New Orleans, Louisiana - BP's catastrophic well may already be permanently sealed and no further cementing necessary through a relief well, US spill chief Thad Allen said on Thursday.
BP performed a static kill last week that suppressed the gushing oil with mud and blocked the main pipe with cement but was expected to conduct a final "bottom kill" operation to seal off the reservoir in the coming days.
But Allen said tests were under way that could show that the remaining area that needs sealing - the annulus between the well pipe and the outer well bore - was already cemented during the static kill.
After drilling 5 426m below sea level, a relief well is 30 feet from the stricken Macondo well, but Allen said they may decide not to intercept if the cement job is already thought to be done.
"The pressure tests will run for about four hours. The data from the pressure tests will be analysed by the BP engineers and our science team," he said, after the tests began around 19h40 GMT.
"I wouldn't rule out anything at this point. We think that it's a low probability that we would not finish the relief well and cement, but we need to run the tests and analyse the data," Allen said.
"One way or the other the annulus will be sealed from the reservoir. Our question is did it already happen and do we know that and will these pressure tests tell us?"
The well ruptured when the BP-leased Deepwater Horizon rig sank on April 22, two days after a massive explosion that killed 11 workers.
At 4.9 million barrels - or enough oil to fill 311 Olympic-sized swimming pools - the disaster is the biggest maritime spill on record.
It threatened the fish and wildlife-rich US Gulf coast with environmental ruin and plunged residents of coastal communities into months of anguish over their livelihoods and the region's future. - Sapa-AFP