Trapping and Safety

It has been demonstrated that CO₂ can be stored permanently with proper site selection, design and management.

The Intergovernmental Panel on Climate Change says that risks are extremely low and that CO₂ could be trapped for millions of years. Well-selected storage sites are considered likely to retain more than 99 per cent of their injected CO₂ in the original rock formation for more than 1,000 years.

There are several mechanisms that work to trap CO2 in deep geologic formations and the safety of CO₂ storage increases with time.

• Structural Trapping – When the CO2 is pumped deep underground, it is initially more buoyant than water and will rise up through the porous rocks until it reaches the top of the formation where it becomes trapped by an impermeable layer of cap rock, such as shale.
• Residual Trapping– Reservoir rocks act like a tight, rigid sponge. Liquid CO2 pumped into a rock formation becomes stuck within the pore spaces of the rock and does not move even under high pressure.
• Dissolution Trapping– Some of the CO2 dissolves in brine (extremely salty water). Because the brine with CO2 is heavier than the surrounding water it sinks to the bottom and is held in place by the water pressure above.
• Mineral Trapping – CO2 dissolved in the saltwater brine is weakly acidic and can react with the minerals in the surrounding rocks, forming new minerals, as a coating on the rock (much like shellfish use calcium and carbon from seawater to form their shells). This process can be rapid or very slow (depending on the chemistry of the rocks and water) and it effectively binds the CO2 to the rocks.