What are the characteristics of a storage site?
A storage site is defined as an underground geologic location where carbon dioxide (CO2) can be potentially stored. The main characteristics of a CO2 storage site refer to its potential to safely and permanently store large amounts of CO2 underground. These characteristics include capacity, injectivity, integrity, and depth.
A storage site needs to have enough capacity to contain large amounts of compressed CO2. The storage capacity varies depending on the location of the site and the different geologic formation properties and structures in the area. Some areas may not be suitable for CO2 storage or limited to a small amount of storage, like a few hundred tons. The storage sites of greatest interest are those which have the potential to store millions of tons of CO2. The capacity of a storage site is directly related to porosity. A porous formation, which acts like a sponge, can allow the injected CO2 to fill the void areas without damaging its surrounding layers.
Similarly, the injectivity of the CO2 is directly related to the permeability of the formation. Permeability is the ability of a porous material to allow fluids to flow across them. A porous formation without adequate permeability will not allow much CO2 to be injected and therefore would be a poor storage site.
||Geologic storage formation types being investigated for potential CO2 storage.
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The integrity of a CO2 storage site deals with its ability to confine CO2 safely within a predetermined volume. A storage site must have one or more confining zones above the injected formation. These confining zones contain non-porous, low permeability layers of rock that can prevent CO2 from rising to the surface or from potentially contaminating underground water sources. There are also various chemical and physical processes that take place in the storage formation that assist in permanently trapping the CO2 underground.
In most cases, CO2 will be injected as a supercritical fluid (below 2,800 feet), where it will behave more like a liquid than a gas. For comparison, almost all underground sources of drinking water are found at depths of 100-300 feet. In addition, many geologic storage formations, such as the Mt. Simon reservoir in the Midwest, begin at a depth of about 5,500 feet. This depth can be compared to 10 Washington Monuments stacked on top of each other. The answers to questions concerning reservoir capacity, injectivity, and integrity can be learned, in part, by characterization of the formations in the area of the proposed geologic storage site. Reservoir characterization is an evolving science that integrates many different scientific disciplines (geology, geophysics, mathematical modeling, computational science, seismic interpretation, well log and core analysis, etc.) in order to build a conceptual model of a formation. The decision to select a particular geologic unit for geologic storage usually depends on having a detailed understanding of the reservoir characteristics and the behavior and fate of the injected fluids and their impact on the geologic strata receiving the fluids. Critical factors include: economic analysis of the location of the site, the distance from the CO2 source to the site, the depth of the reservoir (which influences drilling and injectivity of CO2), the volume of CO2 that the site can contain, the trapping mechanisms and sealing capacity, and the ultimate fate of the stored CO2. Many of these issues will be affected by the different classes of reservoirs that are being targeted for injection.