Contractor Halliburton has admitted that it skipped a crucial test on the final formulation of cement used to seal the BP oil well that blew out in the Gulf of Mexico.
The company, which was BP's cementing contractor, said BP at the last minute increased the amount of a critical ingredient in the cement mix. And while an earlier test showed the cement was stable, the company never performed a stability test on the new blend.
The cement's failure to prevent oil and gas from entering the Deepwater Horizon well has been identified as one of the causes of the April 20 disaster which killed 11 workers and led to the largest offshore oil spill in US history.
Halliburton made the admission in a six-page statement issued in response to findings by US president Barack Obama's oil spill commission.
Commission investigators said on Thursday night that tests performed before the deadly blowout should have raised doubts about the cement used to seal the well.
It was the first finding from the commission looking into the causes of the explosion.
That appeared to conflict with earlier statements made by Halliburton, which said its tests showed the cement mix was stable. The company instead blamed BP's well design and operations for the disaster.
The cement mix's failure to prevent oil and gas from entering the well has been identified by BP and others as one of the causes of the accident.
BP and Halliburton decided to use a foam slurry created by injecting nitrogen into cement to secure the bottom of the well, a decision outside experts have criticised.
The panel said that of four tests done in February and April by Halliburton, only one - the last - showed the mix would hold. But the results of that single successful test were not shared with BP and may not have reached Halliburton before the cement was pumped, according to a letter sent to commissioners by chief investigative counsel Fred Bartlit.