NEW ORLEANS – The only thing keeping millions more gallons of oil out of the Gulf of Mexico right now is a rush job: an experimental cap that has held for more than two weeks but was never meant to be permanent. As soon as this week, crews will pump in some insurance.

Engineers are preparing to launch a so-called static kill as early as this evening, shoving mud and perhaps cement into the blown-out well to make it easier to plug the gusher forever and end the Summer of the Spill.

The effort carries no certainty, and BP PLC engineers still plan to follow it up days later by sending a stream of mud and cement into the bottom of the mile-deep underground reservoir through a relief well they’ve been digging for months.

But the oil giant’s engineers and petroleum experts say it’s the clearest path yet to choke the blown-out well and make it even easier for the crews drilling the relief well to ensure oil can never again erupt from the deep-sea well, which has spewed as much as 184 million gallons since the rig connected to it blew up in April and killed 11 workers.

“It could be the beginning of the end,” said Darryl Bourgoyne, director of the Petroleum Engineering Research Lab at Louisiana State University.

When it begins, crews will slowly pump heavy mud through lines installed last month straight down the throat of the leaky well. If the mud forces the oil back into the massive underground reservoir and scientists are confident the pressure stays stable, then engineers can pump in fresh cement to seal it.

“The only thing that separates the oil from the sea now is the valve. This puts thousands of feet of mud and cement in between,” said Eric Smith, associate director of the Tulane Energy Institute. “The idea is to have as many barriers as possible between the ocean and the reservoir. We’re adding an extra level of safety.”

Officials may then begin the process of choking the underground reservoir feeding the well by pumping mud and then cement down an 18,000-foot relief well. BP officials have long said the process is the only sure way to choke the well for good — plugging up the source of the oil, not just its route to the sea.

No oil has leaked from the busted well since engineers were able to fix a tightly fitting cap over its outlet two weeks ago, and boats skimming the oil and spraying subsea dispersant have been able to contain some of the spill.

But critics have raised questions about the long-term effects of the dispersant on sea life, and congressional investigators said Saturday that the Coast Guard routinely approved BP requests to use thousands of gallons of chemicals a day despite a federal directive to cut its use.

Retired Coast Guard Adm. Thad Allen said Sunday that federal regulators did not ignore environmental guidelines, but that some field commanders were given the authority to allow more dispersants to be used on a case-by-case basis.

BP’s engineers, meanwhile, focus their time on their latest bid to stymie the oil for good. If the static kill attempt sounds familiar, that’s because it is. The company tried a similar process, called a top kill, to choke the well with mud in May. It failed partly because the mud couldn’t overcome the flow of the oil.

There’s reason to hope this time will be different. For one, the oil is no longer freely flowing from the well, thanks to the temporary cap that has contained the out-of-control gusher for two weeks. That means that engineers won’t have to pump in mud with as much force, said Kent Wells, a BP senior vice president.