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A Dilbit Primer: How It's Different from Conventional Oil

Bitumen extracted from tar sands has the consistency of peanut butter and must be diluted to flow through pipelines. And that's just the beginning.

Jun 26, 2012
A handful of Canadian oil sands

When emergency responders rushed to Marshall, Mich. on July 26, 2010, they found that the Kalamazoo River had been blackened by more than one million gallons of oil. They didn't discover until more than a week later that the ruptured pipeline had been carrying diluted bitumen, also known as dilbit, from Canada's tar sands region. Cleaning it up would challenge them in ways they had never imagined. Instead of taking a couple of months, as they originally expected, nearly two years later the job still isn't complete.

Dilbit is harder to remove from waterways than the typical light crude oil—often called conventional crude—that has historically been used as an energy source.

While most conventional oils float on water, much of the dilbit sank beneath the surface. Submerged oil is significantly harder to clean up than floating oil: A large amount of oil remains in the riverbed near Marshall, and the cleanup is expected to continue through the end of 2012.

InsideClimate News spent seven months investigating what made the Marshall spill different from conventional oil spills. Part of the challenge was that there has been little scientific research on dilbit; most of the studies that have been done were conducted by industry and considered proprietary information.

The information we did find comes from government records and publicly available industry studies, plus dozens of interviews with industry analysts, federal and state officials, and several university researchers who've worked with the oil industry. We also interviewed watchdog groups that have focused on increasing dilbit regulations, including the Pipeline Safety Trust, the Natural Resources Defense Council and the Pembina Institute, a well-respected Canadian think tank that supports sustainable energy.

Experts at the University of Alberta and the University of Calgary, where tar sands research has been done, did not return requests for comment. InsideClimate asked the American Petroleum Institute and the Canadian Association of Petroleum Producers to put us in contact with their experts, but neither organization provided scientists or engineers for interviews.

What is dilbit?

Dilbit stands for diluted bitumen.

Bitumen is a kind of crude oil found in natural oil sands deposits—it's the heaviest crude oil used today. The oil sands, also known as tar sands, contain a mixture of sand, water and oily bitumen. The tar sands region of Alberta, Canada is the third largest petroleum reserve in the world.

What makes bitumen different from regular or conventional oil?

Conventional crude oil is a liquid that can be pumped from underground deposits. It is then shipped by pipeline to refineries where it's processed into gasoline, diesel and other fuels.

Bitumen is too thick to be pumped from the ground or through pipelines. Instead, the heavy tar-like substance must be mined or extracted by injecting steam into the ground. The extracted bitumen has the consistency of peanut butter and requires extra processing before it can be delivered to a refinery.

There are two ways to process the bitumen.

Some tar sands producers use on-site upgrading facilities to turn the bitumen into synthetic crude, which is similar to conventional crude oil. Other producers dilute the bitumen using either conventional light crude or a cocktail of natural gas liquids.

The resulting diluted bitumen, or dilbit, has the consistency of conventional crude and can be pumped through pipelines.

What chemicals are added to dilute the bitumen?

The exact composition of these chemicals, collectively called diluents, is considered a trade secret. The diluents vary depending on the particular type of dilbit being produced. The mixture often includes benzene, a known human carcinogen.

If dilbit has the consistency of regular crude, why did it sink during the Marshall spill?

The dilbit that spilled in Marshall was composed of 70 percent bitumen and 30 percent diluents. Although the dilbit initially floated on water after pipeline 6B split open, it soon began separating into its different components.

Most of the diluents evaporated into the atmosphere, leaving behind the heavy bitumen, which sank under water.

According to documents released by the National Transportation Safety Board—a federal agency that is investigating the spill—it took nine days for most of the diluents to evaporate or dissolve into the water.

Can conventional crude oil also sink in water?

Yes, but to a much smaller extent.

Every type of crude oil is made up of hundreds of different chemicals, ranging from light, volatile compounds that easily evaporate to heavy compounds that will sink.

The vast majority of the chemicals found in conventional oil are in the middle of the pack—light enough to float but too heavy to gas off into the atmosphere.

Dilbit has very few of these mid-range compounds: instead, the chemicals tend to be either very light (the diluents) or very heavy (the bitumen).

Because bitumen makes up 50 to 70 percent of the composition of dilbit, at least 50 percent of the compounds in dilbit are likely to sink in water, compared with less than 10 percent for most conventional crude oils.

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