Since 1992, there has been intense interest in using carbon storage to prevent carbon from entering the Earth’s atmosphere, leading to the United Nations Framework Convention on Climate Change (UNFCCC).
The goal of that Convention was the “stabilization of greenhouse gas concentrations in the atmosphere at a level that prevents dangerous anthropogenic interference with the climate system.”
In the day and age of climate change, carbon storage is an essential practice to help us meet this goal.
Carbon storage stabilizes carbon in solid and dissolved forms so that it doesn’t cause the atmosphere to warm.
In this blog, we’ll discuss how this process can reduce the human “carbon footprint” and what you need to know about carbon storage (also known as carbon sequestration).
1. What is carbon?
Carbon is a chemical element that is a basic building block of biomolecules.
It exists on Earth in solid, dissolved, and gaseous forms.
2. What is carbon storage?
Carbon storage, also known as carbon sequestration, is the process of capturing carbon dioxide formed during fossil fuel power generation and industrial processes.
This carbon is then stored so that it is not emitted into the atmosphere.
Carbon storage technologies have the strong potential to reduce carbon dioxide emissions in energy systems.
3. Why is carbon storage necessary?
Carbon storage enables industry to continue operations while reducing greenhouse gas emissions.
Carbon storage is a powerful tool that allows us to address anthropogenic carbon dioxide in the atmosphere.
Safe carbon storage is an environmentally sustainable and potentially cost-effective way of mitigating our carbon emissions.
4. What are the types of carbon sequestration?
There are three types of carbon sequestration: biological, geological, and technological.
Biological: Biological carbon sequestration is the storage of carbon dioxide in vegetation, like grasslands or forests, soils, and oceans.
Oceans absorb roughly 25 percent of carbon dioxide.
Carbon goes in both directions in the ocean:
- Positive atmospheric flux – carbon dioxide is released into the atmosphere from the ocean.
- Negative atmospheric flux – the ocean absorbs carbon dioxide
Carbon is also sequestered in soil by plants through photosynthesis and can be stored as soil organic carbon (SOC).
About 25 percent of global carbon emissions are captured by plant-rich landscapes such as forests, grasslands, and rangelands.
Note: When the plant’s leaves and branches fall or the plant dies, the stored carbon will release into the atmosphere or is transferred into the soil.
Geological: Geological carbon sequestration is the process of storing carbon dioxide in underground geologic formations (rocks).
Carbon dioxide is usually captured from an industrial source (i.e., steel or cement production) or an energy-related source (i.e., power plant or natural gas processing facility).
Then, the carbon dioxide is injected into porous rocks for long-term storage.
Geological carbon capture and storage (CCS) is the primary method used to offset industrial and utility CO2 emissions until ways to permanently reduce such emissions are found.
Technological: Scientists have started to explore additional ways of storing carbon using innovative technologies, such as:
1. Graphene production – This is the use of carbon dioxide as a raw material to produce graphene (a technological material), which is used to create screens for smartphones and other tech devices.
While this is limited to specific industries, it illustrates how carbon dioxide can be used as a resource and solution to reduce emissions.
2. Direct air capture (DAC) – This is a means to capture carbon directly from the air using advanced technology plants.
This is energy-intensive and expensive ($500-$800 per tons of carbon removed).
As such, direct air capture can be effective, but is generally too expensive to be widely used at the moment.
3. Engineered molecules – Scientists are currently engineering molecules capable of singling out and capturing carbon dioxide from the air.
The molecules act as a filter and attract the element that they were engineered to seek.
5. What are the three steps in the industrial carbon storage process?
The three steps in the carbon storage process are as follows:
Capture: The carbon dioxide is separated from other gases that are produced in industrial processes.
Transport: The carbon dioxide is then compressed and transported via pipelines, road transport, or ships to a site for storage.
Storage: The carbon dioxide is injected into rock formations deep underground for permanent storage.
6. How can carbon dioxide be stored underground?
Carbon dioxide can be stored underground as a supercritical fluid.
This means that the carbon dioxide is at a temperature of more than 31.1 degrees Celsius (88 degrees Fahrenheit) and pressure over 72.9 atm.
This temperature and pressure define the critical point for carbon dioxide.
At this high temperature and pressure, carbon dioxide has some properties like gas and others like liquid.
It is dense like a liquid, but with a viscosity like a gas.
When you store carbon dioxide in a supercritical condition, the storage volume is significantly less than if the carbon dioxide were at “standard” (room-pressure) conditions.
At depths below about 800 meters (or roughly 2,600 feet), the natural temperature and fluid pressure are more than the critical point for carbon dioxide.
Thus, the carbon dioxide injected at this depth or deeper will remain in the supercritical condition given the temperatures and pressures present.
Overall, you should not expect carbon dioxide gas to behave the same way when it’s injected deep underground as when it’s in the atmosphere.
The elevated temperatures and pressure that exist at the depths where carbon dioxide is injected change its characteristics.
These altered characteristics allow for the storage of greater volumes of carbon dioxide.
7. How is carbon trapped in the subsurface?
“Trapping” refers to how we keep carbon dioxide underground in the location where it’s injected.
There are four primary mechanisms that trap injected carbon dioxide into the subsurface.
Here are each of the trapping mechanisms.
Structural trapping: This is the physical trapping of carbon dioxide in the rock.
It is also the mechanism that traps the largest amount of stored carbon dioxide.
Once it’s injected in porous rock formations, the supercritical carbon dioxide is more buoyant than other liquids.
Therefore, the carbon dioxide will migrate upwards through the porous rock until it reaches (and becomes trapped) an impermeable layer of seal rock.
Thus, the impermeable rock layers above the storage formation where the carbon dioxide is injected act as a seal, which prevents carbon dioxide from moving out of the storage formation.
Residual trapping: This refers to the carbon dioxide that remains trapped in the porous space between rock grains as the carbon dioxide plume migrates through the rock.
The existing porous rock will act like a rigid sponge.
When the supercritical carbon dioxide is injected into the formation, it displaces existing fluid as it moves through the porous rock.
The carbon dioxide will continue to move, and small portions of carbon dioxide will be left behind as residual droplets in the pore spaces.
These are essentially immobile (like water in a sponge).
Solubility trapping: This refers to the portion of injected carbon dioxide that will dissolve into the brine water that is present in the pore spaces within the rock.
At the carbon dioxide and brine water interface, some of the carbon dioxide molecules dissolve into the brine water within the rock’s pore space.
Some of the dissolved carbon dioxide will combine with the available hydrogen atoms to form HCO3.
Mineral trapping: This refers to a reaction that occurs when the carbon dioxide dissolves in the rock’s brine water and reacts with the minerals in the rock.
When CO2 dissolves in water it forms a weak carbonic acid and then bicarbonate.
Over extended periods, this weak acid can react with the minerals in the surrounding rock to form solid carbonate minerals that permanently trap/store the portion of the injected carbon dioxide.
While it’s generally thought that there’s nothing preventing injected carbon dioxide from migrating to Earth’s surface through the overlying rock, this isn’t true.
Carbon dioxide leakage isn’t inevitable.
The four mechanisms above help trap carbon dioxide in the subsurface and prevent it from migrating to the surface.
8. What are the characteristics of a subsurface carbon storage complex?
A “subsurface storage complex” refers to the geologic storage site that is targeted to store injected carbon dioxide underground safely and permanently.
Here are some of the primary characteristics.
Storage resource: The storage site must have sufficient storage space to contain large amounts of compressed carbon dioxide.
“Large amounts” typically equate to millions of metric tons.
Injectivity: Injectivity refers to the rate at which carbon dioxide can be injected into the subsurface.
Injectivity of carbon dioxide is directly related to the permeability of the formation.
Permeability is a measure of the ability of fluid to flow through it.
Integrity: This refers to the ability to confine carbon dioxide safely within a predetermined volume without a breach.
A solid storage complex will have one or more confining zones above the injected carbon formation that are intact and do not have leakage pathways.
Depth: The carbon dioxide storage zone needs to be located at a sufficient depth and pressure so that carbon dioxide can be injected as a supercritical fluid.
Because supercritical carbon dioxide is dense and behaves more like a liquid than a gas, it can be stored in higher concentrations by volume.
9. What are the different storage types for geologic CO2 storage?
Suitable storage formations can be found both onshore and offshore.
Each type of geologic formation presents different opportunities and challenges.
The U.S. Department of Energy is currently researching five different types of underground formations for geologic carbon storage.
Porous formations filled with brine (salty water) that span large volumes deep underground.
Oil and natural gas reservoirs
Oil and natural gas reservoirs can be found around the world.
Once the oil and natural gas is extracted from an underground formation, it leaves a permeable and porous volume that can be readily filled with carbon dioxide.
Unmineable coal seams
If coal cannot be mined because of geologic, technological, or economic factors, then it can be used as a location to store carbon dioxide.
CO2, once injected, is absorbed by the coal, which provides another mechanism to permanently store the carbon.
To be considered, the coal seam must have sufficient permeability and be considered unmineable.
Basalt formations are formations that were deposited when large flows of lava spread from volcanoes and then cooled and solidified.
The chemical and physical properties of basalt make them good candidates for carbon storage systems.
Shale formations are found around the U.S.
Like depleted gas and oil reservoirs, these formations can be well-suited to confine carbon.
Some shales also have similar properties to coal and can trap carbon dioxide through absorption producing methane that can be harvested for later use.
10. Where around the world is carbon storage happening today?
Carbon storage is currently occurring both in the United States and around the world.
There are large, commercial carbon storage projects being developed in Norway (The Sleipner CO2 Storage Site) and Canada (the Weyburn-Midale CO2 Project).
These projects store more than 1 million metrics of CO2 a piece each year.
China, Australia, and countries in Europe are also beginning to roll out projects.
In addition, there are many projects underway within the U.S. that are helping with geologic carbon storage.
11. What are the legal and regulatory issues associated with carbon storage?
Because carbon storage is still relatively new, the current knowledge regarding legal and regulatory requirements is still limited.
There is no appropriate framework to facilitate the implementation of geological storage while accounting for long-term liabilities.
The legal constraints still need to be clarified, and there are still knowledge gaps that scientists are working to resolve regarding the methodologies for emissions inventories and accounting.
As the world continues to navigate the climate crisis, understanding how carbon storage works is critical.
Scientists are working to expand our knowledge on how to better implement this tactic for long-term use.
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