AFTER 76 days, 190 million gallons of oil, and a $22.5bn clean-up and compensation bill so far, BP is poised to plug its leaking well in the Gulf of Mexico. Drilling engineers have only one chance to get it right.

One wrong move as engineers break through the cement and steel pipe of the Macondo well could increase the torrent of oil into the Gulf. In the worst case scenario, it could even trigger a blow-out in the relief well.

“They pretty much have one shot,” said Wayne Pennington, the chair of geophysical engineering at Michigan Tech University. “Once they hit it and they try to kill it they really just have that one chance.”

Pennington and other experts agree the chances of such a disaster are remote. But it cannot be ruled out entirely as BP moves into the most delicate phase of its relief well operation. Nor can the prospect of unexpected delays, due to technical glitches or forecasts for a very active hurricane season.

For now though, the operation is about a week ahead of schedule.

The first of two relief wells is within striking distance of the Macondo, about 4.5m away from the pipe and 200m or so above the reservoir, after weeks of drilling. The second, ordered by the Obama administration as a safety back-up, is some weeks behind.

But BP and the administration were wary of predicting that the well would be finished sooner than expected.

“There is a chance — a slight chance — they could nick the wellbore,” Thad Allen, the coast guard commander, said. “We shouldn't come off that mid-August date until we know they've actually gone through” the leaking well, he told a White House briefing.

The most important thing is establishing a clear connection with the Macondo so they can begin pumping in the heavy drilling mud according to Mark Proegler, a BP spokesman. A nick risks starting a new small leak or possibly even a collapse of a section of the pipe given that it was damaged in the explosion in ways still not fully understood.

Those challenges are still some days away as BP continues to find the optimal point to break into the well, a process known as ranging. “We have many days ahead of us of ranging runs,” said Proegler. The process involves lowering a device down the relief well that bounces electromagnetic waves through the rock to try to measure the distance to the metal pipe of the Macondo, a target barely seven inches (18mm) in diameter.

— The Guardian, London