As BP's initial efforts to stem the flow of crude oil into the Gulf of Mexico at its Deepwater Horizon drilling site have fallen by the wayside, the company said Monday it is implementing a plan in the next two weeks to permanently plug the leaking well. If successful, this so-called "junk shot" option—which involves clogging the well's failed blowout preventer with a variety of objects, including golf balls, tires and tennis balls—will be covered with a layer of cement that ensures the well is never used again.



"Our number one priority is how do we shut the flow off," Kent Wells, a BP senior vice president for exploration and production, said in a Webcast posted to BP's Web site Monday. "This is an unprecedented technical challenge. We've never had a blowout at 5,000 feet."



Although the prospect of solving such a big problem by using seemingly miscellaneous objects to do what the more sophisticated blowout preventer could not, "there's some science to this," Wells said. "We're trying to get a mix of different sizes of material that will wedge in…and hold."



The junk, for lack of a better word, will be pumped down into the wellbore and into the flow of escaping oil. From there, BP is hoping that the combination of balls, tires and other objects will follow the oil up to the blowout preventer, clogging it and shutting off the leak. "This technique, by the way, has been used all over the world," including in Kuwait after Iraq sabotaged that country's oil wells, Wells said. The challenge, as with everything else BP has tried to shut down the Deepwater leak, is that it has never been attempted under 1,500 meters of ocean.



In the procedure that BP refers to as a "junk shot," well operators use a solid, deformable sealant or mixture of sized solids to create a seal or bridge to stop a leak, John Rogers Smith, an associate professor of petroleum engineering at Louisiana State University, explained in an e-mail to Scientific American. The procedure is used when there is a leak to be sealed that cannot be accessed directly. "It is like pumping leak sealant into a tire (instead of using a patch or a plug) or putting a leak sealant into a radiator (instead of draining it and repairing the leak)."



When a well is drilled, a surface hole is created and reinforced with piping held in place by cement. A high-pressure connector is placed on top of this threaded pipe. The blowout preventer, a large valve used to control the flow of the gas or oil reservoir that has been tapped, is designed to plug the well in an emergency. The Deepwater rig's crew was unable to activate the blowout preventer after an April 20 explosion that sunk the rig and killed 11 crew members.



Wells said that BP will be able to try the junk shot procedure multiple times if necessary to clog the blowout preventer. If this proves successful, BP will then pump a mixture called "kill mud" down the wellbore to push the oil and gas back into the reservoir. "We'll follow that with cement, and we will permanently abandon this well," Wells said.



Cement is used as a permanent sealant in oil and gas wells. The process is relatively routine if it can be pumped into the leak path and held in place until it hardens, according to Smith. For cement to successfully seal a leak, these conditions are required: it must be isolated from contaminants; it must completely displace all of the other fluids from the path to be sealed; it must be used away from areas where it might complicate subsequent operations (by plugging a path that needs to stay open or cementing a tool into the well); and it must remain stationary in the spot it is intended to seal until it sets.



"Meeting these criteria can be difficult," Smith adds.



The junk shot has become an option only after a four-story, 100-ton concrete-and-steel containment box installed on the ocean floor over the largest leak, where 85 percent of the oil is escaping, failed over the weekend. The box was supposed to funnel oil to the surface but had to be removed after a build-up of gas hydrates resembling ice accumulated inside the device and blocked the exit at the top.



There appears to be a significant amount of natural gas bubbling out of the well with the oil, says Paul Bommer, a senior lecturer in the University of Texas at Austin's Department of Petroleum and Geosystems Engineering. Under the right conditions, with lower temperatures, the water in the gas begins to freeze. "If you put a containment dome over the leaking gas, then it's going to try to get to the top of the dome, where it will expand, cool off and possibly freeze," he adds.



In addition to the junk shot, BP is planning to place a smaller box—called a top hat and about the size of a barrel of oil cut in half —over the leak by the end of the week. The top hat has a better chance of working than the larger containment box because it will cover less seawater. "The volume of hydrate will by definition be much less because the volume of water entering the containment will be much less," BP Group CEO Tony Hayward said Monday during the Webcast. The company is also drilling relief wells to alleviate some of the pressure and provide an alternative route for the oil to flow.