After a 16-hour pause to evaluate the effects of the "top kill" operation, BP plans to start pumping mud again this evening in an attempt to staunch the flow of oil from the MC 252 well in the Gulf of Mexico. In the meantime, "the well continues to flow," said BP chief operating officer Doug Suttles during a press briefing on May 27. "We have not yet stopped the flow so the operation has not yet achieved its objective."

The pumping did succeed in stopping the flow of oil and natural gas while it was ongoing, however, and "we pumped some mud into the well bore but we clearly need to pump more and that's why we're making some changes," Suttles explained. Those changes include potentially pumping at higher or lower rates as well as using some of the materials outlined in a so-called "junk shot," ranging from tiny particles to large, dense rubber balls to break up the flow of oil and gas and allow more of the drilling mud to silt up the gushing well.

In fact, the pressure of that gushing pushed some significant fraction of the roughly 15,000 barrels of drilling mud employed to date back out of the various leak points. "We obviously pumped a lot of mudout of the riser… We will continue to [pump mud] as long as we believe this technology could be successful," Suttles said. "It could be the next 24 hours or it could take longer."

He added: "At some point, you stop pumping and see what the pressure does. If the pressure begins to rise you start pumping again or evaluate your options." Even if top kill succeeds and a cement cap can be put in place, the well will not be permanently secured until the relief wells are drilled—in August, at the earliest.

Image: The mobile offshore drilling unit Q4000 holds position directly over the damaged Deepwater Horizon blowout preventer as crews work to plug the wellhead using a technique known as "top kill," May 26, 2010. The procedure is intended to stem the flow of oil and gas and ultimately kill the well by injecting heavy drilling fluids through the blow out preventer on the seabed down into the well. A nearby vessel sprays sea water near the surface of Q4000 to keep oil and fumes from interfering with operations. U.S. Coast Guard photo by Petty Officer 3rd Class Patrick Kelley.