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.