Are there risks to transporting carbon dioxide in pipelines?

CO₂ pipeline accidents have been rare, but as with pipelines carrying oil, natural gas, or hazardous chemicals, if ruptures are allowed to happen they can be very serious.

July 10, 2024

The United States is currently home to about 5,000 miles of pipelines that carry carbon dioxide around the country (compared to about 3 million miles of pipeline for natural gas).¹ Today, CO₂ is mostly transported for oil and gas drilling: companies pump CO₂ underground to loosen more fossil fuels from the ground, a tool called “enhanced oil recovery.” But if many more CO₂ pipelines are built in the future, it will likely be because they can help address climate change. CO₂ is a climate-warming greenhouse gas, and through a technology called carbon capture and storage, it can be collected from smokestacks or removed from the air itself. We can then transport this CO₂ through pipelines to storage locations, and bury it underground where it won’t warm the planet.

Are these CO₂ pipelines safe? “​​Nothing is risk-free. But we have lots and lots of pipelines criss-crossing the United States,” says Howard Herzog, a Senior Research Engineer at the MIT Energy Initiative. “Compared to at least the materials in those pipelines, transporting CO₂ is relatively safe.” The U.S. maintains about 75,000 miles of pipelines that carry other hazardous liquids, including toxic or flammable ones like ammonia and propane, and more than 84,000 carrying crude oil.²

Researchers have found that the failure rates for CO₂ pipelines are similar to those for pipelines carrying oil and gas—which is to say, failures are relatively rare, but are worth taking serious precautions to prevent. From 1986 to 2021, the U.S. recorded around three accidents related to onshore CO₂ pipelines per year.³ (Although it’s important to remember that our small network of CO₂ pipelines means we have limited safety data.) And while big ruptures could release large amounts of CO₂, which would be both dangerous to people and a problem for the climate, the data shows smaller leaks have been much more common. From 1994 to 2021, leaks accounted for 48 percent of CO₂ pipeline incidents while ruptures accounted for 3 percent.³

Serious ruptures can happen, though—for instance, if a natural disaster, like an earthquake or flood, disrupts the integrity of a pipeline. This is what happened in the only mass-exposure event from a CO₂ pipeline failure to date, when a landslide damaged an underground pipeline carrying CO₂ near the village of Satartia, Mississippi, in 2020.

If a pipeline is compromised, CO₂ can leak from it. CO₂ is transported at high pressures in a “supercritical” liquid phase, but in the open air, it turns to gas as it rushes out of a ruptured pipe. Dry ice—the solid form of CO₂—may also form at the opening, which could further damage the pipeline.

As a gas, carbon dioxide is heavier than air. When large amounts of it are released, it hugs the ground and can displace oxygen—including in people's lungs. “The biggest risk is it being an asphyxiant,” says Herzog. Whether that happens depends on the amount of CO₂ that escapes, the landscape of the region, and the weather. In addition to asphyxiation, breathing in concentrated CO₂ can cause headaches, dizziness, sweating, increased heart rate, and other maladies. If people can get to fresh air, these symptoms typically pass (although treatment with oxygen is recommended)—but if high levels of CO₂ sit in low-lying areas on a windless day, or build up indoors, people could be hurt or killed. After the rupture in Satartia, forty-five people went to the hospital for treatment.

If other substances are mixed with the CO₂, that can also present dangers. The rupture that impacted Satartia, for instance, also released hydrogen sulfide, a toxic chemical that can cause convulsions, coma, and even death. “I think with high probability it was the hydrogen sulfide that sent people to the hospital,” says Herzog. “Patients reported a green gas and rotten egg smell, which could only be hydrogen sulfide, and this is a highly toxic chemical. Fortunately, CO₂ pipelines carrying toxic materials are the exception, not the rule. As CO₂ pipelines get built for carbon capture and storage, as opposed to enhanced oil recovery, there will be no reason to transport toxic chemicals in them.”

In April 2024, another section of CO₂ pipeline in Mississippi, owned by the same company as the Satartia pipe, also failed, though the cause is still being investigated.

Building CO₂ pipelines far from people, and ensuring emergency procedures are in place, can reduce risks if ruptures do happen. Prior to the rupture in Satartia, there had been one injury associated with carbon pipelines in the U.S. in the last two decades.⁴

Regulations and careful management can also help make pipelines as safe as possible, says Herzog. Operators must “dry” CO₂ and monitor it for impurities, including even water, before it enters a pipeline, so that reactions in the pipe don’t produce acids that may cause corrosion.⁵ Checking valves and fittings regularly should let companies know whether equipment is damaged or leaking. And regulators have a responsibility to make sure companies follow these practices, monitor for leaks, and accurately report them. (Research has shown oil and gas pipeline leaks across the U.S. have gone undetected in the past.⁶)

As carbon capture becomes more common, companies are expected to build out many more pipelines—up to 66,000 miles of them by 2050⁷—to ferry CO₂ around the country. To avoid future failures, the U.S. Department of Transportation’s Pipeline and Hazardous Materials Safety Administration recently began the process to create new rules for CO₂ pipelines and emergency preparedness.⁸ Those regulations will be important, because many scientists agree we will need to remove some carbon from the atmosphere to meet the world’s climate targets and minimize the harms of climate change. The transport of carbon for underground storage will need to be safe and effective at scale if we expect to include carbon removal in our toolkit of climate solutions.

Thank you to Hans of Sydney, Australia, for the question.

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