Halliburton Co. acknowledged late Thursday that it skipped doing a critical 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 that BP at the last-minute increased the amount of a critical ingredient in the cement mix. 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 well has been identified as one of the causes of the April 20 disaster.
Halliburton said that a successful test was performed on a mixture different than the one eventually used. While some tests were conducted on the new formulation requested by BP, those tests did not include a foam stability test, the company said.
Halliburton Co. made the admission in a statement issued Thursday in response to findings by the presidential oil spill commission.
A commission staff letter issued earlier Thursday said BP and Halliburton knew weeks before the Deepwater Horizon explosion that the cement mixture they planned to use to seal the new well was unstable but still completed the work.
That improper cement work "may have contributed to the blowout" on April 20 that killed 11 workers and led to the largest offshore oil spill in history, the staff stated in the first official finding of responsibility for the disaster.
The letter was sent to commissioners Thursday by chief investigative counsel Fred H. Bartlit Jr.
Other factors have also been cited as possibly contributing to the explosion, among them a faulty blowout preventer and BP's decisions to use fewer stability rings on the well piping and seawater instead of heavier mud to plug the well until it was to be used for production.
The staff noted, however, that had the cement done its job it would "have prevented hydrocarbons from entering the well" and triggering the explosion.
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The commission staff said that "Halliburton and BP both had results in March showing that a very similar foam slurry design to the one actually pumped at the Macondo well would be unstable, but neither acted upon that data."
Commission staff also cited Transocean, the owner of the drilling rig, as sharing some blame.
"The oil industry has developed tests, such as the negative pressure test and cement evaluation logs, to identify cementing failures" the team said, but "BP and/or Transocean personnel misinterpreted or chose not to conduct such tests at the Macondo well."
The letter also stated that independent testing of the type of cement used by Halliburton showed it was not stable.
"Halliburton (and perhaps BP) should have considered redesigning the foam slurry before pumping it at the Macondo well," the investigators added.
BP, as part of its own internal investigation, also conducted tests that showed the cement mix was flawed, but its analysis was criticized by Halliburton, which said it was not the correct formula.
By contrast, the commission obtained proprietary additives from Halliburton as well as a recipe to recreate the slurry that was used on the well.
A spokeswoman for Halliburton said the company was reviewing the findings and would have a response later Thursday. Halliburton has said tests showed the cement mix was stable.
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The staff letter said the first three of four cement tests by Halliburton showed that it did not meet industry standards.
Two of those tests were conducted in February, and two more in April.
The staff said the results from one February test were sent to BP on March 8 "in a technical report along with other information. There is no indication that Halliburton highlighted to BP the significance of the foam stability data or that BP personnel raised any questions about it. There is no indication that Halliburton provided the data from the other February test to BP."
For the last test, the staff wrote, "Halliburton personnel again modified the testing procedure, and this time — for the first time — the data indicated the foam slurry design would be stable. We are not yet certain when Halliburton reported this data internally or whether the test was even complete prior to the time the cement job was poured at the Macondo well. Halliburton reported this data to BP after the blowout."
The Associated Press contributed to this report.