Halliburton Co. is denying that unstable drilling cement foam triggered the April 20 deadly explosion at the Macondo well, although the Houston-based oil-field services company admits it did not test the final cement mixture for stability before using it.

On Oct. 28, Fred Bartlit Jr., the lead investigator for the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, issued a report on Oct. 28 stating the cement foam was unstable and may have contributed to last spring’s blowout. Bartlit reported that tests performed by Chevron Corp. on a cement-slurry, or drilling mud, mixture— similar to that used on BP’s Macondo— indicated the material was unstable.

Additionally, Bartlit says, documents from Halliburton indicate that it and BP knew the mixture was not stable.

Cement slurry is used as a barrier between the well’s casing and interior. Halliburton’s Macondo team injected the slurry with nitrogen, turning it into a type of foam with bubbles that expand and contract in the hole, depending upon heat and pressure.

Halliburton responded quickly to the report. The firm fired back in an Oct. 28 statement that claimed the tests were invalid because they were performed using off-the-shelf products and additives. The Macondo well mixture, by contrast, was a “unique blend of cement and additives … on the rig at the time Halliburton’s tests were conducted.” The company adds that further comment on Chevron’s tests is premature, pending “careful study and understanding of the tests by Halliburton and other industry experts.”

Halliburton also says that while its own stability tests in February and April showed the mixture was unstable, the firm had created a stable mixture before using it on the well.

Mixture tests in February applied different parameters, including depth, pressure and temperature and with different additives, the company says. Halliburton says a fourth test in April included eight gallons of retarder per 100 sacks of cement and showed a stable foam.

BP then instructed Halliburton to increase the amount of retarder to nine gallons per 100 sacks of cement. Drillers used that formulation after tests for thickening time and compressive strength. However, Halliburton says there was no stability test of that formulation.

A small amount of the actual mixture used in the well remains and is being held as evidence. A federal judge recently released that remaining mixture for testing to a joint marine panel investigating the explosion.