The safety of storage and long-term reliability of CO2 storage sites have been areas of particular focus and attention. Clearly the ability of storage sites to retain injected CO2 is essential to the success of any CCS project. Storage sites will therefore be very carefully selected. The site will not only need to show large capacity for storage, but also that a suitable impervious layer of rock is situated above the porous rock (into which the CO2 will be injected) in order to prevent any upward migration of CO2.
The main options for storing CO2 are depleted hydrocarbon reservoirs and saline aquifers. Storage will generally take place below 800m and with pressure and temperatures such that CO2 will be in the liquid or supercritical phase. It is likely that depleted oil and gas reservoirs will be used more frequently in early projects as extensive geological and hydrodynamic assessments will already have been made. Deep saline aquifers represent the largest potential CO2 storage capacity in the long term.
There is already much experience with injecting CO2 deep underground: It has been used for over 30 years in Enhanced Oil Recovery Projects and storage projects are on-going, with, for example, the Sleipner project operating since 1996 (see image below). Other projects include BP’s In Salah project and the Weyburn-Midale project. Experience to date has shown that the risk of leakage is likely to be very small.
It
is also worth drawing attention to the fact that storage sites evolve over
time, becoming inherently more and more secure as the role of different CO2
trapping mechanisms changes. This is shown in the diagram. Physical (structural
and stratigraphic) trapping is initially the main means of preventing upward
migration of CO2 in geological formations. This is achieved by an impermeable
barrier of cap rock (and other structural features) acting as a seal. Stratigraphic
traps are formed by changing rock types and CO2 is trapped in the porous structure
by residual-gas trapping. Another form of physical trapping – hydrodynamic
trapping – occurs in saline formations, where fluids migrate very slowly
over long distances. CO2 in the storage site can then undergo a series of
geochemical interactions with the rock and water in the formation. Solubility
trapping occurs when CO2 dissolves in the water. The CO2 can then form ionic
species as the rock dissolves, before undergoing mineral trapping in which
it forms stable (solid) carbonate minerals.
In a special report on CCS, the Intergovernmental Panel on Climate Change (IPCC) concluded that in storage sites which have been carefully selected and managed the fraction of CO2 retained is likely to exceed 99% over 1000 years. This estimate of the risk that CO2 may be released and contribute to climate change was made based on observations and analysis of current CO2 storage sites, natural systems, engineering systems and models. The IPCC noted that similar fractions retained are likely for even longer periods of time given the evolving roles of different trapping mechanisms further reducing risks of leakage.
It is clear that climate change must be addressed and many options will be needed to do so. Capturing and storing CO2 is one of these options and at present this looks to be very attractive.
2008 CCSA © | Last Updated: Feb 2009
