The heavy drilling mud forced down into the ruptured well appears to have stopped the oil leak in the Gulf of Mexico, as company engineers keep watch to make sure the well remains stable. The next step would be deciding whether to cement the well.
BP said Wednesday that the heavy mud pumped down the blown-out Gulf well has stoppered the flow of oil and that the pressure is stable, calling it "a milestone" toward ending the worst spill in U.S. history.
Crews began injecting mud through the temporary cap and into the well itself, nearly a mile under the Gulf, on Tuesday afternoon. After eight hours, they managed to achieve what engineers call a "static condition" in which the pressure of the mud from the surface and that of the oil pushing from the bottom are equalized.
"It's a milestone. It's a step toward the killing of the well," said BP spokeswoman Sheila Williams.
Engineers must now decide whether to also pour in cement, she said.
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 below. BP has said that may be enough by itself to seal the well.
However, work on an 18,000-foot relief well will continue in a second effort to seal the well. The goal is to kill the well from the bottom so that it will be sealed off in two directions. A 75-ton cap placed on the wellhead last month has helped keep oil from spilling into the Gulf, but it was considered only a temporary fix.
BP also 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.
A BP executive of the British oil giant suggested this week that the "bottom kill" might not be necessary. That prompted retired Coast Guard Adm. Thad Allen, the government’s point man on the spill, to demand that the company continue with the procedure regardless of the static kill's success.
"There should be no ambiguity about that," Allen said. "I'm the national incident commander, and this is how this will be handled."
The apparent disagreement reflects some concern about how to safely kill the well once and for all, NPR science correspondent Richard Harris said.
"The mud is a secure solution, but not a permanent one," he said, adding that there are differing opinions over how to inject the cement to make the seal permanent.
"One possibility is to put cement on top of the heavy fluid that's in the well right now and push that down into the well," Harris said. "That would, however, push the heavy fluid into the rock [around the well bore], because the fluid has to go somewhere. Some geologists are concerned that if that's not done very carefully, it may actually fracture the rock somewhat."
The other option, Harris said, is to complete the relief well, which only has a few hundred more feet to go, and then pump the cement from down there. "The risk is that the relief well might miss on the first try and that the procedure could also be interrupted by hurricanes," he said.
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.
White House energy adviser Carol Browner said on morning TV talk shows Wednesday that a new assessment found that about 75 percent of the oil has 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.
The latest government estimate said roughly 4 million barrels of oil spewed into the Gulf after the April 20 explosion aboard the Deepwater Horizon rig that killed 11 workers. Copyright 2010 National Public Radio. To see more, visit http://www.npr.org/.