D1 CO2 storage

CO2 storage, also called CO2 sequestration, refers to processes that keep CO2 from being emitted to the atmosphere by storing it in a location where it will remain trapped for thousands of years or longer. Most carbon sequestration processes store CO2 away from the atmosphere, at great depth in the ocean or within the earth’s crust. However, one carbon storage process, called terrestrial storage, stores CO2 in plant cell material and in soil organic matters that are in close contact with the atmosphere, but locked in a solid form.

There are four main methods that are either being considered or presently being used for carbon sequestration:

  • Geological Storage, within rock formations deep in the earth’s crust;
  • Ocean Storage, within deep ocean basins;
  • Beneficial Reuse, where CO2 is used for practical purposes such as EOR that also have a component of carbon storage; and
  • Terrestrial Storage, including the storage of carbon in forests, grasslands, algae, and other plant-based systems.

In this section these various ways of storing CO2 to reduce GHG emissions to the atmosphere are examined. Advantages or limitations are considered, and the status of current storage projects is summarised by looking at specific examples.

Figure D-1 Relationship showing density and depth of CO2 injection Source: CO2CRC, 2009

The density of CO2 increases with depth of injection, becoming constant below 1.5 km. The relative volume occupied by CO2 decreases in proportion to the increase in density. Most methods of CO2 storage (except terrestrial storage) take advantage of the physical properties of CO2 that allow its density to increase as pressure increases. When the pressure is high enough, CO2 takes on a liquid-like form that is much denser than CO2 gas. This means that a higher mass of CO2 occupies a smaller volume of space, and therefore can be stored in a smaller volume of reservoir. This liquid-like form of CO2 is called “supercritical” or “dense phase.” This dense phase CO2 has peculiar physical properties, namely displaying high densities like liquids and low viscosities like gases. Dense phase CO2 flows like a liquid and is not very compressible. In the dense phase, the liquid CO2 does not mix with water, but forms a separate layer that floats above the water when the two fluids are combined, much as oil floats as a separate layer above water. This occurs because the dense phase CO2 is less dense than water, like oil is less dense than water. Although dense phase CO2 does not mix with water, some of the CO2 will dissolve in water forming dissolved CO2 gas, much like a soft drink, carbonic acid, or dissolved ions such as bicarbonate and carbonate. Some of the dissolved forms of CO2 will also react with other elements to form rock minerals such as calcite (limestone). However, this is a relatively small proportion of the total amount of CO2 present.