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Behind the Special ‘Drilling Mud’ That Helps Clog the Oil Leak

The key to stemming the stream of oil gushing into the Gulf of Mexico may be as clear as mud. Spill officials said this morning that blasting the leaking well with “heavy drilling mud” seem to be working to hold back the flow of oil.

But the full success of the “top kill” effort may not be evident until sometime over the weekend.

Since Wednesday, crews have intermittently blasted the undersea blowout preventer with highly pressurized mud and other debris in attempt to stop the oil flow long enough to seal the well with cement.

The so-called “drilling mud” isn’t mud at all, but a slurry of compounds typically used to lubricate and cool wells during the normal drilling process. BP officials have said they are using a water-based drilling fluid, rather than a more toxic oil-based mud, which is regulated for offshore drilling by the EPA.

The mud is probably made with a sea water base, fortified with the mineral barite and bentonite clay to make it more viscous, said Martin Chenevert, a petroleum engineering senior lecturer who specializes in drilling fluids at the University of Texas at Austin.

“This particular type of mud is classified as non-toxic,” he said. “Certain types of muds that we use, we are not allowed to use offshore. The type of mud that is acceptable, you don’t have to have any special permission to dump it if you had to. If you spill a little bit, it’s OK. I don’t think that mud is going to be damaging to the ocean fish or wildlife.”

Some difficult drilling situations require the use of synthetic drilling fluids or fluids with a diesel or mineral oil base, which are more toxic to the ecosystem.

While a typical drilling operation uses about 100,000 gallons of mud, BP has stockpiled more than two million gallons ships in the gulf for this operation, an unnamed BP official told the New York times.

Chenevert said the mud currently in use is probably around 16 pounds per gallon, or about twice as heavy as of a gallon of milk. At that weight, it has a better chance of suppressing the quick-flowing oil.

While some environmental advocates have fought the use of chemical dispersant and other substances at the site of the spill, the “top kill” method also carried risks of exacerbating the flow of oil.

“I think contamination from drilling mud is a tiny risk compared to the risk of continuing not to cap the well,” said David Pettit, a lawyer for the National Resources Defense Council.

“It’s certainly in BP’s financial interests to do something quickly, but I don’t think they’ve chosen this particular method lightly.”

If the ‘top kill’ mud mixture is successful at plugging the leak, it’s likely the cleanup–and the political mud-slinging–are just beginning.

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