What is a saline aquifer?

Why are saline aquifers important for CCS?

Saline aquifers are essential for CCS because they offer abundant storage capacity. Their depth, combined with impermeable caprock layers, makes them a secure option for trapping CO₂, thereby preventing it from reaching the atmosphere.

How is CO₂ stored in saline aquifers?

CO₂ is injected deep underground into the aquifer through specialized wells. The CO₂ is compressed into a supercritical state, allowing it to spread efficiently within the porous rock. Over time, the CO₂ is trapped through a combination of physical, dissolution, and mineralization processes.

Are saline aquifers safe for CO₂ storage?

Yes, saline aquifers are generally safe for CO₂ storage when properly managed. They are typically capped by impermeable rock layers, which help prevent CO₂ from migrating upward. Additionally, monitoring systems are used to ensure CO₂ remains securely stored.

What makes saline aquifers different from other CO₂ storage sites?

Saline aquifers are different because they contain non-potable, salty water, making them ideal for CO₂ storage without competing with water sources meant for human or agricultural use. They are also generally more abundant and widespread than other storage options like depleted oil and gas reservoirs.

How much CO₂ can a saline aquifer store?

The CO₂ storage capacity of a saline aquifer varies depending on its depth, porosity, and permeability. However, saline aquifers have a very high storage potential, theoretically capable of holding hundreds of gigatons of CO₂ globally, enough to support long-term storage needs.

What are the main benefits of using saline aquifers for CCS?

Saline aquifers offer several benefits:

• Capacity: They have large storage potential, often located close to major emission sources.
• Isolation: Located at great depths, saline aquifers reduce the risk of CO₂ leakage.
• Non-competition: They contain salty water, so they don't compete with freshwater resources.

What are the risks of storing CO₂ in saline aquifers?

The main risks include potential CO₂ leakage if the storage site is not properly monitored. However, with rigorous site selection and advanced monitoring techniques, these risks are minimized, and leakage is generally very low.

How is CO₂ kept from escaping saline aquifers?

CO₂ is kept in place by multiple trapping mechanisms, including:

1. Caprock Sealing: An impermeable rock layer prevents CO₂ from migrating upward.
2. Dissolution Trapping: CO₂ dissolves in the saline water, creating a stable mixture.
3. Mineralization: CO₂ eventually reacts with the surrounding rock, forming stable minerals.

What is supercritical CO₂, and why is it used in saline aquifer storage?

Supercritical CO₂ is CO₂ that has been compressed to a state where it has properties of both a gas and a liquid. This form allows CO₂ to be injected and stored more efficiently within the rock’s pore spaces, maximizing the storage potential of saline aquifers.

How long does CO₂ stay stored in saline aquifers?

CO₂ can stay stored in saline aquifers for thousands to millions of years. Over time, it becomes increasingly secure as it dissolves in brine and mineralizes, forming stable compounds that lock the CO₂ into rock formations.

What monitoring techniques are used for CO₂ stored in saline aquifers?

Monitoring techniques include:

• Seismic surveys: to detect CO₂ movement within the aquifer.
• Pressure monitoring: to ensure stable conditions in the storage site.
• Chemical sampling: to assess CO₂ concentrations and detect leakage.

Can saline aquifers be found everywhere?

Saline aquifers are widespread and found around the world, though not all are suitable for CO₂ storage. Suitable aquifers must be deep enough, have adequate porosity and permeability, and be covered by impermeable caprock to ensure secure storage.

How do saline aquifers compare to other CCS storage options?

Saline aquifers offer more storage capacity than depleted oil and gas fields and are generally more abundant. However, oil and gas fields have a known geological history, which can sometimes make them easier to monitor and manage. Both options play a role in CCS, but saline aquifers are crucial for long-term, large-scale CO₂ storage.

Are saline aquifers the future of CCS?

Yes, saline aquifers are expected to play a significant role in the future of CCS because of their capacity, widespread availability, and isolation from potable water sources. With continued research and development, they are likely to become a primary storage option in CCS strategies worldwide.

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