OREANDA-NEWS. Light, sweet crude blends out of Cushing, Oklahoma, could be contributing to fouling at some refineries, leading to increased maintenance and costs.

The problem appears to be related to asphaltenes, a component common to heavier crude oil that when refined can be transformed into petroleum coke. But when the asphaltenes fall out of the crude solution they become precipitated asphaltenes, and are more likely to turn into coke in parts of the refinery where they are unwelcome, including in refinery treatment and heating systems. Refiners can face more frequent, costly maintenance and lost operating efficiency.

Refiners contacted by Argus declined to comment on how the contaminant has affected operations.

Precipitated asphaltenes are nothing new, as the industry has been noting them in crude for many years. Dozens of crudes produced around the world have varying levels of precipitating asphaltenes.

But the precipitated asphaltenes are appearing with greater frequency in samples of crude delivered from Cushing, where the US benchmark oil price is set. This has prompted the industry consortium Crude Oil Quality Association (COQA) to set up a subcommittee to study and potentially resolve the issue.

While the subcommittee's work is still in the early stages, it is possible the blending of light sweet crude in Cushing with heavy or sour crudes could be causing the problem. Precipitated, or undissolved, asphaltenes commonly result from the blending of a small amount of heavy or sour crudes with mostly lighter crudes. As shale crude gets lighter it must be blended with heavier crude to reduce the API gravity to meet the WTI benchmark specification.

Asphaltenes are not common in WTI at the wellhead in west Texas' Permian basin, said Irwin Weihe, president of Soluble Solutions, the company hired by COQA to test samples for the study. Less than 10pc of Permian samples submitted to Weihe contained undissolved asphaltenes. And while WTI in Cushing has long been a mix of different crudes, precipitated asphaltenes have not been an issue until 2011. Since then, about 75pc of the WTI samples Weihe has received contained precipitated asphaltenes.

The issue does not seem to be going away. Seven of the nine WTI samples submitted so far in 2016 contain precipitated asphaltenes, according to Wiehe.

The COQA subcommittee is testing samples of WTI originating from multiple locations and shipped on various pipelines, in an effort to determine the source and cause of the problem. Given the infrequency of the issue in samples directly from west Texas compared to Cushing-delivered samples, blending appears to be the most likely cause in Cushing, while contamination is the probable cause for west Texas crude.