Sidebar: 20 years ago this month: What's heavy oil and what ain't

Kristen Nelson

Petroleum News

In most places viscous oil equals heavy oil, but that isn't the case on the North Slope, where the term viscous is a better fit than heavy for the shallower, colder crude oil BP and ConocoPhillips are tapping at Milne Point, Kuparuk and Prudhoe Bay.

BP's Milne Point asset manager, Ed LaFehr, told the Alaska Support Industry Alliance April 8, 2004, that while heavy oil and viscous oil are not always "directly correlated, most of the time they are," and so in most places it's just called heavy oil.

But not on the North Slope, he said.

So what's the difference?

BP spokesman Daren Beaudo said on the North Slope viscous oil is generally any crude oil with an American Petroleum Institute gravity of less than 22 degrees and a viscosity greater than 10 centipoises.

Conventional North Slope crude oils, the lighter oils, have higher API gravities. The Alaska Oil and Gas Conservation Commission lists oil from the main Prudhoe Bay reservoir at 28 degree API and oil from Kuparuk at 24 degrees API. Oil from the Alpine and Northstar fields is much lighter, 40 degrees and 44 degrees respectively.

A centipoise is a measure of the viscosity of a liquid -- how it flows. Water has a centipoise of 1 at atmospheric pressure and temperature, and anything with a higher number isn't as fluid as water.

"Heavy oils," Beaudo said, "will typically have much lower API gravities, but could have similar viscosities as the reservoir temperature in many basins is higher than the North Slope.

"The distinction between heavy and viscous is really that on the North Slope we are producing relatively light oil at higher reservoir viscosities -- thus viscous oil."

LaFehr said North Slope viscous oil is "low sulfur, low metals." And, he said, because there is so much light oil going into the trans-Alaska pipeline from Alpine and Northstar, viscous oil "actually helps the blend" in the pipeline and keeps the overall crude a good fit for West Coast refineries.

--KRISTEN NELSON

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