3.1.1 Structural trapping

The injected carbon dioxide tends to rise towards the top of the formation due to buoyancy forces, where it will be trapped by an almost impermeable caprock, such as shale. This is referred to as 'structural trapping'. A sealing cap rock is required to allow accumulation of CO2 in a geological trap over a long time period. If the caprock contains fractures and/or faults, which may be connected to another permeable layer, it cannot act as a sealing boundary. Besides, it should be at a desired depth (> 800 m) to keep the CO2 in supercritical state. The modelling of the amount and shape of the plume accumulated under a cap-rock depends heavily on the earth model.