What is an aquifer ?
Sedimentary basins are areas where mineral or organic particles collected and solidified. The process of sedimentation can last for tens of million years, creating several layers, each varying from the conditions at the time of sedimentation. Some of these layers – aquifers – are made of porous and permeable rocks charged with water. The top part of aquifers is often used as a source of drinking water, whereas deeper underground the salinity that is found exceeds that of sea water, which makes it unfit for drinking.
The Paris basin spreads from East of Brittany to Alsace and is 200 million years old. It is particularly fit for CO2 storage as it is made of numerous layers of aquifers.
For what capacity ?
Of all types of reservoirs that are considered for storage, deep saline aquifers are the ones that present the most storage capacity (400 to 10 000 CO2 Gton). Albeit widely spread around the globe, their structure and potential to permanently trap CO2 remains very hard to assess; therefore a significant research effort must be undertaken to bring answers on their long-term potential for the prospects of CCUS.
Sleipner (Norway), North Sea
A first experience started in 1996 with a million tons of CO2 per year being injected into a deep aquifer. It is the first industrial mission of CO2 geological storage with environmental intent – to fight climate change.
The CO2 comes from the natural gas field of Sleipner located in the North Sea at about 200km off the Norwegian coast and run since 1996 by Statoil. The natural gas, mainly made of methane, also contains 4 to 10% of CO2, a rate that must be cut down to 2.5% in order to respect the trade standards. This procedure of reduction takes place offshore.
The extracted CO2 is directly injected into the largest local saline aquifer, about 1 000 meters deep under the seabed, in the Utsira sand formation. Every year, a million tons of CO2 is buried in the oceanic substratum instead of being rejected to atmosphere as it usually is the case. The injection fees are offset by the Norwegian tax on offshore CO2 emissions.
Caption: First world CO2 storage site in deep saline aquifer in Sleipner, North Sea (Norway). Natural gas is extracted 2 500m deep from the drilling platform and separated from the CO2 it contains on the gas processing platform. The CO2 is then injected into the sand aquifer of Utsira, 1 000m deep.