MIAMI — BP on Tuesday finally began slowly strangling its blown-out well at the bottom of the Gulf of Mexico, injecting heavy drilling mud slowly but steadily into the well in a plan to drive the crude back into the deep rock formations from which it first surged more than three months ago.
The company and the Obama administrations cautioned it would take another step and a week or more to officially pronounce the monstrous gusher dead, but a successful "hydrostatic kill" operation would drive one huge nail in the coffin.
Early on, the signs from a mile below the Gulf's surface were encouraging. BP said it began the process, which injects a dense "drilling mud'' tipping the scales at 13.2 pounds a gallon to muscle oil and gas back down its ancient reservoir, around 4 p.m. Eastern Time after what BP Vice President Kent Wells called some "text book'' tests.
Those tests, which officials called the "injectivity test," showed that the oil could be successfully backed back into the reservoir.
How long the kill procedure, known as bullheading, would last was uncertain. Wells, in a briefing to reporters 30 minutes after the procedure began, didn't say how fast the mud was being pumped into the well. During the "injectivity test," he said, various rates were tried, from one barrel of mud per minute to seven barrels of mud a minute. A barrel is equal to 42 gallons.
Those rates suggested that filling the 2,000-barrel volume of the wellbore — 84,000 gallons — could take from five hours at seven barrels a minute to 33 hours at one barrel a minute.
Wells, in an earlier tele-conference, and federal officials estimated it would take several days to assess whether the operations had permanently plugged a well that spewed nearly 5 million barrels of crude into the Gulf, a volume some 20 times larger than the nation's previous largest offshore oil spill.
Both Thad Allen, the retired Coast Guard admiral who leads the federal response task force, and Robert Gibbs, President Barack Obama's press secretary, stressed that there would be no declaring victory until BP completes a relief well and delivers a final ‘‘bottom kill."
"And there should be no ambiguity about that," Allen said. "I'm the National Incident Commander, and that's the way this will end."
Though a massive 75-top "stacking cap'' sealed the well in July, Allen said it remained unclear where the flow was coming from inside a well running some 2½ miles below the sea floor.
Bullheading can plug the well's inner casing, he said, but an internal rupture also might be allowing oil or gas up the annulus, the open space between the casing that normally carries oil and gas and the larger bore hole surrounding it.
Earlier in the week, Wells had suggested that the static kill alone might be enough to finish off the well.
But Allen, supported by a team of federal scientists led by Energy Secretary Steven Chu, have argued that the only assured permanent plug is for the relief well to penetrate the annulus and pump in more mud and cement.
"We need to go into the bottom to make sure we fill the annulus, the casing, and any drill pipe there and then follow that with cement," Allen said. "This thing won't truly be sealed until those relief wells are done."
Earlier in the day, Allen said BP had completed cementing in casing for its primary relief well, which is just 100 feet from its target 18,000 feet below the Gulf's surface.
Wells said the relief well may not reach its target before Aug. 15.