New Orleans, La. – A major unknown is whether the mud can push the oil back directly down the well and into the undersea reservoir. Coast Guard Admiral Thad Allen says pressure readings will be closely watched as each barrel of mud makes its way into the containment cap at the wellhead.
"My duty as the national incident commander is to give you my best, you know, view and it may be a little conservative but I think we need to understand we don't know the condition of the well until we start to put mud in it."
Allen says if pressure holds to safe levels, more and more mud will be pumped at higher and higher rates. It's a process called a static kill. BP is claiming it could close up the busted well, but the government insists a relief well that's almost finished be used to cement it closed later this month.
For NPR News, I'm Eileen Fleming in New Orleans.