In the past years, people have come to know it as one of the top three greenhouse gases – carbon dioxide, CO2. Carbon sequestration, the storage of CO2 gas, has been spoken of as a great way to achieve this.

Currently, many projects are being conducted in the Williston Basin to see if this would be a possibility. Carbon sequestration alone, however, is not the main interest. Instead, there’s value-added sequestration, where enhanced oil recovery in concordance with carbon sequestration benefits both the environment as well as energy companies.

According to Jon Harju, assistant director of research for University of North Dakota’s Energy and Environmental Research Center, the naturally occurring CO2 reservoirs gave the idea for carbon sequestration, storing CO2 gas in rocks beneath the earth. “There’s a couple of general places CO2 comes from,” Harju explains. One way that is being studied for use in value-added sequestration is CO2 as a by-product of gas processing. However, not all gas processing plants produce enough CO2 to be used.

“The CO2 stream needs to be big enough to be an economically viable quantity to be used in enhanced oil recovery [EOR],” Harju says.

In EOR, CO2 is injected into an oil reservoir. The CO2 causes the oil to swell which helps mobilize the oil trapped in the rock’s pores.

Right now, most of the CO2 used for EOR is purchased from CO2 reservoirs.

“Capturing CO2 is very energy intensive,” Harju says. For instance, he says capturing CO2 from a power plant takes so much energy the plant then has less power to sell.

The amount of CO2 needed for EOR is quite a large amount. The expense of CO2 is also somewhat expensive, so energy companies try to re-inject the CO2 as many times as possible.

At the beginning of an EOR project, says Harju, companies purchase around a ton of CO2 for every incremental barrel to be recovered. Although each reservoir is different, commonly half of all CO2 injected remains in the reservoir by converting to mineral, dissolving in the reservoirs salt water or hardening in the pores. The other half of the CO2 comes to the surface with the extracted oil. The CO2 is then separated and re-injected into the reservoirs.

How long this process can go on and still be economically viable remains unknown.

“Most EOR projects have not been conducted to their full extent,” Harju said. The first reservoirs to undergo CO2 flooding in the Permian Basin in the 1970s are still producing oil. Thirty years ago, this gas was first being used in enhanced oil recovery in the Permian Basin of west Texas. The oil reservoir was flooded with CO2 injected into it. The CO2 was taken from CO2 reservoirs close by.

“It [carbon sequestration] is never the objective in the traditional sense,” Harju says of value-added sequestration.

Because carbon capturing is currently so expensive, value added sequestration is not very common. “The cost of carbon capturing is close to the cast of capture,” Harju says. If the practice is not economically viable, the practice will not be used. “Right now, the cost of capture is greater than EOR,” Harju says.

Adding value to carbon sequestration, such as combining with EOR, will make it more appealing for companies. The more value added to sequestration the more it will be implemented.

Presently there are studies to combine CO2 enhance oil recovery with carbon sequestration, referred to as value-added sequestration.

The Williston Basin’s longest running EOR project is in Weyburn, Sask. It has been running for more than six years, and energy companies believe it has a big future. The CO2 used in Weyburn is captured and piped to Weyburn from the Great Plains Synefuels Plant in Beulah, N.D.

The Energy and Environment Research Center, in conjunction with many partners, has also completed a study on the sequestration potential for oil fields in the Williston Basin. The study showed the potential for CO2 storage is estimated at 3.75 billion tons.