It's a little-noticed part of BP's internal investigation of the Deepwater Horizon blowout, but it could have larger implications: Buried in an appendix to BP's Bly report, the company concluded that the properties of an unusual fluid mixture that it chose to pump into the Macondo well may have skewed its reading of a crucial pressure test, as The Washington Post reported.

(As we've noted, the decision to proceed with finishing the well despite "a very large abnormality" in the pressure test was one of the few issues for which BP took some blame in the report.)

The fluid in question was a gooey mixture of two products that were left over after the well was drilled, according to the Post. If BP found a way to use them in the well, it wouldn’t have to bring the product to shore for disposal as hazardous waste. So according to the Post, BP simply poured the waste back down the well:  

During the April 20 pressure test, BP used it as a "spacer" to separate seawater from dense drilling fluid called "mud" in a column of fluids pumped into the well.

"They didn't want to have to dispose of them," [drilling fluid specialist Leo] Lindner said of the leftover Form-A-Set and Form-A-Squeeze.

The BP report does not discuss the disposal advantage that Lindner described. The report said the decision to use the material as a spacer "was driven by the opportunity for the beneficial re-use of the materials."

[BP’s chief investigator Mark] Bly told reporters that using such a mixture was "not an uncommon thing to do."

Linder, a fluid specialist who worked for M-I SWACO, a contractor on the rig, testified in July before a government panel that the fluid was unusual in both its mixture and its quantity.

As the Post pointed out, a BP manager also testified in August that he’d never before used a similar mixture as a spacer, or a buffer between the lighter seawater and the heavy drilling mud.

BP's report doesn't go into detail about the composition of the fluid that was used, nor does it mention that the properties of the unusual mixture may have had an effect on the pressure test. But in an appendix to the report, BP concluded that either “solids from the spacer” or “the viscosity or gel strength of the spacer” could have skewed the negative pressure test.

The appendix also acknowledged that the two leftover products had “no history or testing for use as a spacer,” and that mixing them together and using them as a spacer “was not standard.” (Read Appendix Q.)

BP’s Bly told the Post that using the leftover products “would have been fine” if the mixture had not penetrated a part of the well where it did not belong.

As we've noted, BP's internal investigation into the Gulf spill took some responsibility, but generally deemphasized the degree to which its well design choices and its decision to use fewer centralizers may have contributed to the disaster.