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Carbon Sequestration
 FAQ Information Portal

What is carbon sequestration?

Q What is carbon sequestration?
A Carbon sequestration is the placement of CO2 into a repository in such a way that it will remain permanently sequestered. Efforts are focused on two categories of repositories: geologic formations and terrestrial ecosystems.
   
Q What is geologic sequestration?
A
  CO2 EOR Technology Brochure Cover
   
Geologic sequestration involves injecting CO2 into underground reservoirs that have the ability to securely contain it. Geologic CO2 storage R&D focuses on five types of geologic formations: oil and gas reservoirs, deep saline formations, unmineable coal seams, oil- and gas-rich organic shales, and basalts. Oil and gas reservoirs are layers of porous rock formations that have trapped crude oil or natural gas for millions of years. An impermeable, overlying rock formation forms a seal that traps the oil and gas; the same mechanism would apply to CO2 storage. As a value-added benefit, CO2 injected into these reservoirs can facilitate recovery of oil and gas resources left behind by earlier recovery efforts. CO2 can increase oil recovery from a depleting reservoir by an additional 10-20 percent of the original oil in place. CO2 enhance oil recovery (EOR) accounts for 4 percent of the Nation's oil production, and DOE studies have indicated that a widespread CO2 EOR program in large, favorable reservoirs could significantly boost U.S. oil production.
  Saline formations are composed of porous rock saturated with brine and capped by one or more regionally extensive impermeable rock formations, enabling trapping of injected CO2. Compared with coal seams or oil and gas reservoirs, saline formations are more common and offer the added benefits of greater proximity to emission sources, higher CO2 storage capacity, and fewer existing well penetrations. On the other hand, much less is currently known about the potential of saline formations to store and immobilize CO2.
  Unmineable coal seams, at depths beyond conventional recovery limits, represent another promising opportunity for CO2 storage and can result in enhanced coalbed methane recovery (ECBM). Most coals contain adsorbed methane, but will preferentially adsorb CO2, causing the methane to desorb. Similar to the by-product value gained from EOR, the recovered methane provides a value-added revenue stream to the carbon capture and storage process, reducing overall net costs. CO2 injection is known to displace methane, and a greater understanding of the displacement mechanism is being developed to optimize CO2 storage and to understand the problems of coal swelling and decreased permeability.
  CO2 storage in coal seams represents a promising sequestration pathway, and research is underway along several fronts to overcome technical, economic, and environmental barriers: 1) storage capacity in deep, unmineable coal seams, including guidelines for defining unmineable coals; 2) geologic and reservoir data defining favorable settings for injecting and storing CO2 in coal seams; 3) enhanced understanding of the near-term and longer-term interactions between CO2 and coals, particularly the ability to model coal swelling (reduction of permeability) in the presence of CO2; 4) reliable, high-volume CO2 injection strategies and well-spacing patterns that could reduce the number of wells required for storing significant volumes of CO2; and 5) integrated CO2 storage and ECBM recovery.
  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 shales contain 1–5 percent organic material, and this hydrocarbon material provides an adsorption substrate for CO2 storage, similar to CO2 storage in coal seams. Research is focused on achieving economically viable CO2 injection rates, given shales' generally low permeability.
  Basalt formations 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 isolating it from the atmosphere permanently. 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 emissions sources.
   
Q What is a geologic “seal?”
A In the context of geologic sequestration of CO2 in deep formations, the term “seal,” or “caprock,” is used as a general term for one or more layers of rocks that separate the CO2 injection reservoir from surrounding strata, especially the freshwater zones nearer the ground surface.  These relatively impervious layers overlie the injection reservoirs and act to prevent movement of CO2 and other fluids beyond the injection zones or immediate buffer zones.  These layers have very low permeability—that is, their ability to transmit fluids and gases is extremely low.  For example, many sandstones are good storage reservoirs because there is enough interconnected pore space between the sand grains that fluids, such as brine, or saltwater, flow easily through them.  On the other hand, most shales (made of smaller, clay particles) have very little interconnected pore space and thus do not readily allow fluid movement, making them a good sealing layer.
   
Q What is terrestrial sequestration?
A
 
Carbon analysis of soil using inelastic neutron scattering instrument at USDA Facility in Auburn, AL
 

Carbon analysis of soil using inelastic neutron scattering instrument at USDA Facility in Auburn, AL

Terrestrial carbon sequestration is the net removal of CO2 from the atmosphere by plants and microorganisms in the soil and the prevention of CO2 net emissions from terrestrial ecosystems into the atmosphere. There is significant opportunity to use terrestrial sequestration both to reduce CO2 emissions and to secure additional benefits, such as habitat and water quality improvements that often result from such projects.

In principle, terrestrial sequestration is the enhancement of the 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 includes the development of fast-growing trees and grasses and deciphering the genomes of carbon-storing soil microbes.  NETL's terrestrial sequestration R&D is focused on reforesting and amending minelands and other damaged soils and analyzing various land management techniques, including no-till farming, reforestation, rangeland improvement, wetlands recovery, and riparian restoration.

 
   
Q How many acres of forest land does it take to offset the CO2 emissions from a medium-sized coal-fired power plant?
A Roughly speaking, about 220,000 acres would be required to offset emissions from an average-sized power plant. This assumes an average coal power plant from the existingfleet and a forest uptake rate of 3 tons of carbon per acre per year.
  Terrestrial sequestration is conceptualized for use in conjunction with CO2 capture and storage to provide fossil-fired power generation with zero net greenhouse gas emissions.  It is expensive to capture the last 5-10 percent of CO2 emissions from a fossil fuel conversion plant, due to the law of diminishing returns.  A cost-effective approach for zero emissions is to capture 90 percent of emissions and offset the remaining 10 percent with forest land.  Moreover, afforestation and other terrestrial sequestration approaches offer many collateral benefits, including flood protection, wildlife/endangered species habitat, restored ecosystems, etc.
   
Q What is soil carbon?
A

Soil carbon is both organic and inorganic carbon contained in soil.  During photosynthesis, plants convert CO2 into organic carbon, which then is deposited in the soil through their roots and as plant residue.  Organic carbon is found in the top layer of soil, the A horizon.  Inorganic soil carbon comprises carbonates that form through non-biological interactions.  They are a minor amount compared with organic carbon, but are considered more permanent.  Large plant roots, such as those of trees, are considered biomass and not part of the soil, but the organic matter, if you look closely, includes many fine root hairs, where much of the  CO2 "exchange" from the plant to the soil occurs.
   
Q What are the Regional Carbon Sequestration Partnerships?
A
 

Regional Carbon Sequestration Partnerships

 
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Formed by DOE, the Regional Carbon Sequestration Partnerships (RCSPs) are a government/industry effort tasked with determining the most suitable technologies, regulations, and infrastructure needs for carbon capture and sequestration in different regions of the United States and Canada. The energy sectors of both countries are very closely related. Geographical differences in fossil fuel use and sequestration potential across the United States and Canada dictate regional approaches to sequestration of CO2 and other greenhouse gases. The RCSPs are examining regional differences in geology, land practices, ecosystem management, and industrial activity that can affect the deployment of carbon capture and storage technologies. The seven RCSPs that form this network currently include more than 350 state agencies, universities, and private companies spanning 40 states; three Indian nations; and four Canadian provinces. In addition, agencies from six member countries of the Carbon Sequestration Leadership Forum are participating.
  The RCSPs' effort has three distinct phases: 1) Characterization (2003–2005), 2) Validation (2005–2009), and 3) Deployment (2008–2017). The Characterization Phase began in September 2003 with seven RCSPs working to develop the necessary framework to validate and potentially deploy carbon sequestration technologies. The partnerships produced several dozen reports on projects conducted during this phase. At the end of the Characterization Phase, the RCSPs had succeeded in establishing a national network of companies and professionals working to support sequestration deployments, creating a National Carbon Sequestration Database and Geographic Information System (NATCARB), and raising awareness and support for carbon sequestration as a GHG mitigation option. The Regional Carbon Sequestration Partnerships Phase I Accomplishments paper contains additional information.
  The Validation Phase focuses on validating the most promising regional opportunities to deploy sequestration technologies by building upon the Characterization Phase accomplishments. Two different sequestration approaches are being pursued in this phase: geologic and terrestrial. Efforts are being made to validate and refine current reservoir simulation for CO2 injection; collect physical data to confirm capacity and injectivity estimates; demonstrate the effectiveness of MM&V (monitoring, mitigation, and verification) technologies; develop guidelines for well completion, operations, and abandonment; and develop strategies to optimize the storage capacity of various sink types.
  The Deployment Phase will consist of several large-volume sequestration tests. These tests are designed to demonstrate that sequestration sites have the potential to store hundreds of years of regional CO2 emissions. The large-volume sequestration tests in this phase will be conducted to address issues such as sustainable injectivity; well design for both integrity and increased capacity; and formation behavior with respect to prolonged injection.
  Included in the RCSPs' programs will be Monitoring, Mitigation, and Verification projects. Detailed information about the RCSPs' contributions to carbon capture and storage is provided in DOE/NETL's Carbon Sequestration Atlas of the United States and Canada. This atlas presents the first coordinated assessment of carbon capture and storage potential across the majority of the U.S. and portions of western Canada. The RCSPs also contributed to other pertinent publications and programmatic information about DOE/NETL's carbon sequestration R&D available here.
   
Q What is the regulatory environment for carbon sequestration?
A Although there is not yet a comprehensive federal legal and regulatory framework for carbon storage, the Environmental Protection Agency (EPA) has jurisdiction under the Safe Drinking Water Act of 1974 (SDWA) to regulate most types of underground injection.  According to the EPA, the injection of CO2 for underground storage is included.  Specific regulations are brought together in the Underground Injection Control (UIC) Program, which regulates underground injection in five different classes of injection wells.  States are allowed to assume primary responsibility for implementing the UIC requirements within their boundaries, as long as the state program is consistent with EPA regulations and has received EPA approval.  The SDWA itself authorizes any state to assume primary responsibility for controlling underground injection related to oil and gas recovery and production by demonstrating that its program meets SDWA requirements and represents an effective program.
  The EPA announced in March 2007 through its “Guidance” procedure that it recommends using an experimental well category (Class V) for permitting pilot carbon sequestration projects.  This special classification is aimed at pilot and demonstration projects with experimental goals.  The EPA expressly recognizes that in the future, the technology surrounding CO2 will no longer be considered experimental, but expects by then to have made a decision on a strategy to address CO2 injection on a commercial scale.  A different classification or an exemption of CO2 in a manner analogous to that accorded natural gas may be called for in the long term.  In any event, large commercial carbon sequestration operations raise broad issues of site selection criteria, monitoring for subsurface migration, injection well design standards, conditions attaching to any abandonment of the site, and standards for halting CO2 injection if a loss of containment should occur.  In short, as the need for carbon sequestration projects grows, comprehensive regulation will be required, addressing access to pipeline networks, pricing of transportation and storage, policies regarding monopoly control, and the mix of federal and state authority over the safety aspects of transportation and storage facilities.  The legislative history of existing regulations pertaining to oil and natural gas transportation and storage will be instructive.
   
Q How are geologic sequestration sites chosen?
A In the United States, DOE’s Regional Carbon Sequestration Partnerships will manage this process in conjunction with the EPA.  In general, goals for geological CO2 storage selection are to analyze how much CO2 can be stored at a potential storage site, and to demonstrate that the site is capable of meeting required storage performance standards.  This requires collection of the myriad geological data needed to reach these criteria.  By its nature, the process will be site specific in most respects.  Much of the data will be integrated into geological models that will be used to simulate and predict the performance of the storage site.  Other considerations are whether the site can be operated safely and how to accomplish it; whether there is a legal and regulatory framework within which the storage project can be undertaken; and whether the project is economically feasible. 
  Risks of underground CO2 storage will be no greater than those associated with natural gas storage and enhanced oil recovery.  The equipment used to monitor CO2 storage will be the same as that in use to monitor natural gas stored underground.  Similarly, the risks posed by CO2 pipeline transport are expected to be lower than those imposed by oil and gas pipelines operating across the country.  Risk management for geological storage involves four interrelated activities:  (1) scrupulous site selection involving performance and risk assessment, as well as examination of socioeconomic and environmental factors; (2) monitoring to assure performance is as expected and provide warning leaks are detected; (3) effective regulatory oversight; and (4) prescription of remediation measures to eliminate the causes of leakage should it occur. 

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