What is CCS?
Once the carbon dioxide (CO2) has been transported, it is stored in porous geological formations that are typically located several kilometres under the earth’s surface, with pressure and temperatures such that carbon dioxide will be in the liquid or ‘supercritical phase’. Suitable storage sites include former gas and oil fields, deep saline formations (porous rocks filled with very salty water), or depleting oil fields where the injected carbon dioxide may increases the amount of oil recovered. Depleted oil and gas reservoirs are more likely to be used for early projects as extensive information from geological and hydrodynamic assessments is already available. Deep saline aquifers represent the largest potential carbon dioxide storage capacity in the long term, but are currently less well understood.
At the storage site the carbon dioxide is injected under pressure into the geological formation. Once injected, the carbon dioxide moves up through the storage site until it reaches an impermeable layer of rock (which can not be penetrated by carbon dioxide) overlaying the storage site; this layer is known as the cap rock and traps the carbon dioxide in the storage formation. This storage mechanism is called “structural storage”.
Structural storage is the primary storage mechanism in CCS and is the same process that has kept oil and natural gas securely trapped under the ground for millions of years providing confidence that carbon dioxide can be safely stored indefinitely. As the injected carbon dioxide moves up through the geological storage site towards the cap rock some of it is left behind in the microscopic pore spaces of the rock. This carbon dioxide is tightly trapped in the pore spaces by a mechanism known as “residual storage”.
Over time the carbon dioxide stored in a geological formation will begin to dissolve into the surrounding salty water. This makes the salty water denser and it begins to sink down to the bottom of the storage site. This is known as “dissolution storage”. Finally “mineral storage” occurs when the carbon dioxide held within the storage site binds chemically and irreversibly to the surrounding rock.
As the storage mechanisms change over time from structural to residual, dissolution and then mineral storage the carbon dioxide becomes less and less mobile. Therefore the longer carbon dioxide is stored the lower the risk of any leakage.
There is already considerable experience with injecting carbon dioxide deep underground for storage at a number of industrial-scale CCS projects (see below). These storage sites have been carefully selected and the evidence from monitoring suggests that the carbon dioxide has been completely and safely locked into the geological formations. Carbon dioxide has been stored 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 in Algeria and the Weyburn-Midale project in Canada.
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 carbon dioxide retained is likely to exceed 99% over 1000 years. This estimate of the risk that carbon dioxide may be released and contribute to climate change was made based on observations and analysis of current carbon dioxide 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.