Saline aquifer have been proposed as a long term storage site for captured atmospheric CO₂ due to their large storage capacity
Summary
Deep saline aquifers show promise for long-term CO₂ storage due to their natural geological features. These underground rock formations contain brine-filled porous rock capped by impermeable layers, preventing upward migration of injected CO₂. Primary storage mechanisms include physical trapping under caprock, dissolution in brine, and mineral formation through chemical reactions. While offering massive global storage potential and reduced infrastructure needs through natural dissipation, challenges include ensuring caprock integrity over time and high initial drilling costs. Monitoring requirements and lack of existing infrastructure add to implementation complexities.
Saline aquifers have been proposed as long-term storage sites for captured atmospheric CO₂. Saline aquifers are deep, porous rock formations filled with briny (salt-rich) water. Due to the highly saline nature of the water, saline aquifers are not suitable for other uses such as agriculture or human consumption.
Characteristics of saline aquifer storage
Saline aquifers have the natural potential to store large amounts of CO₂ due to the following characteristics:
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High porosity and permeability: Saline aquifers are typically composed of porous rock formations, which allow the CO₂ to dissipate throughout the rock formation 1.
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Impermeable caprock: Saline aquifers typically have an impermeable/low-permeable layer on their top surface known as a caprock.
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Depth: Many saline aquifers are located at depths of more than 1,000 meters below the surface 1.
Trapping Mechanisms
There are multiple mechanisms that assist in trapping CO₂ in saline aquifers:
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Structural/stratigraphic trapping: Structural trapping is the dominant trapping mechanism for saline aquifers. Structural trapping is simply the trapping of CO₂ under the impermeable caprock. As the density of the CO₂ is lower than the density of the brine, the CO₂ will naturally rise to the surface and be trapped under the caprock 1. This is analogous to taking an empty cup, placing it upside down, and then submerging it in water. If the cup is impermeable, the air will be trapped inside.
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Residual trapping: Residual trapping is the trapping of CO₂ in the remaining pore space after the CO₂ has been injected into the aquifer. This occurs when the CO₂ is injected into the aquifer and displaces the brine. When the displaced brine starts to return to the rock, the CO₂ will be trapped in the remaining pore space.
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Solubility trapping: Solubility trapping is the trapping of CO₂ into the brine as the CO₂ dissolves 1.
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Mineral trapping: Mineral trapping is the trapping of CO₂ via the precipitation of carbonate minerals. This is achieved through the formation of carbonic acid (H₂CO₃) in the brine, which reacts with other minerals present in the rock formation such as calcium (CaCO₃) and magnesium (MgCO₃).
Advantages
The main advantage of saline aquifer carbon dioxide storage is the extremely large storage capacity that they offer due to the ubiquity of saline aquifers on Earth 1. Another advantage of saline aquifer storage is that due to their porous nature, there is relatively large dissipation of CO₂ into the rock formation, meaning that fewer injection sites are required 1.
Disadvantages
The main disadvantage of saline aquifers is the stability of their caprock. Long-term storage of CO₂ in saline aquifers requires a stable caprock to prevent CO₂ from escaping to the surface, and it can be difficult to accurately evaluate the quality of a caprock, as well as monitor it for new faults 1. A secondary disadvantage is the high cost of drilling and constructing the injection sites, which typically do not already contain existing injection infrastructure 1.