After a delay Monday night, BP conducted a successful flow test today, clearing the way for the static kill to begin. This afternoon, the company began pumping heavy kill mud down the well, pushing the oil deep underground and back into the reservoir.
Watch 'World News' for the latest on the oil spill tonight on ABC.
At the spill site today, Capt. Keith Schultz said the procedure was going well, saying he's "very confident we'll be able to kill this well."
BP workers will continue to pump mud until the pipe that runs 13,000 feet from the floor of the Gulf of Mexico to the reservoir below is filled. The process is expected to be complete no later than Thursday.
It involves simple physics: the oil weighs four pounds per gallon, the kill mud weighs 13.2 pounds per gallon, By pumping the mud into the top of the well the oil is forced back down into the reservoir. Once that is done the well will technically be dead because the lighter oil will have no way to push the mud out of the well pipe.
But that is not the end.
Once the mud is in place, BP will inject cement to close the well permanently. The mud could either be pumped in from the top of the well through the stacking cap, or the bottom through two relief wells. The relief wells are still at least a week away from meeting their target.
"I would say the chances of this working are pretty much up around 95 to 99 percent," said Don Van Niewenhuise, director of petroleum geosciences at the University of Houston.
No matter what, "We want to end up with cement in the bottom of the hole," said BP executive Kent Wells.
Until now, the only thing holding back oil from spewing into the Gulf is the containment cap that was installed on July 15. It was never intended as a permanent fix.
New Estimate Shows Spill was Far Larger Than Previously Thought
The federal government acknowledged today that the amount of oil that has actually poured into the Gulf during this crisis is far worse than the public had been led to believe.
On April 21st, Rear Admiral Mary Landry said, "We don't see a major spill emanating from the source at this time."
By April 30th, the estimate had risen to 5,000 barrels per day, and by June, it had climbed to anywhere from 12 to 25,000 barrels.
Scientists now say that at the start of the crisis, 62,000 barrels per day flooded the Gulf. That brings the total size of the spill to 205,800,000 gallons of oil -- enough to fill 76 floors of the Empire State Building. It is officially the worst accidental oil spill in history, exceeded only by the intentional spill ordered by Saddam Hussein at the end of the first Persian Gulf War 20 years ago.
The Associated Press contributed to this report.