The "storage" in Carbon Capture and Storage is the critical final step that ensures captured CO₂ is permanently removed from the atmosphere. The Carbon Capture and Sequestration storage process involves injecting compressed CO₂ into deep underground rock formations for permanent isolation .
Geological Storage Mechanisms
CO₂ is typically stored in porous rock formations, such as sandstone, that contain tiny spaces called pores . These formations act as natural storage reservoirs and must be covered by a layer of solid, impermeable rock called caprock, which works like a natural seal to prevent the CO₂ from escaping to the surface . This is the same geological mechanism that has trapped natural gas and oil for millions of years.
CO₂ is converted into a supercritical fluid before injection, taking up to 400 times less space than as a gas and behaving like both a liquid and a gas . It is injected deep underground, often at depths of one kilometer or more, where heat and pressure are high enough to maintain this state.
Types of Storage Formations
The most common storage sites are deep saline aquifers or formations, which are porous rock formations filled with very salty water . These are considered excellent storage sites because they are widely available and not connected to freshwater sources. Depleted oil and gas reservoirs are also ideal because their geological integrity has been proven over millions of years and existing infrastructure can often be repurposed.
The total accessible geologic storage capacity is estimated to be between 8,000 and 55,000 gigatonnes . The storage potential in coal seams is also significant, with coal-rich countries like India having the capacity to store billions of tonnes of CO₂ . In Indian coalfields, the Raniganj Coalfield's Barakar Formation has the highest simulated CO₂ storage potential, reaching 1282 Mt .
Storage Processes
There are four key mechanisms for trapping CO₂ in a sandstone formation within a saline aquifer :
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Local trapping: CO₂ bubbles are trapped in the tiny pore spaces of the sandstone
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Dissolving: CO₂ mixes with the salty water in the sandstone
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Mineralisation: CO₂ reacts with the rock to form solid minerals
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Physical trapping: A layer of rock acts as a seal to stop the CO₂ from moving upward
These mechanisms, especially mineralisation, become more effective over time, ensuring long-term permanent storage .
Operational Considerations
The injection of CO₂ into coal seams can also enhance methane recovery (CO₂-ECBM) because CO₂ has 2-3 times higher preferential adsorption than CH₄ at the same pressure . This creates a dual benefit: storing CO₂ while improving methane recovery from unminable coal seams. The injection of CO₂ into these formations is carefully managed, with well-established techniques for monitoring the CO₂ plume and ensuring storage integrity. The Oil & Gas Magnetic Ranging Market is expected to achieve robust growth by 2035, and the development of carbon storage infrastructure will continue to create demand for precise subsurface measurement technologies.
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