Prior to sequestration, captured CO 2 must be compressed at high temperature and pressure to its supercritical phase. In this phase, CO 2 has properties of both a liquid and a gas and is therefore denser. This is economically desirable for CO 2 transportation, which is typically accomplished via pipeline.

There are two principal options for the injection of CO 2: deep saline aquifers and depleted oil and gas fields. Both options have advantages and disadvantages. Deep saline aquifers are estimated to have a sequestering capacity of 1000-10,000 gigatonnes of CO 2, whereas depleted oil and gas fields are estimated to have a 675- to 900-gigatonne capacity for sequestration. An additional option is to inject captured CO 2 into producing, so-called mature, oil and gas fields. This approach, which is employed in the Permian basin of Texas and the Weyburn-Midale basin
of Saskatchewan, enhances oil production from mature oil fields and results in significant (approximately 50%) of injected CO 2 being sequestered underground during the initial injection. . CO 2 that returns to the surface with produced oil is separated from the oil and re-injected (and sequestered). Another section of this webpage will address this approach, termed enhanced oil recovery.

In evaluating a geological formation for its sequestering potential, there are several important criteria:

• porosity. Greater porosity (in a depleted oil and gas field) permits greater volumes of CO 2 to be stored.

• prevention of CO 2 leakage. As the figure below illustrates (from Kaldi and colleagues; see For more information for complete reference), structural/stratigraphic (depleted oil and gas fields) and solubility (deep saline aquifers) mechanisms are initially responsible for trapping CO 2. The structural and stratigraphic mechanism is actually that responsible for confining oil and gas prior to their extraction. In the long term, mineral trapping assumes a more significant role. Mineral trapping is the incorporation of CO 2 into carbonate minerals. This mechanism is especially secure in that it renders CO 2 immobile.

• depth of the geological formation for sequestration. Deep sequestration is required to prevent interference with relatively shallow groundwater aquifers. If CO 2 leaks into aquifers, acidity of groundwater increases and this increased acidity can mobilize metals such as arsenic and lead. This would have dire public health consequences. Such consequences dictate extensive monitoring, both direct and with remote sensing, of CO 2 movement underground.