The saga of BP's runaway Deepwater Horizon well has entered a crucial phase that will determine whether the Gulf of Mexico gusher ends in mid-August or persists, perhaps for months.

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WASHINGTON — The saga of BP’s runaway Deepwater Horizon well has entered a crucial phase that will determine whether the Gulf of Mexico gusher ends in mid-August or persists, perhaps for months.

Unlike the public drama, this act will unfold miles below the seabed, as drill technicians begin delicately maneuvering a relief well that they hope will pierce and cap the gushing oil well.

Last week, BP began using sensitive electronic equipment to detect differences in the rock’s electromagnetic field in an effort to pinpoint the metal pipes inside the wellbore. Based on what they find, they’ll make adjustments every few hundred feet in an effort to intercept those pipes and kill the gusher by pumping it full of tons of heavy drilling mud and then concrete.

The stakes riding on those adjustments are enormous, and the chance of failure, at least on the first try, is huge.

“The engineers will tell you that they have a 95 percent chance of success” in killing a runaway gusher with a relief well, said Bruce Bullock, the director of the Maguire Energy Institute at Southern Methodist University (SMU) in Dallas. “But that depends on how you define success. It’s quite unlikely they’ll hit it on the first stab.”

“They’re aiming at a salad plate thousands of feet down,” Bullock said: a 7-inch pipe buried in concrete, 12,000 feet below the seafloor.

Every time a relief well misses, its crew must back up the drill bit and try again. Last year, a relief well aimed at capping a blowout in the Timor Sea off Australia missed its target four times before connecting. Each new effort took an average of another week of drilling, for a total delay of 27 days after the drillers began closing on their target.

A similar delay at the Deepwater Horizon site would mean more than 68 million gallons more crude dumped into the Gulf if the latest government estimates of the flow are accurate.

Many obstacles lie in the way of the relief well, not least of which are the same vagaries of subsurface strata and gas pockets that put the Deepwater Horizon 43 days behind schedule before bad decisions and equipment failures sent it to the bottom of the Gulf.

And Tropical Storm Alex, which formed in the western Caribbean on Saturday, could end up forcing a shutdown of the whole oil-collection operation, depending on its course. It was unclear whether the storm would affect the area near the blown-out well, but Adm. Thad Allen of the Coast Guard, the federal commander on the scene, said the most recent forecast showed the storm heading toward Mexico and missing the spill. “We know that these tracks can change,” Allen said, “and we’re paying very close attention to it.”

Drillers were reminded Wednesday how quickly success can give way to desperation.

A day earlier, a jury-rigged containment system had captured more than 1 million gallons of crude. Then a robot vehicle nudged the “top hat” containment dome, closing a vent and sending what workers feared was flammable natural gas up a line to a drill ship on the surface.

Mindful that an unexpected surge of methane had doomed the Deepwater Horizon, workers quickly yanked the containment dome away, leaving the well to gush, unimpeded, until the dome was put back in place 10 hours later.

Uncertainty waits even once the drillers pierce the Deepwater Horizon pipe. That’s when they’ll try to plug the well with drilling mud and concrete. The idea is that the heavy mud will counteract the pressure that’s forcing the oil up the well pipe.

However, such plans have failed before, according to studies of previous relief well attempts, either because the mud didn’t weigh enough or the drillers didn’t have enough of it.

Officials also acknowledge that the plan could be defeated if the well were seriously damaged by the explosion of methane that sank the Deepwater Horizon rig April 20, killing 11 workers, and took a mile of pipe down with it.

Congressional investigators have accused BP of causing the blowout by cutting corners on the well’s design, not using enough spacers to hold the well pipe in the center of the bore to save money and selecting a single-pipe system for the final leg of the well, instead of the usual one-pipe-inside-another approach.

Those choices could undermine efforts to kill the well, for example by allowing pathways for the drilling mud and crude oil to seep into the surrounding rock instead of filling the well pipe.

Adding to the uncertainty of the relief-well effort is the fact that, like so much of the Deepwater Horizon containment effort, there’s no good precedent for comparison.

The last great gusher in the Gulf, the Ixtoc 1 blowout off the coast of Mexico in 1979, ran wild for 10 months before relief wells found it and killed it. That, however, was before the latest electromagnetic technology was developed in the mid-1980s, when petroleum engineers realized that they could increase the accuracy of their ranging efforts by running electricity through the runaway well’s metal pipe.

The blowout off Australia last August is a poor guide. The well was in 250 feet of water, not the 5,067 feet where the Deepwater Horizon was drilling, and the oil reservoir is less than half as deep as the Deepwater Horizon’s Moncado field in the Gulf. Moncado is 13,670 feet below the seafloor.

At those depths, adjusting the drill bit to make a second, third or fourth approach to the well is likely to take more time. “The operations at the well itself are so complicated,” SMU’s Bullock said. “The mechanics take a while to do.”

The high-stakes are not lost on the hundreds of men drilling the wells at the Deepwater Horizon site.

“I’m glad to be part of the operation to try to stop this well,” said Wendell Guidry, a senior tool pusher aboard the Development Driller II. The spill, he said, is “not only affecting me, but my family, my kids, later on down the line.”

Material from The New York Times is included in this report.