ON THE GULF OF MEXICO (AP) " BP claimed a key victory Wednesday in the effort to plug its blown-out well and the government said much of the spilled oil is gone " though what's left is still at least quadruple the amount that poured from the Exxon Valdez.
BP PLC reached what it called a significant milestone overnight when mud that was forced down the well held back the flow of crude.
Also, White House energy adviser Carol Browner said on morning TV talk shows that a new assessment found that about 75 percent of the oil has either been captured, burned off, evaporated or broken down in the Gulf.
"It was captured. It was skimmed. It was burned. It was contained. Mother Nature did her part," Browner told NBC's "Today" show. On ABC's "Good Morning America," she said about 25 percent remained.More on the Gulf oil crisis: Time to scrap BP brand? Gas-station owners divided Wildlife rescues increase near Michigan oil spill Oil spill news & multimedia
It was not clear if she was referring to 25 percent of what gushed from the well " about 205 million gallons based on new government estimates released this week " or 25 percent of what made it into the water, about 172 million gallons. The rest was either burned, skimmed or siphoned in the days after the April 20 explosion aboard the offshore drilling rig Deepwater Horizon.
Using the lower number, 25 percent would be about 43 million gallons. Even if the Gulf well had leaked only that much to begin with, it would still be among the worst oil spills in history. By comparison, the Exxon Valdez tanker spill that wreaked environmental havoc in Alaska in 1989 spilled 11 million gallons.
In the Gulf, workers stopped pumping mud in after about eight hours of their "static kill" procedure and were monitoring the well to ensure it remained stable, BP said.
"It's a milestone," BP PLC spokeswoman Sheila Williams said. "It's a step toward the killing of the well."
The next step would be deciding whether to cement the well.
The pressure in the well dropped quickly in the first 90 minutes of the static kill procedure Tuesday, a sign that everything was going as planned, wellsite leader Bobby Bolton told The Associated Press. Bolton said Tuesday night that the procedure was going well. "Pressure is down and appears to be stabilizing," he told the AP then.
Browner told NBC it was good news that the static kill was working but that "we remain focused on the relief well."
The static kill " also known as bullheading " involved slowly pumping the mud from a ship down lines running to the top of the ruptured well a mile below. BP has said that may be enough by itself to seal the well.
But the mud that was forced down the broken wellhead to permanently plug the gusher is only half the story. To call the mission a success, crews working on a flotilla of vessels on a desolate patch of water need to seal off the well from two directions.
An 18,000-foot relief well BP has been drilling for the past three months will be used later this month to execute a "bottom kill," in which mud and cement will be injected into the bedrock 2 1/2 miles below the sea floor to finish the job, retired Coast Guard Adm. Thad Allen said.
"There should be no ambiguity about that," Allen said. "I'm the national incident commander, and this is how this will be handled."
A 75-ton cap placed on the well in July has been keeping the oil bottled up inside over the past three weeks, but is considered only a temporary measure. BP and the Coast Guard want to plug up the hole with a column of heavy drilling mud and cement to seal it off more securely.
A previous, similar effort failed in May when the mud couldn't overcome the unstemmed flow of oil.
BP won't know for certain whether the static kill has succeeded until engineers can use the soon-to-be-completed relief well to check their work.
The task is becoming more urgent because peak hurricane season is just around the corner, Allen said. Tropical Storm Colin formed then dissipated far out in the Atlantic on Tuesday, but early forecasts say it will travel toward the East Coast rather than the Gulf.
Weber reported from aboard the Q4000. Bluestein reported from New Orleans. Associated Press writers Bernard McGhee in Atlanta, Robert Barr in London and Ramit Plushnick-Masti in Houston contributed to this report.
Copyright 2010 The Associated Press.