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.