Retired Coast Guard Adm. Thad Allen said crews are having trouble finding patches of the crude that had been washing up on beaches and coating delicate coastal wetlands since the Deepwater Horizon offshore drilling rig exploded April 20, killing 11 people.
Crews have taken a crucial step toward readying the relief well they need to permanently stop the oil, removing a plug they had popped in to keep the well safe ahead of Tropical Storm Bonnie.
Allen also said Wednesday that a temporary cap put on the busted well two weeks ago is holding firm. Before that, it spewed 94 million to 184 million gallons of oil.
Though no one knows for sure how much oil might be lurking below the surface, most of what was coming ashore has broken up or been sucked up by skimming boats or burned.
"The oil that we do see is weathered, it is sheen," Allen said.
Barring a calamity, the oil won't start flowing again before BP PLC can permanently kill the well, which could happen as soon as mid-August. Allen said the Coast Guard expects oil to keep showing up on beaches four to six weeks after that happens.
Then, he said, the Coast Guard may start redeploying some of the 11 million feet of boom, 811 oil skimmers and 40,000 people that have been part of the oil spill response. Many of the workers are fishermen who have lost their livelihoods because of the spill.
Crews are taking every precaution as they work toward the permanent fix it's hoped the relief well will deliver.
"We have always asked for a backup plan for the backup plan," he said. "This relief well, while it is deep, it is something that has been done before. Obviously the depth is an issue here. But we are confident we are going to get this thing done."
Drilling the relief well has been a monthslong task, and BP had used several other techniques to stop the leak that had never been attempted before in mile-deep waters. Some were utter failures and none was totally successful until a carefully fitted cap was placed over the well and the leak stopped in mid-July.
The cap has stopped the flow but is only a temporary measure while crews finish the relief well that will plug up the gusher from below.
The work had to stop last week because of Bonnie, which passed through in weakened form without doing any major damage.
Now that the plug is out, the relief well must be flushed out with drilling mud before casing can be dropped in and cemented. All that should be done around Monday, Allen said, though he cautioned that was just an estimate.
Once everything is in place, crews will begin a procedure known as a static kill, pumping heavy mud straight down the well though the temporary cap and failed blowout preventer. If the well casing is intact, the mud will force the oil back down into the natural petroleum reservoir. Then workers will pump in cement to seal the casing.
The static kill is on track for completion some time next week. Then comes the "bottom kill," where the relief well will be used to pump in mud and cement; that process will take days or weeks, depending on the success of the static kill.