Top Kill

BP Plc's (BP.L) next attempt to stanch the flow of oil from its blown-out Gulf of Mexico well will be a delicate balancing act for engineers pumping dense fluids from the surface, and is deemed as likely to fail as succeed.

Few can offer any firm predictions about it because a "top kill," as the method is known, has never before been attempted nearly a mile beneath the ocean surface.

After more than a month of oil gushing into the Gulf, BP remains hopeful but realistic about its chances of success. The top kill could start as early as Wednesday.

The procedure will make use of tubes already on the blowout preventer (BOP) to get near the well at the bottom of the BOP. A heavy fluid known as drilling mud will be injected in a bid to reverse the well's outward flow and push the oil back down.

To pull this off, BP says it has 30,000 hydraulic horsepower of pumping capacity at its disposal.

"The real issue is can you get enough momentum against the flow of this well to actually push it back?" Dave Roberts, head of worldwide upstream operations at Marathon Oil Corp (MRO.N), told the Reuters Global Energy Summit in Houston. "I think they have the horses on the surface to take care of it."

But if the mud alone doesn't do it, BP can inject bridging material, which can include golf balls or pieces of tires, that would then be pushed up into the BOP by the oil and gas flowing out of the well.

The hope is this material gets stuck and impedes the oil flow enough to let the mud to do its job, and allow the well to be plugged with cement. The trick will be monitoring pressure levels to see whether more bridging material needs to go in to create enough of a seal, but not so much as to cause unwanted blockage.

CONCERN OVER BIGGER LEAK

"If it's wide open up to the hole, it's going to be difficult to get that pressure to build," said Pierre Conner, head of sales, trading and research at Capital One Southcoast in New Orleans, and a former drilling engineer. "Success is going to be dependent on watching those pressures."

Holden Zhang, a University of Tulsa associate professor of petroleum engineering who has a doctorate in fluid mechanics, was optimistic about the top kill working, but also highlighted the problem of the oil flow simply pushing the mud straight out the top of the BOP. "That's a possibility," he said.

Apache Corp (APA.N) Chief Executive Steve Farris, whose company operates in the shallow waters of the Gulf of Mexico, told the Energy Summit that the top kill was "doable" even if the depth and pressure pose unique challenges.

"One of their biggest concerns is to create a bigger leak than they've got right now," he said. "I don't blame them."

Conner put the chances of success at about 50 percent, but believed there was a low probability of making the situation any worse. "I don't see a lot of downside," he said.

Analysts at Tudor Pickering Holt, a Houston-based brokerage specializing in energy, see the chances of a successful top kill at less than 50 percent, worrying about the possibility of debris in the BOP impeding the injection of mud.

BP may or may not broadcast the top kill from its underwater camera, but the world should not have to wait long to find out how it goes. If the process starts at daylight, Conner said they would have some idea whether the mud was coming back up by midday.

BP does have other options if the top kill fails, including the installation of a dome along with a seal to prevent the formation of ice -- which made the last dome ineffective -- and the installation of another BOP on top of the existing one.