The oldest of these wells were abandoned in the late 1940s, raising the prospect that many deteriorating sealing jobs are already failing.
The AP investigation uncovered particular concern with 3,500 of the neglected wells - those characterized in federal government records as "temporarily abandoned."
Regulations for temporarily abandoned wells require oil companies to present plans to reuse or permanently plug such wells within a year, but the AP found that the rule is routinely circumvented, and that more than 1,000 wells have lingered in that unfinished condition for more than a decade. About three-quarters of temporarily abandoned wells have been left in that status for more than a year, and many since the 1950s and 1960s - even though sealing procedures for temporary abandonment are not as stringent as those for permanent closures.
As a forceful reminder of the potential harm, the well beneath BP's Deepwater Horizon rig was being sealed with cement for temporary abandonment when it blew April 20, leading to one of the worst environmental disasters in the nation's history. BP alone has abandoned about 600 wells in the Gulf, according to government data.
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There's ample reason for worry about all permanently and temporarily abandoned wells - history shows that at least on land, they often leak. Wells are sealed underwater much as they are on land. And wells on land and in water face similar risk of failure. Plus, records reviewed by the AP show that some offshore wells have failed.
Asked in multiple requests over several weeks how often abandoned wells have failed, the U.S. Environmental Protection Agency acknowledged Tuesday - as this story was being released - that it has had to deal with leaks at abandoned wells in shallow state waters of Louisiana and Texas. The U.S. Bureau of Ocean Energy Management, Regulation and Enforcement - which oversees wells in federal waters - also acknowledged Tuesday that it has dealt with "a few" failed abandoned wells farther out in the Gulf. But the information was released only through the public affairs offices and neither agency provided experts for follow-up.
Experts say abandoned wells can repressurize, much like a dormant volcano can awaken. And years of exposure to sea water and underground pressure can cause cementing and piping to corrode and weaken.
"You can have changing geological conditions where a well could be repressurized," said Andy Radford, a petroleum engineer for the American Petroleum Institute trade group.
Whether a well is permanently or temporarily abandoned, improperly applied or aging cement can crack or shrink, independent petroleum engineers say. "It ages, just like it does on buildings and highways," said Roger Anderson, a Columbia University petroleum geophysicist who has conducted research on commercial wells.
Despite the likelihood of leaks large and small, though, abandoned wells are typically not inspected by industry or government.
Oil company representatives insist that the seal on a correctly plugged offshore well will last virtually forever.
"It's in everybody's interest to do it right," said Bill Mintz, a spokesman for Apache Corp., which has at least 2,100 abandoned wells in the Gulf, according to government data.
Officials at the U.S. Interior Department, which oversees the agency that regulates federal leases in the Gulf and elsewhere, did not answer repeated questions regarding why there are no inspections of abandoned wells.
State officials estimate that tens of thousands are badly sealed, either because they predate strict regulation or because the operating companies violated rules. Texas alone has plugged more than 21,000 abandoned wells to control pollution, according to the state comptroller's office.
Offshore, but in state waters, California has resealed scores of its abandoned wells since the 1980s.
In deeper federal waters, though - despite the similarities in how such wells are constructed and how sealing procedures can fail - the official policy is out-of-sight, out-of-mind.
The U.S. Minerals Management Service - recently renamed the Bureau of Ocean Energy Management, Regulation and Enforcement - relies on rules that have few real teeth. Once an oil company says it will permanently abandon a well, it has one year to complete the job. MMS mandates that work plans be submitted and a report filed afterward.
Unlike California regulators, MMS doesn't typically inspect the job, instead relying on the paperwork.
The fact there are so many wells that have been classified for decades as temporarily abandoned suggests that paperwork can be shuffled at MMS without any real change beneath the water.
With its weak system of enforcement, MMS imposed fines in a relative handful of cases: just $440,000 on seven companies from 2003-2007 for improper plug-and-abandonment work.
Companies permanently abandon wells when they are no longer useful. Afterward, no one looks methodically for leaks, which can't easily be detected from the surface anyway. And no one in government or industry goes underwater to inspect, either.
Government regulators and industry officials say abandoned offshore wells are presumed to be properly plugged and are expected to last indefinitely without leaking. Only when pressed do these officials acknowledge the possibility of leaks.
Despite warnings of leaks, government and industry officials have never bothered to assess the extent of the problem, according to an extensive AP review of records and regulations.
That means no one really knows how many abandoned wells are leaking - or how badly.