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Geologic Sequestration of Carbon Dioxide

Geologic sequestration involves collecting and placing carbon dioxide into suitable underground formations for storage.

Sequestration Processes and Geologic Reservoirs

Geologic sequestration involves three main processes: capturing carbon dioxide, transporting carbon dioxide, and placing the carbon dioxide in a geologic formation for permanent or semi-permanent storage. The carbon dioxide is placed into the geologic formation by means of a system of injection wells. An injection well is like an oil well or a water well, except that instead of drawing material (oil or water) out of the ground, carbon dioxide is injected into the well. Injection wells are also used for the disposal of various types of wastes and to enhance oil recovery in some areas.

The geologic formation should have a number of characteristics to make it suitable for placement and retention of the carbon dioxide. The formation should be deep enough and be covered with a formation that will prevent the carbon dioxide from migrating to the surface. The depth of injection would depend upon the depth of the formation being targeted for injection. Candidate formations can be thousands of feet deep. The formation has to have adequate porosity and permeability to accept the injected carbon dioxide. If the formation also contains nonpotable groundwater or is comprised of rock that would react chemically with the carbon dioxide and form a compound that will stay fixed in the subsurface, that increases its suitability for geologic sequestration. Some underground formations that could be used for the geologic sequestration of carbon dioxide are:

  • Depleted oil and gas reservoirs;
  • Unmineable coal seams;
  • Deep formations containing salty water; and
  • Basalt formations.

In general, geologic sequestration can be thought of as a more permanent process for storing carbon than terrestrial sequestration. Except in cases where timber is used to create long-lived wood products, the carbon dioxide captured during terrestrial sequestration will return to the atmosphere once the plant used for the terrestrial sequestration project dies and the soil carbon returns to the atmosphere. Geologic sequestration effectively places the carbon back into the subsurface from which most of our fossil fuels come. Geologic sequestration is permanent if the geologic formations targeted for sequestration do not release the carbon dioxide after it is injected.

Carbon Capture, Storage, Transport, and Injection

Geologic sequestration requires the capture of carbon dioxide before it can be transported and placed into the subsurface formation. During terrestrial sequestration, plants collect and sequester the carbon via photosynthesis. During geologic sequestration, however, carbon dioxide must be collected from fuels before they are burned or from the smokestack, so to speak, after the fuels are burned. The collection (also known as "capture") of carbon dioxide adds to the cost of producing energy, but carbon capture is an important first step in geologic sequestration. In general, carbon capture needs to happen at or near where the energy is being generated because that is where the combustion occurs. The technology used for carbon capture depends on the concentrations of carbon dioxide, the temperature of the emissions, and the corrosiveness of the emissions. Most capture technologies use either chemical or physical means to capture carbon dioxide.

Although underground formations suitable for geologic sequestration are located throughout North America, the power plants or factories where the carbon dioxide is created may not be located near those suitable formations. As a result, once the carbon dioxide is captured, it has to be transported to where the underground injection is going to occur. In some cases, the captured carbon dioxide may have to be stored prior to transport. Under pressure, carbon dioxide is a liquid, so pressurized carbon dioxide can be stored in tanks and transported easily by pipeline. There are no large-scale geologic sequestration projects in North America, although small-scale research projects on geologic capture and sequestration have been funded and are active.

Major Carbon Dioxide Sources and Pipelines in the U.S. Rocky Mountain Region
Major Carbon Dioxide Sources and Pipelines in the U.S. Rocky Mountain Region
Source: Utah Geological Survey
Major Carbon Dioxide Sources and Pipelines in the U.S. Rocky Mountain Region
Source: Utah Geological Survey
Click to enlarge

In addition, carbon dioxide is being captured and transported by pipeline for injection into near-depleted oil reservoirs to push out the last few drops of oil, and carbon dioxide is being injected into coal beds as part of coal bed methane extraction projects. For example, carbon dioxide captured from a coal gasification project is being piped to an enhanced oil recovery (EOR) site at the Weyburn Oil Field in Saskatchewan, Canada. The project, which was started in 2000, is expected to extend the life of the Weyburn field by 25 years, enabling an additional 130 million barrels of oil to be produced, and sequestering 1 million metric tons of carbon per year. Another example of an EOR project is the Aneth Oil Field Project. Although not a project used for geologic sequestration only, the Aneth Oil Field Project which is located in the Paradox Basin in Utah, is injecting carbon dioxide into the ground to enhance the oil recovery in an oil field on Navaho land near Bluff Utah. The size of the field makes it a candidate for pilot scale and larger sequestration operations. The field is partially owned by the Navajo Nation Oil and Gas Company. The figure above depicts the Paradox Basin, major carbon dioxide sources, and pipelines for carbon dioxide transport.