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  • Dick writes about Earth and planetary science for Science magazine.
 

How to Kill a Well With Gravity

27 May 2010 12:23 pm
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Oil giant BP plc has a very long straw stuck 3048 meters into the Gulf of Mexico sea floor with oil and gas spouting out the top at several thousand pascals. How do BP engineers stop the flow when none of the control valves at the top is working and there's no way to put a stopper in the straw's end? The only option is using gravity, notes petroleum engineer Paul Bommer of the University of Texas, Austin. To get gravity on their side, engineers yesterday started trying to quiet down the well by pumping drilling "mud" into the side of the leaky straw as hard as they could. "If they get it quiet," says Bommer, "gravity will take over."

If all goes well, the stopper that holds in the oil and gas will be 3048 meters of drilling mud—a colloid of water, clay, and other solids—filling the well. Twice as dense as water and far more dense than the hydrocarbons coming up the well, the weight of a well full of drilling mud would more than balance the force of fluids trying to rise from the well bottom. This is the way drillers normally control well pressures.

The trick, of course, is filling the well with mud after a blowout has filled it with rushing oil and gas. "The first thing [the mud] probably wants to do is go with the flow," says Bommer, which is out through the malfunctioning blowout preventer and a short length of kinked riser pipe. But the 30,000-horsepower mud pumps on ships on the surface and the weight of 1524 meters of mud between the surface and the wellhead would boost the backpressure in the wellhead that is created by resistance to flow in the blowout preventer and riser. If that increased backpressure can greatly slow the flow or even stop it, the mud will begin to sink down the well under its own weight, says Bommer. In a matter of hours, the mud would fill the well and kill it.

Kill it, that is, if the resistance maintaining the heightened backpressure holds up. But if that resistance erodes under the higher pressure and faster flow escaping the wellhead, it could be time for a "junk shot." That's the injection of large particles of, um, ... stuff intended to clog leaks and let the backpressure rise again. No word on what sort of stuff might be used, but shredded tires and golf balls have come up.

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