BP has finally begun the so-called “static kill” procedure that should soon plug its damaged well for good, 106 days into the oil crisis in the Gulf of Mexico.

BP conducted a successful flow test Tuesday, after a delay Monday night, clearing the way for the static kill to begin. The company began pumping heavy kill mud down the well Tuesday afternoon, pushing the oil deep underground and back into the reservoir.

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.”

Until the pipe that runs 13,000 feet from the floor of the Gulf of Mexico to the reservoir below is filled, BP workers will continue to pump mud. 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. The oil is forced back down into the reservoir by pumping the mud into the top of the well. The well will technically be dead once that is done because the lighter oil will have no way to push the mud out of the well pipe.

(Thanks to ABC News and Wikipedia)