Coast Guard Petty Officer Carlos Vega said the accident happened about 8 p.m.
A statement from Swift said the well in January 2008 was shut. Its last production test, shortly before that, showed 18 barrels — 756 gallons — of oil, 3 barrels of water and 59 thousand cubic feet of natural gas per day, the company said.
The site is about 50 miles west of where the BP-leased drilling rig Deepwater Horizon was drilling at the Macondo well when it exploded in April 2010, killing 11 workers on the rig and setting off the nation's worst offshore oil disaster.
Capping the BP well was hampered by the force of oil and gas pushing out of the well — an estimated 200 million gallons of oil eventually escaped — and because the well was about a mile under the Gulf surface. It took months for engineers to stop oil from flowing into the Gulf.
A trial to determine responsibility for the BP spill is going on in U.S. District Court New Orleans, about 70 miles north of the Swift wellhead.