One relief well is now within about 20 feet of BP's ruptured well in the Gulf, but historically these fixes haven't always worked on the first try--even on blowouts in far shallower waters.

The Ixtoc well in the Gulf of Mexico was only 150 feet below the surface, and it took nine months, two relief wells, and several tries before a relief well finally stopped that spill. That was in 1979.

And the Montara well, off the coast of Australia, was in 250 feet of water and flowed for 10  weeks last fall. A relief well took five attempts. At one point engineers even lost control of the relief well, causing a fire that destroyed the original rig, according to The Times-Picayune of New Orleans.

BP's well in the Gulf, by comparison, is 20 times deeper than the Montara well and more than 30 times deeper than the Ixtoc well. Two relief wells are being drilled--one as a backup.

Initial failure is "almost a certainty," David Rensink, president of the American Association of Petroleum Geologists, told Bloomberg. "It would be like winning the lottery to get it on the first shot."

The Financial Times points out in a piece today that each failure will likely "take days or weeks," since the hard part is not drilling the main well--it's intercepting the ruptured well.

The Los Angeles Times has more on the technical aspects of how BP's relief wells are supposed to work. A BP spokesman told the Associated Press that the company has a high degree of confidence in the relief wells, and that expectations of completion in early August are still on target.