Reuters reported that while imports of medium-heavy and heavy grades of crude oil (with specific gravity of less than 30 degrees) have remained roughly constant at 4.5 to 5 million barrels per day since 2007, imports of medium-light and light oils have dropped from 6 million barrels per day to just over 2 million.
Imports of the lightest grades of oil, the closest substitutes for domestic shale production, have been reduced from 2.5 million barrels in 2007 per day to just 500,000 in the first seven months of 2014, according to US Energy Information Administration (EIA).
The sudden change in the grades of crude oil processed by the refineries were said to have threatened the capacity of the plants to blend the different grades to derive the required quality of refined products.
The refineries are said to be conscious of the quality and density of crude oil as “crude varies considerably in terms of density, acidity, type of hydrocarbon molecules they contain, and presence of impurities such as sulphur and heavy metals such as nickel and vanadium.”
For instance, if the crude oil contains too much acid or salt, the refinery’s equipment will be damaged by corrosion, while with too many heavy metals, the catalysts that aid refining will be poisoned.
On the other hand, too much sulphur will make the crude too hard to meet specifications for petroleum products.
Also, if the crude oil is of the wrong density, it will be impossible to maximise the efficiency of the refinery’s distillation tower and other units.
The average density of crude oil processed in the US refineries since 1985 has been fairly steady and in statistical terms, the weighted average specific gravity has been 31.1 degrees with a standard deviation of just 0.7 degrees.
But according to EIA’s US crude oil production forecast, analysis of crude oil types released in May 2014, “roughly 96 percent of the 1.8 million barrels per day growth in (domestic) production between 2011 and 2013 consisted of grades with American Petroleum Institute (API) gravity of 40 or above.”
To handle the lighter shale oil, the US refiners need to reconfigure their plants to handle a lighter average blend, but that would take time and involves costly investment.
The simpler option, it was learnt, would be to lift the ban on crude oil exports and allow US refiners to continue to import and refine more of the heavier oil they prefer.