CO2 Storage Formations

CO2 Storage Resource Methodology

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DOE's Regional Carbon Sequestration Partnerships (RCSPs) were charged with providing a high-level, quantitative estimate of carbon dioxide (CO2) storage resource available in subsurface environments of their regions. Environments considered for CO2 storage were categorized into five major geologic systems: oil and gas reservoirs, unmineable coal areas, saline formations, shale, and basalt formations. Where possible, CO2 storage resource estimates have been quantified for oil and gas reservoirs, saline formations, and unmineable coal in the fourth edition of the United States Carbon Utilization and Storage Atlas (Atlas IV). Shale and basalt formations are presented as future opportunities and are not assessed (please see disclaimer on the NATCARB homepage).

The methodology employed by the RCSPs is based on volumetric methods for estimating subsurface volumes. Subsurface storage volume estimates depend on geologic properties and storage efficiency. Storage efficiency for this methodology was determined using Monte Carlo sampling, which includes efficiency terms to define the pore volume that is amenable to geologic storage and displacement terms to define the pore volume immediately surrounding a single CO2 injection well. 

Methodologies used in Atlas IV are intended to produce high-level, regional- and national- scale CO2 resource estimates of potential geologic storage. At this scale, the estimates of CO2 geologic storage have a high degree of uncertainty. Because of this uncertainty, estimates from Atlas IV are not intended to be used as a substitute for site-specific characterization and assessment. As CO2 storage sites move through the site characterization process, additional site-specific data is collected and analyzed, reducing uncertainty. Incorporation of this site-specific data allows for the refinement of CO2 storage resource estimates and development of CO2 storage capacities by future potential commercial project developers.

All data, metadata, and high resolution jpgs are available on NATCARB's Data Download and Custom Maps Request webpage.

Sedimentary Basins

Sedimentary Basins

The Regional Carbon Sequestration Partnerships have identified and examined the location of potential CO2 injection formations in different sedimentary basins throughout the United States and Canada. These sedimentary basins collected sediments that lithified to become sedimentary rocks. If these sedimentary rocks are porous or fractured, they can be saturated with brine (water with a high total dissolved solids concentration), oil, or gas. If the sedimentary rock is permeable (e.g., many sandstones), it could be a target for CO2 injection. If it is impermeable (e.g., many shales), it could act as a confining zone to prevent migration of CO2. Necessary conditions for a CO2 storage site are the presence of both a reservoir with sufficient injectivity and a seal to prevent migration. 

CO2 Storage Geologic Formations

Oil and Gas Reservoirs

Oil and Gas Reservoirs

Oil and gas reservoirs are porous rock formations (usually sandstones or carbonates) containing hydrocarbons (crude oil and/or natural gas) that have been physically trapped. There are two main types of physical traps: (1) stratigraphic traps, created when changes have occurred in rock types, and (2) structural traps, in which the rocks have been folded or faulted to create a trapping reservoir. Oil and gas reservoirs are ideal geologic storage sites because they have held hydrocarbons for thousands to millions of years and have conditions suitable for CO2 storage. Furthermore, their architecture and properties are well known as a result of exploration for and production of these hydrocarbons. In addition, due to the industrialization of these sites, infrastructure exists for CO2 transportation and storage.

While not all potential mature oil and gas reservoirs in the United States have been examined, DOE's RCSPs have documented the location of approximately 225 billion metric tons of CO2 storage resource. For details on unmineable coal storage by state, see Appendix D of the 2012 United States Carbon Utilization and Storage Atlas (Atlas IV).

CO2 Storage Resource Estimates for Oil and Gas Reservoirs by RCSP
RCSP Billion Metric Tons Billion Tons
BSCSP 1 1
MGSC 1 1
MRCSP 14 15
PCOR 25 28
SECARB 32 35
SWP 149 164
WESTCARB 4 4
Total 226 248
*Data current as of December 2012.

Unmineable Coal

Unmineable Coal

Coal that is considered unmineable because of geologic, technological, and economic factors (typically too deep, too thin, or lacking the internal continuity to be economically mined with today?s technologies) may have potential for CO2 storage. Coal preferentially adsorbs CO2 over methane, which is naturally found in coal seams, at a ratio of 2 to 13 times. This property, known as adsorption trapping, is the basis for CO2 storage in coal seams. Methane gas is typically recovered from coal seams by dewatering and depressurization, but this can leave significant amounts of methane trapped in the seam. The process of injecting and storing CO2 in unmineable coal seams to enhance methane recovery is called enhanced coalbed methane (ECBM) recovery. Enhanced coalbed methane recovery parallels CO2-EOR because it provides an economic benefit from the recovery and sale of the methane gas, which helps to offset the cost of CO2 storage. However, for CO2 to be stored in coals, the coal must have sufficient permeability, which controls injectivity. Coal permeability depends on the effective stress and usually decreases with increasing depth. Furthermore, studies have shown that CO2 injection can impact coal permeability and injectivity. 

While not all unmineable coal has been examined, DOE's RCSPs have documented the location of approximately 56 to 114 billion metric tons of potential CO2 storage resource in unmineable coal. For details on unmineable coal storage by state, see Appendix D of the 2012 United States Carbon Utilization and Storage Atlas (Atlas IV).

CO2 Storage Resource Estimates for Coal by RCSP
  Low High
RCSP Billion Metric Tons Billion Tons Billion Metric Tons Billion Tons
BSCSP 1 1 1 1
MGSC 2 2 3 3
MRCSP 1 1 1 1
PCOR 7 8 7 8
SECARB 33 36 75 83
SWP 1 1 2 2
WESTCARB 11 12 25 28
Total 56 61 114 126
*Data current as of December 2012.

Saline Formations

Saline Formations

Saline formations are layers of sedimentary porous and permeable rocks saturated with salty water called brine. These formations are fairly widespread throughout North America, occurring in both onshore and offshore sedimentary basins, and they have potential for CO2 storage. It is important that a regionally extensive confining zone (often referred to as caprock or seal) overlies the porous rock layer. Trapping mechanisms include the CO2 dissolving in the brine (solubility trapping), reacting chemically with the minerals and fluid to form solid carbonates (mineral trapping), or becoming trapped in the pore space (volumetric trapping).

While not all saline formations in the United States have been examined, DOE's RCSPs have documented an estimated CO2 storage resource ranging from approximately 2,102 billion metric tons to more than 20,043 billion metric tons of CO2. For details on unmineable coal storage by state, see Appendix D of the 2012 United States Carbon Utilization and Storage Atlas (Atlas IV).

CO2 Storage Resource Estimates for Saline Formations by RCSP
  Low High
RCSP Billion Metric Tons Billion Tons Billion Metric Tons Billion Tons
BSCSP 98 108 1,237 1,364
MGSC 11 12 158 174
MRCSP 95 105 123 136
PCOR 174 192 511 563
SECARB 1,376 1,516 14,089 15,530
SWP 266 293 2,801 3,088
WESTCARB 82 90 1,124 1,239
Total 2,102 2,316 20,043 22,094
*Data current as of December 2012.

Future Opportunities for CO2 Geologic Storage

Basalt Formations 

Basalt Formations

Another potential CO2 storage option DOE is investigating are geologic formations of solidified lava called basalt formations. The relatively large amount of potential storage resource in basalts, along with their geographic distribution, make them an important formation type for possible CO2 storage, particularly in the Pacific Northwest and the southeastern United States. These formations have a unique chemical makeup that could potentially convert all of the injected CO2 to a solid mineral form, thus isolating it permanently from the atmosphere. Before basalt formations can be considered viable storage targets, a number of questions relating to the basic geology, the CO2 trapping mechanisms and their kinetics, and monitoring and modeling tools need to be addressed. As such, Atlas IV presents a map of these potential future storage opportunities, but provides no CO2 storage resource values for basalt formations.

Shale Basins

Organic Shale Basins

Organic-rich shales are another geologic storage option. Shales are formed from silicate minerals, which are degraded into clay particles that accumulate over millions of years. The plate-like structure of these clay particles causes them to accumulate in a flat manner, resulting in vertical rock layers with extremely low permeability. Therefore, shales are most often used in geologic storage as a confining zone or caprock. Before organic-rich shale basins can be considered viable storage targets, a number of questions relating to the basic geology, the CO2 trapping mechanisms and their kinetics, and monitoring and modeling tools need to be addressed. As such, Atlas IV presents a map of these potential future storage opportunities, but provides no CO2 storage resource values for organic-rich shale basins. 
 

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