CCS explained > Carbon storage
In order to have an impact on man-made emissions very large volumes of captured CO2 must be safely stored and excluded from the atmosphere.
The largest available sink we have is subsurface geological formations, where CO2 can be stored in the pore space of sedimentary rocks. These formations mainly consist of highly porous sandstones which are capped by low permeability rock formations such as shales. In order to maximise the security of geological storage reservoirs are typically greater than 800 m below the Earth’s surface. At this depth CO2 is in a dense supercritical state which is less buoyant than the gas phase. Being in a dense phase is also advantageous to storage capacity. A tonne of CO2 at STP (0˚C and 1 atm) has a volume of 509m2, if the geothermal and lithostatic gradients are 35 ˚C/km and 22.5MPa/km respectively then the same mass of CO2 will have a volume of only 2.5m2 if it is stored 1km below the surface.
The UK has a huge potential for offshore geological storage with numerous potential storage reservoirs within the northern and central North Sea Basin, the southern North Sea Basin and the East Irish Sea Basin. It has been estimated that the UK has 16-20Gt (i.e. 16 – 20,000,000,000 tonnes) and 19-716Gt worth of storage capacity in abandoned hydrocarbon fields and saline aquifers respectively. This is enough to store over 500 years of the UK’s annual emissions. Important aspects of geological storage not only include storage security but also injection strategies and monitoring techniques.
There are four principle geological processes which can physically or chemically trap injected CO2 within the storage reservoir. Structural and stratigraphic trapping involves low permeability layers, such as a shale caprock, or geological structures, such as anticlines. These low permeability layers prevent the buoyant ascent of CO2 as they have a high capillary entry pressure which basically means that the pore fluid in the low permeability layer is at a significantly higher pressure than ascending CO2. Solubility trapping occurs when CO2 dissolves into brine (pore water containing large amounts of salt) and becomes an aqueous phase. This brine/CO2 mixture is denser than the surrounding brine and so will sink towards the bottom of the reservoir. Dissolution of CO2 into brine produces a mild acid which can then undergo chemical reaction with silicate minerals rich in Ca, Mg and Fe to form solid carbonate minerals. This process, known as mineral trapping, is the most stable and permanent form of storage, however it is a slow process that takes place over hundreds to many thousands of years. Residual trapping occurs when blobs of CO2, at a range of scales, become isolated as reservoir brine flows into the tail of a migrating CO2 plume. This trapping mechanism could well prove to be the most important as experimental work has shown that up to 70% of injected CO2 can be immobilised in this manner.