CO2 Storage Geologic Project Descriptions Storage of CO2 in the Geologic Formations in the Ohio River Valley Region The Coal-Seq II Consortium: Advancing the Science of CO2 Sequestration in Deep, Unmineable Coal Seams Geological Sequestration of CO2: The GEO-SEQ Project CO2 Sequestration in Basalt Formations Strategies for Controlling Coal Permeability in CO2-Enhanced Coal Bed Methane Recovery Enhanced Coal Bed Methane Production and Sequestration of CO2 in Unmineable Coal Seams Optimal Geological Environments for Carbon Dioxide Disposal in Saline Aquifers CO2 Sequestration Potential of Texas Low-Rank Coals Terrestrial Project Descriptions Application and Development of Appropriate Tools and Technologies for Cost-Effective Carbon Sequestration Enhancement of Terrestrial Carbon Sinks through Reclamation of Abandoned Mine Lands in the Appalachians Restoring Sustainable Forests on Appalachian Mined Lands for Wood Products, Renewable Energy, Carbon Sequestration, and Other Ecosystem Services Carbon Sequestration on Surface Mine Lands Carbon Sequestration in Reclaimed Mine Soils of Ohio Enhancing Carbon Sequestration and Reclamation of Degraded Lands with Fossil-Fuel Combustion By-Products Optimal Geological Environments for Carbon Dioxide Disposal in Saline Aquifers In Field, Continuous, Non-Invasive Soil Carbon Scanning System World Projects Reference Shelf
	Interactive Geologic Sequestration Model To view geologic sequestration in action, click here to save the executable file to your desktop or run it from its location. Once the model is displayed, select the interactive choices in the left hand corner to display how CO2 concentrations will change over time.

Carbon Dioxide (CO₂) storage is the placement of CO₂ into a repository in such a way that it will remain stored and not release to the atmosphere.  Efforts to store CO₂, often referred to as carbon sequestration, are focused on two categories of repositories:  geologic formations and terrestrial ecosystems.

Geologic Formations
Geologic formations considered for CO₂ storage are layers of porous rock deep underground that are “capped” by a layer or multiple layers of non-porous rock above them.  Sequestration practitioners drill a well down into the porous rock and inject pressurized CO₂ into it.  Under high pressure, CO₂ turns to liquid and can move through a formation as a fluid.  Once injected, the liquid CO₂ tends to be buoyant and will flow upward until it encounters a barrier of non-porous rock, which can trap the CO₂ and prevent further upward migration.

There are other mechanisms for CO₂ trapping as well: CO₂ molecules can dissolve in brine, react with minerals to form solid carbonates, or adsorb in the pores of the porous rock.  The degree to which a specific underground formation is amenable to CO₂ storage can be difficult to discern.  Research is aimed at developing the ability to characterize a formation before CO₂-injection to be able to predict its CO₂ storage capacity.  Another area of research is the development of CO₂ injection techniques that achieve broad dispersion of CO₂ throughout the formation, overcome low diffusion rates, and avoid fracturing the cap rock.  These areas of site characterization and injection techniques are interrelated because improved formation characterization will help determine the best injection procedure.

There are three priority types of geologic formations in which CO₂ can be stored, and each has different opportunities and challenges:

Depleted oil and gas reservoirs.   These are formations that held crude oil and natural gas over geologic time frames.  In general, they are a layer of porous rock with a layer of non-porous rock above such that the non-porous layer forms a dome.  It is the dome shape that trapped the hydrocarbons.  This same dome offers great potential to trap CO₂ and makes these formations excellent sequestration opportunities.

As a value-added benefit, CO₂ injected into a depleting oil reservoir can enable recovery of additional oil.  When injected into a depleted oil bearing formation, the CO₂ dissolves in the trapped oil and reduces its viscosity.  This “frees” more of the oil by improving its ability to move through the pores in the rock and flow with a pressure differential toward a recovery well.  Typically, primary oil recovery and secondary recovery via a water flood produce 30–40% of a reservoir's original oil in place (OOIP).  A CO₂ flood enables recovery of an additional 10–15% of the OOIP.  CO₂ enhanced oil recovery (EOR) and enhanced gas recovery (EGR) are commercial processes and in demand recently with high crude oil prices.  However, commercial practitioners operate their injections with the goal of minimizing the amount of CO₂ left in the ground so that the CO₂ can be used for another well.  NETL’s work in this area is focused on CO₂ EOR and EGR injection practices that maximize the amount of CO₂ sequestered.  More than 82.4 billion metric tons of sequestration potential exists in mature oil and gas reservoirs identified by each of the Regional Carbon Sequestration Partnerships (RCSPs).  Formed by DOE in 2003, the seven Partnerships span 40 states, three Indian nations, and four Canadian provinces.

Unmineable coal seams.   Unmineable coal seams are too deep or too thin to be mined economically.  All coals have varying amounts of methane adsorbed onto pore surfaces, and wells can be drilled into unmineable coal beds to recover this coal bed methane (CBM).  Initial CBM recovery methods, dewatering and depressurization, leave a fair amount of CBM in the reservoir.  Additional CBM recovery can be achieved by sweeping the coalbed with nitrogen.  CO₂ offers an alternative to nitrogen.  It preferentially adsorbs onto the surface of the coal, releasing the methane.  Two or three molecules of CO₂ are adsorbed for each molecule of methane released, thereby providing an excellent storage sink for CO₂.  Like depleting oil reservoirs, unmineable coal beds are a good early opportunity for CO₂ storage.  More than 180 billion metric tons of CO₂ sequestration potential exists in unmineable coal seams identified by each of the RCSPs.

Coal swelling is a potential barrier to CO₂ ECBM.  It has been observed that when coal adsorbs CO₂, it swells in volume.  In an underground formation swelling can cause a sharp drop in permeability, which not only restricts the flow of CO₂ into the formation but also impedes the recovery of displaced CBM.  Angled drilling techniques and fracturing are possible means of overcoming the negative effects of swelling.

Saline formations.   Saline formations are layers of porous rock that are saturated with brine.  They are much more commonplace than coal seams or oil and gas bearing rock, and represent an enormous potential for CO₂ storage capacity.  However, much less is known about saline formations than is known about crude oil reservoirs and coal seams and there is a greater amount of uncertainty associated with their amenability to CO₂ storage. 

Saline formations tend to have a lower permeability than do hydrocarbon-bearing formations, and work is directed at hydraulic fracturing and other field practices to increase injectivity.  Saline formations contain minerals that could react with injected CO₂ to form solid carbonates.  The carbonate reactions have the potential to be both a positive and a negative.  They can increase permanence but they also may plug up the formation in the immediate vicinity of an injection well.  Researchers seek injection techniques that promote advantageous mineralization reactions.  The RCSPs estimated a range of 919 to 3,300 billion metric tons of sequestration potential in saline formations.

Shale.   Shale, the most common type of sedimentary rock, is characterized by thin horizontal layers of rock with very low permeability in the vertical direction.  Many types of shale contain 1–5 percent organic material, and this hydrocarbon material provides an adsorption substrate for CO₂ storage, similar to where CO₂ can be stored in coal seams.  Given the generally low permeability of shale, research is focused on achieving economically viable CO₂ injection rates.
 
Basalt formations.   Basalts are geologic formations of solidified lava.  Basalt formations have a unique chemical makeup that could potentially convert all of the injected CO₂ to a solid mineral form, thus permanently isolating it from the atmosphere.  Research is focused on enhancing and utilizing the mineralization reactions and increasing CO₂ flow within a basalt formation.  Although oil- and gas-rich organic shales and basalts research is in its infancy, these formations may, in the future, prove to be optimal storage sites for stranded CO₂ emissions.

Terrestrial Ecosystems
Terrestrial sequestration is the enhancement of CO₂ uptake by plants that grow on land and in freshwater and, importantly, the enhancement of carbon storage in soils where it may remain more permanently stored.  Terrestrial sequestration provides an opportunity for low-cost CO₂ emissions offsets.  Early efforts include tree-plantings, no-till farming, and forest preservation.  More advanced research is being conducted to develop fast-growing trees and grasses, and decipher the genomes of carbon-storing soil microbes.  Responsibility for terrestrial sequestration research is shared by many Federal agencies.  Cooperative research in this area in conducted with the DOE Office of Science, U.S. Department of Agriculture, U.S. Environmental Protection Agency, and U.S. Department of Interior Office of Surface Mining.  The scope of terrestrial sequestration options addressed in NETL’s core R&D is limited to the integration of energy production, conversion, and use with land reclamation.  Specifically, this involves reforestation and amendment of minelands and other damaged soils, when possible, using solid residuals from coal combustion.