Relief wells are last hope to stop oil leak
The ultimate solution to stanch oil gushing beneath the Gulf of Mexico relies not on the high-tech, risky robotic maneuvers used since the rig explosion, but on a slow, workaday method that has successfully plugged other giant spills — drilling a relief well.
Despite the name, relief wells are not meant to relieve pressure. They simply provide a path to insert heavy mud and cement in the leaking well about 18,000 feet deep under the seafloor, stopping the flow of oil.
Their construction requires little more than everyday oil-field technology and math no more complicated than high-school trigonometry.
And though not foolproof, they are “as close as it gets to a sure fix,” said David Rensink, president-elect of the American Assn. of Petroleum Geologists and a retired Houston engineer.
The problem is relief wells are built at a snail’s pace: “Straightforward, but time-consuming,” said Jerome Milgram, a retired professor of ocean engineering at MIT. The extreme depths only make the job harder.
“The deeper you get, the longer things take,” said BP spokesman Bryan Ferguson. Every normal step of the drilling process — changing a bit, taking a test reading — takes longer when the equipment has to move several miles out of a well shaft as narrow as a foot across.
In the Gulf of Mexico, BP is drilling two relief wells in case one fails. The first began May 2 and has reached a depth of 12,090 feet. The second began two weeks later and thus far is 8,650 feet deep. The company estimates that the relief wells will be completed in August. The trick to the operation is to make sure the relief wells squarely intercept the gushing well.
The broken well is only about 7 inches wide at the bottom. Hitting it right on the mark may be unlikely the first time. But if BP misses on its first stab, it will plug that hole and jab toward the target again.
“The first attempt will give them a sense of where they are,” Rensink said. “And each attempt will bring them closer.”
Eventually, a square-on hit is what engineers seek, said John Smith, associate professor of petroleum engineering at Louisiana State University. Relief wells are “most likely to be successful if they literally intersect the existing well,” Smith said. “Otherwise, [the cement] could mostly leak off into the rock and not really stop anything.”
The task is painstaking, but not as chancy as it might have been years ago before global positioning systems, said Greg McCormack, director of the Petroleum Extension Service at the University of Texas at Austin.
Some experts have predicted all along that relief wells would prove to be the only answer to the disaster triggered when the Deepwater Horizon oil rig burst into flames April 20, causing the gush. “This has been used before, and we know it works,” McCormack said.
It was relief wells that stanched the flow of oil at last year’s Montara spill in Australia and at the Ixtoc I blowout in Mexico in 1979-80, which, at 3.3 million barrels leaked, is the world’s largest accidental oil spill.
But it took more than nine months to kill the Ixtoc well. Months were devoted to drilling; it took several more to get relief wells operating correctly.
When they get deep enough, drillers will make a gradual bend in the relief wells’ paths, plunging at a 35-degree angle to get within 50 feet of their target. Once in range, they will use magnetic instruments to more precisely home in on the original well casing — or pipe — and fill it with heavy synthetic drilling mud to push oil down.
After that, cement is pumped in to shut off the flow for good.