Management of greenhouse gas emissions has become a major issue for governments and the industries whose emissions they regulate. Concerns about global warming are driving policy decisions that may soon bring about major changes in the way greenhouse gases are managed in the United States. Carbon dioxide (CO2), a product of fossil fuel combustion, is one of the larger contributors to greenhouses gases.
A technology with the potential to mitigate the amount of CO2 released to the atmosphere as fossil fuel emissions is known as Carbon Dioxide Capture and Storage (CCS). This involves the capture of CO2 at point sources such as coal power plants, and injecting it directly into deep underground geological formations. Oil fields, gas fields, saline formations, unmineable coal seams, and saline-filled basalt formations have been suggested as possible storage sites (Figure 1).
Two important geologic factors that can act as trapping mechanisms to prevent leakage of injected CO2 back to the surface include:
- Depth of burial and the presence of low-permeability caprocks or stratigraphic seals;
- Geochemical reactions of CO2 with host rocks and the formation of stable minerals (Gunter and others, 2004).
Figure 1: Options for storing CO2 in deep underground geological formations (after Cook, 1999).
Enhanced Oil and Gas Recovery
The injection of CO2 into deep geological formations has been on-going for over 40 years. It was first undertaken in Texas in the early 1970s as part of enhanced oil recovery (EOR) projects, and has been on-going there and at many other locations around the world ever since. In the U.S., approximately 30 to 50 million metric tons of CO2 are injected annually into oil fields that are declining in oil production (Benson and others, 2005). The CO2 used in the EOR projects is collected, in part, from anthropogenic sources, but is principally derived from naturally occurring geologic sources of CO2. It is transported to the oil-producing fields through a large network of pipelines.
The first large-scale CCS project was initiated in 1996 by the Norwegian company Statoil to remove excess CO2 from natural gas produced from deposits in the North Sea. At Sleipner, the company strips CO2 from the natural gas and stores it in a deep saline aquifer.
In 2000, a coal-fueled synthetic natural gas plant in Beulah, North Dakota became the world's first synfuel plant to capture and store carbon dioxide produced from coal conversion. Dakota Gasification Company captures about 3 million tons of CO2 annually at the Great Plains Synfuels Plant, and transfers it via a 205-mile pipeline to the Weyburn-Midale oil field located in southeast Saskatchewan, Canada. There the CO2 is used for enhanced oil recovery and permanently stored in the depleted oil reservoir. http://ptrc.ca/projects/weyburn-midale.
Assessing Geologic Reservoirs for CO2 Storage
In 2007 the U.S. Geological Survey (USGS) was authorized to conduct a national assessment of potential geologic storage resources for CO2 in cooperation with the U.S. Environmental Protection Agency and the U.S. Department of Energy under the Energy Independence and Security Act (Public Law 110–140). The USGS uses the following criteria for considering geologic formations as potential storage units:
- A minimum depth of 3,000 feet (914 m) below ground surface; CO2 at this depth is typically subjected to temperatures and pressures that maintain the CO2 in a supercritical state. Deep sequestration also helps insure there is an adequate thickness of rock (confining layers) above the potential injection zones to act as a geologic seal;
- Proposed salinity limit of at least 10,000 parts per million (ppm) for total dissolved solids (TDS) of formation waters;
- Geologic formations with the potential to store a minimum of 1 million metric tons (or greater) of CO2.
In 2010-11, Geology and Mineral Resources in cooperation with the USGS conducted a preliminary assessment of the deep geologic formations in Virginia that might serve as permanent storage formations for CO2 (Figure2). The storage formations and potential confining cap rocks were identified and their structure, depth, and thickness were mapped. Physical data pertinent to CO2 storage, such as porosity, formation salinity, and permeability were compiled where the information was available. A comprehensive series of digital maps and databases were constructed. An example of the type of map produced for this study is shown in Figure 3.
Figure 2: Areas of investigation (annotated by boxes) based on locations of wells completed at depths greater than 3,000 ft.
Figure 3: Thickness map of the Berea Sandstone at depths greater than 3,000 ft in southwest Virginia. The thickness of the sandstone greatly increases in the Nora and Break-Haysi Gas Fields in Dickenson and Buchanan Counties.
Relevance and Impact
This research has shown that Virginia has the storage resources available for permanently storing CO2 in deep geological reservoirs, particularly in the gas fields of southwest Virginia. Future work will include further geologic mapping and modeling of additional stratigraphic intervals as well as the refinement of those maps and models developed during this phase of the project.
A final report describing the results of studies completed in cooperation with the USGS is in preparation and will be available in the near future.
This work was supported by a grant from the United States Geological Survey (USGS) as part of the National Geologic Carbon Dioxide Sequestration Assessment.
Cook, P.J., 1999, Sustainability and nonrenewable resources: Environmental Geosciences, 6(4), 185–190.
Gunter W.D., Bachu S., and Benson S., 2004, The role of hydrogeological and geochemical trapping in sedimentary basins for secure geological storage of carbon dioxide, in: Baines S.J., and Worden R.H.,( Eds.), Geological Storage of Carbon Dioxide: Geological Society of London, Special Publication 233, pp 129–145.
Metz, B., Davidson, O., de Coninck, H., Loos, M., and Meyer, L. (Eds.), 2005, IPCC Special Report on Carbon Dioxide Capture and Storage: Intergovernmental Panel on Climate Change (IPCC), Cambridge University Press, U.K., 431p.