Cement slurry used in the Macondo well was unstable and may have contributed to the April 20 blowout aboard the Deepwater Horizon, according to a report issued Thursday by Fred Bartlit Jr., lead investigator for the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling.

In his report to the presidential-appointed commission, Bartlit said that Houston-based Halliburton and BP both knew from previous tests that the nitrogen-based cement, as planned to be used in the Macondo well, would be unstable.

“Halliburton (and perhaps BP) should have considered redesigning the foam slurry before pumping it at the Macondo well,” Bartlit wrote.

BP nor Halliburton had issued a response to the report late Thursday.

As part of its investigation into the explosion that left 11 dead and millions of gallons of oil leaking into the Gulf of Mexico, the commission asked Chevron to test the composition of the materials and the design of the cement job used by Halliburton when it cemented the well on April 19 and April 20. Nine tests performed on the slurry by Chevron for the commission yielded unstable foam every time.

“Although laboratory foam stability tests cannot replicate field conditions perfectly, these data strongly suggest that the foam cement used at Macondo was unstable,” Bartlit writes to the commission. “This may have contributed to the blowout.”

After Chevron’s tests, Bartlit requested data from Halliburton detailing the tests conducted on the slurry before April 20. The documents showed that Halliburton conducted at least four tests on the foam slurry, designed similarly as it was used in the well. Only one of the tests, completed just a day before the explosion, indicated that the slurry design would be stable. Halliburton and BP both had results from previous test that indicated the cement job would be unstable. Neither acted on that information, Bartlit wrote.

Cement slurry is used a barrier between the interior of the well and the well casing. The cement slurry was injected with nitrogen, an unusual, but not uncommon, practice in deepwater drilling.

Halliburton had completed cementing of the final production casing string 20 hours before the blowout. Halliburton has denied responsibility for the accident saying that it conducted tests on the slurry before and after the job. Halliburton and BP officials have said that the cement job completed was successful.

Bartlit’s findings mirror those of BP’s internal investigation released in September, which also blamed the cement as a problem with the well. But Bartlit closed his letter saying that the cement job is not the single cause of the blowout.

“We want to emphasize that even if our concerns regarding the foam slurry design at Macondo are well founded, the story of the blowout does not turn solely on the quality of the Macondo cement job. Cementing wells is a complex endeavor and industry experts inform us that cementing failures are not uncommon even in the best of circumstances. Because it may be anticipated that a particular cement job may be faulty, the oil industry has developed tests, such as the negative pressure test and cement evaluation logs, to identify cementing failures. It has also developed methods to remedy deficient cement jobs.

“BP and/or Transocean personnel misinterpreted or chose not to conduct such tests at the Macondo well,” Bartlit wrote.

The report was released in advance of a Nov. 8 and Nov. 9 public hearing scheduled by the presidential oil spill commission in Washington D.C. to discuss the preliminary findings of Bartlit’s investigation. The commission is led former Gov. Bob Graham, and William K. Reilly, a former administrator of the Environmental Protection Agency.