Producers are supposed to strip out volatile gases before transporting oil. But experts say some may have been 'cheating' and leaving in large amounts.

By Marcus Stern and Sebastian Jones

As federal regulators continue investigating why tank cars on three trains carrying North Dakota crude oil have exploded in the past eight months, energy experts say part of the problem might be that some producers are deliberately leaving too much propane in their product, making the oil riskier to transport by rail.

Sweet light crude from the Bakken Shale formation straddling North Dakota and Montana has long been known to be especially rich in volatile natural gas liquids like propane. Much of the oil is being shipped in railcars designed in the 1960s and identified in 1991 by the National Transportation Safety Board as having a dangerous penchant to rupture during derailments or other accidents.

While there's no way to completely eliminate natural gas liquids from crude, well operators are supposed to use separators at the wellhead to strip out methane, ethane, propane and butane before shipping the oil. A simple adjustment of the pressure setting on the separator allows operators to calibrate how much of these volatile gases are removed. The worry, according to a half-dozen industry experts who spoke with InsideClimate News, is that some producers are adjusting the pressure settings to leave in substantial amounts of natural gas liquids.


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