BP has started what it calls a 'static kill' operation to permanently seal the underwater well in the Gulf of Mexico.
Specially formulated mud will be pumped into the top of the well, pushing the oil back down into the reservoir. The BBC reports the procedure could take up to 2.5 days.
Afterwards, the top of the well could be filled with cement.
The US government estimates the well leaked 4.9 million barrels of oil before being capped last month.
The well ruptured after an explosion and fire on the Deepwater Explorer drilling rig on 20 April, in 11 workers were killed. The leak was finally stopped on 15 July.
Last week, BP reported a record loss of $US17 billion, having set aside $US32 billion to cover the costs of the disaster.