Carbon Storage

WHAT ARE THE CHARACTERISTICS OF A SUBSURFACE CARBON STORAGE COMPLEX?

When assessing a storage site, some of the reservoir characteristics that are studied for long-term CO2 storage include storage resource, injectivity, integrity, and depth. The term ‘subsurface storage complex’ refers to the geologic storage site that is targeted to safely and permanently store injected CO2 underground. It includes a storage formation with at least one, or usually multiple, regionally continuous sealing formations called caprock or seals.

  • Storage Resource - A storage site needs to have enough storage resource (space) to contain large amounts (millions of metric tons) of compressed CO2. The storage resource is a fraction of the pore volume of porous and permeable sedimentary rocks available for storage.
  • Injectivity – This refers to the rate at which CO2 can be injected into the subsurface. Injectivity of the CO2 is directly related to the permeability of the formation. The permeability of a rock is a measure of the resistance to fluid flow through a rock. If fluid can easily passes through the rock, it has “high permeability.”
  • Integrity – This refers to the ability to confine CO2 safely within a predetermined volume without a breach from the storage complex. A storage complex must have one or more confining zones that seal above the injected formation that are intact and do not have leakage pathways.
  • Depth – The CO2 storage zone needs to be located at a sufficient depth and pressure so that CO2 can be injected as a supercritical fluid. Supercritical CO2 is dense and behaves more like a liquid than a gas, allowing for storage of higher concentrations of CO2 by volume.

All of these characteristics are examined in order to determine if a potential storage complex has adequate conditions for CO2 storage.

storage-complexes
Image depicting the features of different types of storage complexes including saline formations, oil and natural gas reservoirs, unmineable coal areas, organic-rich shales, and basalt formations . All of the complexes include: (1) a confining zone that includes a thick (or several) sealing layer(s) above the storage, separating the stored CO2 from drinking water sources and the surface; (2) adequate integrity within the storage formation and sealing layers; (3) sufficient porosity and permeability to store large amounts of CO2, and; (4) are at supercritical depth to allow for concentrated storage.
(mouseover image for more detailed description of portions of the complex)

Myth: Any location that has an injection well can be used to inject and store carbon.
Reality: A specific set of characteristics are needed to make a setting appropriate to act as a storage complex. These characteristics are determined through a rigorous characterization process that includes assessing potential storage risks and meeting the regulations under the EPA’s permitting process that grants permission to inject CO2 for carbon storage purposes.

What are the underground rock types that are most promising for storing CO2?


Power Plants CO2 Pipelines Water Rock Layers Formation Types