After 76 days, 190 million gallons of oil, and a $22.5bn (£15bn) 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 15ft (4.5m) away from the pipe and 600ft or so (200m) 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 (18cm) in diameter.

"They are homing in on that metal or iron signal from the first well," said Julius Langlinais, a former petroleum engineer and professor at Louisiana State University.

The search for the Macondo would go faster if BP were using measurement while drilling tools, whereby sensors installed in the drill string send the appropriate readings back to the surface, said Langlinais. However, that equipment is hugely expensive. Instead, BP is relying on a process that involves swapping the drill bit for the line carrying the sensor.

"They have to pull the drill string out of the well and lower down this sensitive device that looks for magnetic field variations and from that they can tell where the casing of the well is," Pennington said. Then engineers remove the device, replace the drill string and begin all over again. Each shift can take up to two days.

At some point though the engineers will arrive at the right spot on the pipe somewhere between the reservoir and the leak. They will then stop and install metal casing in the relief well, using cement to secure it in the rock.

The intercept could be complicated if it turns out that the oil is flowing around the pipe, between the pipe and the cement of the well bore.

Engineers also have to be spot-on in their calculations as to how much drilling mud – or pressure – to exert on the well to choke it off. A vessel containing 44,000 gallons of mud is on standby.

The mud must be viscous enough to flow down the pipe but also dense enough to slow down the oil bubbling up from below.

That balance will be crucial to gaining control over the well, so that the flow of oil is checked without having to continuously pump in more mud.

"You get a dense enough mud and a tall enough column in that flow path, and the reservoir can't flow any more. It can't buck the pressure," said Darryl Bourgoyne, a petroleum engineer at Louisiana State University.

Then, if all has gone according to plan, operators will install a cement plug, sealing off the well for good.



• This article was amended on 5 July 2010. The original converted converted a measurement of seven inches into metric as "18mm". This has been corrected.