resulting in larger uncertainty about the aquifer structure. Unlike storage in oil fields or coal beds, no side product offsets the storage cost. Trapping mechanisms such as structural trapping, residual trapping, solubility trapping and mineral trapping may immobilize the CO2 underground and reduce leakage risks. Enhanced oil recovery CO2 is occasionally injected into an oil field as an enhanced oil recovery technique, but because CO2 is released when the oil is burned, it is not carbon neutral. CO2 has been injected into geological formations for several decades for enhanced oil recovery and after separation from natural gas, but this has been criticised for producing more emissions when the gas or oil is burned. Leakage risks during storage Long-term retention IPCC estimates that leakage risks at properly managed sites are comparable to those associated with current hydrocarbon activity. It recommends that limits be set to the amount of leakage that can take place. However, this finding is contested given the lack of experience. CO2 could be trapped for millions of years, and although some leakage may occur, appropriate storage sites are likely to retain over 99% for over 1000 years. Mineral storage is not regarded as presenting any leakage risks. Norway's Sleipner gas field is the oldest industrial scale retention project. An environmental assessment conducted after ten years of operation concluded that geosequestration was the most definite form of permanent geological storage method: Available geological information shows absence of major tectonic events after the deposition of the Utsira formation [saline reservoir]. This implies that the geological environment is tectonically stable and a site suitable for CO2 storage. The solubility trapping [is] the most permanent and secure form of geological storage. In March 2009, StatoilHydro issued a study documenting the slow spread of CO2 in the formation after more than 10 years operation. Gas leakage into the atmosphere may be detected via atmospheric gas monitoring, and can be quantified directly via eddy covariance flux measurements. Sudden leakage hazards At the storage site, the injection pipe can be fitted with non-return valves to prevent an uncontrolled release from the reservoir in case of upstream pipeline damage. Large-scale CO2 releases present asphyxiation risks. For example, in the 1953 Menzengraben mining accident, several thousand tonnes were released and asphyxiated a person 300 meters away. Malfunction of a CO2 industrial fire suppression system in a large warehouse released 50 t CO2 after which 14 people collapsed