Carbon Storage

WHAT ARE THE DIFFERENT STORAGE TYPES FOR GEOLOGIC CO2 STORAGE?

Suitable storage formations can occur in both onshore and offshore settings, and each type of geologic formation presents different opportunities and challenges. DOE is investigating five types of underground formations for geologic carbon storage:

  • Saline formations
  • Oil and natural gas reservoirs
  • Unmineable coal areas
  • Organic-rich shales
  • Basalt formations

A complete description of these storage types can be found in the 2012 Carbon Utilization and Storage Atlas, Fourth Edition.

SALINE FORMATIONS
Saline formations consist of porous rock filled with brine, or salty water, and span large volumes deep underground. Saline formations are designated as underground sources of drinking water (USDW) if they contain water that has a total dissolved solids (TDS) concentration of less than 10,000. Any formation that contains these concentrations is excluded from consideration for CO2 storage. CCS focuses on formations that contain brine with TDS levels greater than >10,000 TDS. Studies show that saline formations have the largest potential volume for storing CO2 around the world.

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Image depicting the Saline Storage Resource in the United States and portions of Canada. Extensive saline formations exist in the large sedimentary basins located across the country. (click image to enlarge)

OIL AND NATURAL GAS RESERVOIRS

Oil and natural gas reservoirs can be found in many places in the United States and around the world. Once the oil and natural gas is extracted from an underground formation it leaves a permeable and porous volume that can be readily filled with CO2. Oil and natural 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. Injecting CO2 can also enhance oil production by pushing fluids towards producing wells through a process called enhanced oil recovery (EOR).

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Images depicting the Oil and Natural Gas Reservoir and Unmineable Coal Storage Resources in the United States and portions of Canada. (click image to enlarge)

UNMINEABLE COAL SEAMS
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) may still serve as locations to store CO2. To be considered for CO2 storage, the ideal coal seam must have sufficient permeability and be considered unmineable. Coal seams may also contain methane (CH4), which can be produced in conjunction with CO2 injection in the process called enhanced coal bed methane (ECBM) recovery (see depiction below). In coal seams, the injected CO2 can be chemically trapped by being adsorbed (or adhered) to the surface of the coal while CH4 is released and produced. This trapping mechanism allows for permanent storage of CO2.

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Diagram depicting ECBM and EOR recovery process by which CO2 is injected and used to drive the natural gas or oil towards a recovery well.

BASALT FORMATIONS
Basalt is a type of rock that was deposited when large flows of lava spread from volcanoes cooled and solidified. Over time, they built up thick layers of basalt (with other rock types often layered in between) that have been identified in buried deposits across the United States. The chemical and physical properties of these basalts, as well as the other rock types in between basalt layers, make them good candidates for CO2 storage systems. The chemistry of basalts potentially allows injected CO2 to react with magnesium and calcium in the basalt to form the stable carbonate mineral forms of calcite and dolomite. This mineralization process shows promise to be a valuable tool for CCS since the mineralization process permanently locks carbon in the solid mineral structure, thereby permanently trapping the CO2.

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Image depicting Basalt Formations in the United States. Basalts may offer a highly secure method of CO2 storage because of their potential to allow the CO2 to react with the minerals in basalt to form carbonates, permanently trapping the CO2. (click image to enlarge)

ORGANIC SHALE FORMATIONS
Shale formations are found across the United States and are typically made up of low porosity and low permeability rocks best suited as confining zones. However, some shales have similar properties to coal, having the ability to trap CO2 through adsorption (adherence to the surface), subsequently releasing methane, making them potentially attractive for storage. The advents of new drilling and field technologies that can enable injection of CO2 into shale formations have opened the possibility of shale as a potential option for CO2 storage.

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Image depicting basins containing organic-rich shales in the United States and portions of Canada. (click image to enlarge)

Myth: There are limited options to store CO2 underground, and little is known about these options.
Reality: There are many storage types that can store CO2 and geologic storage of oil, natural gas, and CO2 in the subsurface has been occurring naturally for millions of years.

Is the United States working towards demonstrating the commercial viability of CCS?