Dakota Gasification Company's Great Plains Synfuels Plant is an international leader in technologies that capture, compress, and transport carbon dioxide (CO₂) emissions from a coal gasification process. The Synfuels Plant captures more CO₂ from coal conversion than any facility in the world, and is a participant in the world's largest carbon sequestration project. Dakota Gas sends CO₂ through a 205-mile pipeline to Saskatchewan, Canada, where oil companies use it for enhanced oil recovery operations that result in permanent CO₂ geologic sequestration. The geologic sequestration of CO₂ in the oil reservoir is monitored by the International Energy Agency (IEA) Weyburn CO₂ Monitoring and Storage Project.

Dakota Gas has the ability to capture up to 3 million tons of CO₂ per year, or about 8,000 metric tons of CO₂ daily.

• Dakota Gas captures about two-thirds of the readily available CO₂ when running at full rates.
• Since 2000, CO₂ emissions at the Synfuels Plant have been reduced by about 45%.
• Dakota Gas is making research contributions to CO₂ sequestration technology.
• The plant has captured and transported more than 40 million metric tons of CO₂ for geologic sequestration since 2000.

With increasing demand for the capture and storage of CO₂, Dakota Gas has seen a dramatic reduction in its CO₂ emissions at the Synfuels Plant. As an added environmental benefit, virtually all of the injected CO₂ is expected to remain permanently sequestered in the depleted oil fields long after they have been abandoned.

Capturing carbon dioxide

When coal is burned, it produces sulfur dioxide and nitrogen oxide as well as particulate matter and mercury. Under the Clean Air Act, those pollutants must be removed from exhaust gases that come out of the stack.

Coal combustion also produces carbon dioxide, which is not currently regulated. When coal is gasified, the process removes the sulfur dioxide, mercury and carbon dioxide from the syngas before it is combusted, making the "syngas" cleaner than raw coal. The other gases are lowered in the process, and the carbon dioxide is more concentrated, which makes it easier to capture.

Delivering carbon dioxide

The Great Plains Synfuels Plant has built a 205-mile long, 14-inch and 12-inch carbon steel pipe through western North Dakota into southern Saskatchewan. During construction, the pipeline had to cross the Little Missouri River and Lake Sakakawea.

At first, two compressors pushed the CO₂ to Canada. In 2006, a third compressor was added, so more CO₂ could move through the pipeline.

Storing carbon dioxide

Sequestration means to set apart something for safekeeping. Carbon sequestration encompasses the processes of capture and storage of CO₂ that would otherwise reside in the atmosphere for long periods of time.

In the case of CO₂, many consider it to be a greenhouse gas that contributes to the phenomenon of global warming. There is much debate over this issue with popular opinion supporting the theory that capturing human-produced CO₂ emissions is a step in the right direction. On that basis, enhanced oil recovery via CO₂ flooding is an example of how one project can be a benefit to both industry and the environment.

How enhanced oil recovery works

The gas in the pipeline is at very high pressure (about 152 bar), which makes it a supercritical fluid. Supercritical fluids are gases under such high pressures that the vapor (gas) phase becomes as dense as the liquid phase. Supercritical fluids have high density, but flow easily like gases, so are ideal for transporting through pipelines. The Weyburn oil field has a total of 720 wells. The vertical wells were drilled in a 9-spot grid pattern – eight producing wells in a square around an injection well and typically have a spacing of around 150 meters. The high pressure CO₂ is pumped into 37 injection wells, helping oil to flow towards 145 active producer wells.

The level of purity of the CO₂ supplied is ideal for use in enhanced oil recovery. This is because CO₂ dissolves more readily into oil when small impurities are present. Hydrogen sulfide (H₂S), which makes up 1% of the injection gas, is particularly beneficial at helping CO₂ to mix with oil.

When CO₂ supercritical fluid is pumped at high pressure into the reservoir, the CO₂ mixes with the oil, causing it to swell and become less viscous. The swelling forces oil out of the pores in the rocks, so that it can flow more easily. Water is pumped into the injection wells, alternating with CO₂, to push the released oil towards producer wells. Some CO₂ comes back out of the ground at producer wells; this is recycled, compressed and re-injected along with gas from the pipeline.

It is predicted that the CO₂ enhanced oil recovery operation will enable an additional 130 million barrels of oil to be produced, extending the field's commercial life by around 25 years. It is also anticipated that about 20 million tons of CO₂ will be injected and become permanently stored 1,400 meters underground over the lifetime of this project.

Although using CO₂ to increase oil production is not new, prior to the Dakota Gas and Cenovus Energy project, the CO₂ primarily came from natural sources. Because the naturally occurring ground source CO₂ was removed and injected into the geological structure, there was no net reduction in CO₂ emissions in the atmosphere. Dakota Gas is unique in that its CO₂, which would have previously been emitted into the atmosphere, is permanently injected into a geological sink, thereby reducing the total annual emissions of CO₂.