BP is about to start the first stage of its attempt to permanently seal the leaking Gulf of Mexico oil well, as scientists called the accidental spill the worst in history.
The company plans to pump mud, and then cement, into the well following its successful capping 18 days ago, which has since prevented more oil and gas leaking into the ocean.
Government scientists last night estimated that 4.1 million barrels of oil were released into the ocean before the well was capped, with BP collecting a further 800,000 barrels.
In advance of completing two relief wells, the next step is to attempt a ‘static kill’ of the well using heavy mud to prevent oil rising from the underground reservoir to the seabed.
A leak in the capping stack yesterday forced BP to delay starting this process, but the company hopes to restart operations today.
The first stage of the static kill will be an injectivity test where oil will be pumped back down the pipe at a gradually increasing rate to establish that there is a path for the mud and what pump rate to use, and to monitor pressure.
If the test is successful, engineers will then pump around 2,000 barrels of mud from a ship on the surface into the well.
Admiral Thad Allen, the Coastguard official leading the response to the spill, said: ‘When the mud starts being pumped in through the choke line at the two-gallon-per-minute rate, it will start to press down the well column and offset the weight of the oil that’s there and ultimately drive down towards the reservoir.
‘There will be a small amount of oil that will remain above where the choke line comes in up to the top of the capping stack. That will be compressed slightly as the mud is pumped into the well, but should not result in any significant pressure increase.’
He added that it would take around four hours to evaluate the data from the injectivity test and another five to begin the initial pumping of mud.
BP’s previous attempt to pump mud back down the well failed when it attempted a ‘top kill’ operation in May while the well was still gushing oil.
Kent Wells, BP’s senior vice president, said: ‘During the top-kill procedure, the well was flowing and, therefore, we had to pump mud at very, very high rates and pressures to sort of overcome that.
‘We’re in a very different place today, where we have the sealing cap on it. There is no flow from the well and the approach of the static kill will be very different. It will be at low rate and at pressures nowhere near what we pumped during the top kill.’
Once the static kill is complete, BP will decide whether to follow the mud with cement. ‘[This] will be completely dependent on the assessment of the integrity of the casing and the wellbore,’ said Allen.
If the static kill doesn’t work, BP hopes it will still be able to use the relief wells to seal the main well with cement. Kent Wells also suggested that, if the static kill does work, BP may not need the relief wells – the first time such a possibility has been raised.
‘Whether [the cement] comes from the top or whether that comes from the relief well, those will be decisions that will be made along the way,’ he said.
However, Allen said: ‘I don’t think we can see this as the end all, be all until we actually get the relief wells done.’
The first relief well is now only 4ft away from the main well, but BP’s engineers plan to drill another 100ft down before intercepting it. They expect this to happen sometime between the 11 and 15 August, at about 800ft above the top of the oil reservoir.
The 4.1 million barrels of oil released into the sea beat the world’s previous largest accidental spill, which occurred in 1979, also in the Gulf of Mexico. The biggest spill ever occurred during the 1991 Gulf War when Iraqi forces opened oil pumps, releasing six million barrels.