|What is sequestration?|
It is the process of removing CO2 from the atmosphere and converting it into stored compounds. Types include:
Approximately 13 million metric tons of CO2 have been sequestered since the project began.
Basin Electric's major subsidiary, Dakota Gasification Company (Dakota Gas), provides carbon dioxide (CO2) to the largest carbon sequestration project in the world.
In 1997, Dakota Gas agreed to send at least 95 million standard cubic feet (MMSCF) of 96 percent CO2 from its Great Plains Synfuels Plant to Cenovus Energy oilfields near Weyburn, Saskatchewan, Canada for use in enhanced oil recovery. Dakota Gas began successfully capturing and transporting a portion of its CO2 emissions to Weyburn in fall 2000 and more recently to nearby Apache Canada Ltd. oilfields.
Dakota Gas ships approximately 152 MMSCF or 8,000 metric tons of CO2 to Canada every day. The CO2 is expected to be permanently sequestered in the oil reservoir, which is monitored by the International Energy Agency (IEA) Weyburn CO2 Monitoring and Storage Project.
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 synthesis gas before it is converted into synthetic natural gas (SNG), making the SNG 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.
Sequestration means to set apart something for safekeeping. 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.
Groundbreaking for the CO2 pipeline was held in May 1999 and 205 miles of 14-inch and 12-inch carbon steel pipe was laid through western North Dakota and southern Saskatchewan. Challenges during construction included the crossings of the Little Missouri River and Lake Sakakawea, the latter of which is three miles wide at the point of crossing.
The gas in the pipeline is at very high pressure (about 150 - 180 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 recovered, compressed and re-injected along with fresh CO2 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 to 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 to the atmosphere.