CO2 Capture and Storage
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 percent.
- Dakota Gas is making research contributions to CO2 sequestration technology.
- The plant has captured and transported nearly 35 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 smoke 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 sulphide (H2S), which makes up 1.0 percent 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.