The 14,000 inactive oil and gas wells in the US Gulf of Mexico region can still leak and harm marine ecosystems in the process. However, the cost of securing them could reach $30bn (£23.7bn), according to researchers at the University of California.
The team collected data from the US Bureau of Safety and Environmental Enforcement on the 82,000 wells drilled in the Gulf of Mexico. Although the majority of the wells have been secured, the researchers found evidence that more than 14,000 wells are still unplugged, despite having been inactive for at least five years - a point beyond which they are unlikely to restart production.
Operators are legally required to plug wells once they are taken out of production, which usually involves a cement cap covered with sediment.
"The wells aren't supposed to just be leaking into the environment if they're not actively producing, but sometimes they do," said Mark Agerton, lead author of the study published in the journal Nature Energy.
The team drew special attention to the 13,000 idle wells located in shallow waters close to the shore, near the coastlines of Texas, Louisiana and Alabama, or in federal jurisdiction. These are the wells that would pose the greatest risk to human life, as oil leaks in these locations would be more likely to harm human health. Moreover, any leakages in these wells would be more likely to reach the surface and go up into the atmosphere.
The research found that while 90 per cent of the inactive wells in the Gulf of Mexico were in shallower waters closer to shore, they accounted for only a quarter of the $30bn required to plug all of the remaining wells.
"The policy implication is that you'd likely focus on those shallow ones," said co-author Gregory Upton, of Louisiana State University, in a press briefing.
Both taxpayers and fossil fuel giants would likely be liable for the costs of plugging and abandoning the wells, the researchers said. This is a complex process that includes encasing the opening in concrete to stop oil and the potent greenhouse gas methane from leaking out.
In the case of the Gulf of Mexico wells, the study found that of the total in estimated costs for plugging inactive wells, under $2bn (£1.5bn) were in state waters, meaning the cost would have to be covered by taxpayers.
In contrast, the vast majority of costs were in federal waters and had been owned in the past by "supermajor" companies such as Chevron, Shell, ExxonMobil, ConocoPhillips, BP, Total and Eni.
"I think that points to a very strong conclusion that before a taxpayer would be liable for a well, there's a large oil and gas firm that would be liable," said Agerton.
