The polar blast in Texas earlier this year revealed a dirty secret in the most prolific U.S. oil field: Two under-the-radar natural gas plants that are a persistent source of pollution.

Natural disasters in the state often turn into environmental disasters, and February’s cold wave was no exception. Stricken by power outages and mechanical failures, industrial facilities burned off or released huge quantities of hazardous gases as they shut down. The worst culprits, however, weren’t the vast petrochemical complexes on the Gulf Coast but the two Permian Basin facilities that take raw gas from wells and purify it into sales-quality fuel.

The Wildcat and Sand Hills plants, both run by Houston-based Targa Resources Corp., accounted for almost 20% of the state’s total pollution during the freeze and nearly four times the amount emitted by the country’s biggest refinery, according to an analysis of state records that was carried out by Air Alliance Houston, Environment Texas and Environmental Defense Fund and reviewed by Bloomberg.

The winter storm wasn’t a one-off: The two plants have released hazardous gases above permitted levels more than 400 times since the beginning of 2019, according to filings with the Texas Commission on Environmental Quality. That’s equivalent to about once every two days. Targa didn’t respond to multiple requests for comment.

That two mid-sized processing plants can become super emitters underscores how the vast U.S. gas network is only as strong as its weakest link. Environmental goals adopted by producers and refiners count for little if the facilities holding the system together are vulnerable to breakdowns. The crisis triggered by the freeze has spurred notoriously regulation-averse Texas into action, with state lawmakers weighing bills that may require gas plants and pipelines to be winterized.

“When you have hundreds of emission events a year, is it really an upset or is it just usual operating conditions?” said James Doty, a former TCEQ mobile monitoring manager. “You would obviously expect more emissions from the biggest oil and gas facilities in the world than you would from these types of plants.”

State filings likely don’t even capture the scope of the pollution released during the storm. Companies are required to report hazardous compounds like benzene, hydrogen sulfide and sulfur dioxide but not greenhouse gases such as carbon dioxide and methane.

Targa suffered major outages as a result the storm, with its operations at about half capacity during the 10-day period, Chief Executive Officers Matthew Meloy told investors on a Feb. 23 earnings call. He declined to provide any more detail and didn’t specify whether the problems were caused by the cold, power outages or both.

One reason for the high volume of emissions from gas processing plants is that the facilities are in short supply in the Permian, where supplies of the fuel have swelled as a byproduct of drilling for more-valuable oil in the past decade. As a result, the plants operate under extreme pressure and one mechanical fault can cause outages that lead to flaring, or burning off gas that has nowhere to go.

“Gas processing plants are designed for continuous flow, so if you get unexpected outages on the outbound side, you have to start flaring immediately for all the gas that’s still incoming,” said Artem Abramov, head of shale research at Rystad Energy. “Infrastructure is super stretched, so when one plant goes offline, you have high line pressure“ in the surrounding pipelines that collect gas from wells.

In a vicious cycle, that extra pressure can mean more outages. Loss of gas supply to power plants contributed to blackouts during the freeze, which in turn compounded operational problems at gas processors. There was “a complete collapse of general infrastructure,” Abramov said.

Targa’s TCEQ filings shed some light on what led to the plants becoming the state’s top polluters, emitting more than Exxon Mobil Corp.’s giant Beaumont and Baytown refining and petrochemical complexes combined.

“Sustained freezing temperatures” at Wildcat in Winkler County caused its amine, or acid removal, system to fail, meaning that the plant had to flare incoming gas for seven days straight, Targa said in a TCEQ filing. Separately, Targa reported it released 4.9 million cubic feet of gas over a 10-hour period after a “hydrate”—likely an ice blockage—was found in a pipeline near the plant. That’s about the same amount of gas used by 25,000 U.S. homes each a day.

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