil is eventually lit on fire, making more CO2, and making the climate emergency even worse. For every ton of CO2 stored below ground in an enhanced oil recovery operation, two to five tons are released into the atmosphere.
The new kid on the block is mineralization, which combines CO2 molecules with minerals, turning them into stone—à la the sorcerer Koschei the Deathless in Stravinsky’s The Firebird, who turned the citizenry of his kingdom into garden statuary. Mineralization is energy intensive, very expensive, still in the experimental stages of research and development, yet is the only guaranteed permanent way to imprison CO2 forever.
The CO2 captured from smokestacks and other point sources, as well as the CO2 snatched right out of the atmosphere (so-called Direct Air Capture, DAC), can be sent to any of the three kinds of storage. By historical accident, most DAC CO2, which is also still in the experimental stage, is currently going to mineralization.