Carbon Capture and storage (CSS) has been hailed as a promising way to reduce industrial emissions of CO2 into the atmosphere. It’s gaining traction and investment, with more projects getting approved every day. Once captured, the CO2 is injected underground and stored in rock formations, a process that Oil and Gas sector has perfected (although, in reverse) over time. It seems straightforward. And, according to Energy Minute and the Global CCS Institute, there is ample potential for storage globally (check out their map!). But how do you go about choosing the best place to store CO2 so that it is sequestered safely long-term? You may have guessed that things are never as simple as they sound. When it comes to choosing a reservoir, there are several factors that must be considered.
If you want more background on CCS, check out the first article in our ongoing CCS series, 7 Essential Things to Know about CCS.
Make way! A reservoir needs to accommodate the amount of CO2 that we want to inject. According to the IEA, current projects (once operational) should be able to store approximately 110 Mt CO2/year by 2030 (with 1 200 Mt CO2/year needed to be stored to meet net-zero goals).
How much space does a reservoir have? The space depends on how extensive the reservoir is, and how porous the rock is. For example – if you were to blow into a sponge, you could fill its holes with your breath. If you tried the same exercise with your desk, you might not have the same luck. This is because the sponge has high porosity, and those pores are connected – another factor that is important (keep on reading). Core analysis, well log analysis, and seismic attribute analysis are a few ways that geoscientists can estimate porosity.
The interconnected nature of the spaces in a rock (ie, its permeability) is also important. Permeability will also dictate the amount of CO2 that we can inject into a reservoir. The CO2 requires the ability to flow through the rock to accommodate large volumes of the injected gas. Geophysicists map the spreading (and containment) of CO2 plumes underground using repeated seismic surveys, fiber optic sensing, and microseismic monitoring (for more on this read The Importance of Seismic in CCS Projects).
Once the CO2 is in the rock, it needs to stay put. That is why a seal is critical for long-term containment. The reservoir should have a good caprock, which is a layer of impermeable rock that covers the reservoir and acts as a lid to keep CO2 from escaping. The caprock should have low permeability to prevent CO2 leakage.
The reservoir should be geologically stable, which means it should not have any cracks or faults that could create pathways for CO2 to leak out. A storage site should also have minimal geological activity, such as earthquakes or volcanic eruptions, that could damage the storage site. One way to understand the underground environment, is to use seismic surveys. These are used to map faults, fracture patterns, structures, and geologic changes in a potential storage project area.
The cost (and potential additional emissions) to transport CO2 from its industrial source to a storage site is another big consideration that impacts the economics of a CCS project. Ideally, the closer the reservoir is to the CO2 source, the better! The most common method for transporting CO2 is through pipelines. Similar to natural gas pipelines, dedicated CO2 pipelines transport the captured CO2 from the capture site to the storage site. The Alberta Carbon Trunk Line, for example consists of 240 km of pipeline (read about it here).
For Geoscientists and engineers, the public’s safety is paramount. Projects should not impact freshwater resources, sensitive ecosystems, or human populations. Prior to any project proceeding, a rigorous risk analysis must be completed and satisfy local regulations.
Choosing a suitable storage reservoir is vital for the success of CCS projects. It ensures that we can store CO2 safely and effectively for a long time. By considering factors such as porosity, permeability, storage capacity, caprock integrity, geological stability, proximity to emission sources, and environmental considerations, we can find the best places to store CO2 underground and reduce the impact of industrial operations.
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