CO2 is stored underground by injecting it, in a supercritical state, directly into suitable geological formations.
Understanding Underground CO2 Storage
To effectively store CO2 underground, it is first converted into a supercritical fluid, which behaves like a runny liquid similar to WD40. This state allows for a higher density and thus more efficient storage. Then, it is injected deep underground into specific types of rock formations.
Suitable Geological Formations for CO2 Storage
Several types of geological formations are considered suitable for long-term CO2 storage:
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Sedimentary Rocks: These are the primary targets for CO2 injection. They feature porous spaces that can trap the injected CO2. These rocks may be located in:
- Old oil fields
- Gas fields
- Saline formations: Rocks with porous spaces filled with salty water.
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Unmineable Coal Seams: These seams, which are not economically viable for mining, can also serve as storage sites.
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Volcanic Rocks: Some volcanic rock formations are also being explored for their storage potential.
Injection Process
Supercritical CO2 is pumped deep underground via injection wells into these formations. The porous nature of the rocks allows the CO2 to permeate the rock structure, where it can be trapped by various mechanisms, including:
- Structural Trapping: Impermeable rock layers above the porous storage formation prevent the CO2 from migrating upwards.
- Residual Trapping: CO2 becomes trapped in the pore spaces of the rock, similar to water remaining in a sponge.
- Solubility Trapping: The CO2 dissolves into the formation water (e.g., saline water in saline aquifers), reducing its mobility.
- Mineral Trapping: Over long periods, the CO2 can react with the rock minerals, forming stable carbonate minerals and permanently locking away the CO2.
Advantages of Underground Storage
Underground CO2 storage offers several advantages:
- Large Capacity: Geological formations possess immense storage potential.
- Long-Term Security: With appropriate site selection and monitoring, CO2 can be stored safely for thousands of years.
- Reduced Emissions: Capturing and storing CO2 underground prevents it from being released into the atmosphere, mitigating climate change.
