CCS enables industry to continue to operate while emitting fewer greenhouse gases CHGs , making it a powerful tool for addressing mitigation of anthropogenic CO 2 in the atmosphere. However, storage must be safe, environmentally sustainable, and cost-effective.
Suitable storage formations can occur in both onshore and offshore settings, and each type of geologic formation presents different opportunities and challenges. Geologic storage is defined as the placement of CO 2 into a subsurface formation so that it will remain safely and permanently stored. The U. Department of Energy DOE is investigating five types of underground formations for geologic carbon storage:.
Myth: Carbon capture and storage is not a feasible way to reduce human CO 2 emissions. Reality: Developing the technologies and know-how to successfully capture and store CO 2 emissions will allow for a viable industry that will reduce the human contribution to atmospheric CO 2 levels. Carbon dioxide CO 2 can be stored underground as a supercritical fluid. Supercritical CO 2 means that the CO 2 is at a temperature in excess of At such high temperatures and pressures, the CO 2 has some properties like a gas and some properties like a liquid.
In particular, it is dense like a liquid but has viscosity like a gas. At depths below about meters about 2, feet , the natural temperature and fluid pressures are in excess of the critical point of CO 2 for most places on Earth. This means that CO 2 injected at this depth or deeper will remain in the supercritical condition given the temperatures and pressures present.
Myth: The CO 2 gas behaves the same in the atmosphere as it does when injected deep underground. Reality: The elevated temperatures and pressures that exist at the depths where CO 2 is injected changes its characteristics, allowing for storage of much greater volumes of CO 2 than at the surface.
Trapping refers to the way in which the carbon dioxide CO 2 remains underground in the location where it is injected. There are four main mechanisms that trap the injected CO 2 in the subsurface. Each of these mechanisms plays a role in how the CO 2 remains trapped in the subsurface. The following provides a description of each type of trapping mechanism. Structural Trapping — Structural trapping is the physical trapping of CO 2 in the rock and is the mechanism that traps the greatest amount of CO 2.
The rock layers and faults within and above the storage formation where the CO 2 is injected act as seals, preventing CO 2 from moving out of the storage formation. Once injected, the supercritical CO 2 can be more buoyant than other liquids present in the surrounding pore space.
Therefore, the CO 2 will migrate upwards through the porous rocks until it reaches and is trapped by an impermeable layer of seal rock. Diagram depicting two examples of structural trapping. The top image shows the CO 2 being trapped beneath a dome, preventing it from migrating laterally or vertically.
The bottom image shows that CO 2 is prevented from migrating vertically by the overlying seal rock and a fault to the right of the CO 2. Residual Trapping — Residual trapping refers to the CO 2 that remains trapped in the pore space between the rock grains as the CO 2 plume migrates through the rock. The existing porous rock acts like a rigid sponge. When supercritical CO 2 is injected into the formation, it displaces the existing fluid as it moves through the porous rock.
As the CO 2 continues to move, small portions of the CO 2 can be left behind as disconnected, or residual, droplets in the pore spaces which are essentially immobile, just like water in a sponge. Diagram depicting the pockets of residually trapped CO 2 in the pore space between the rock grains as the CO 2 migrates to the right through the openings in the rock.
Solubility Trapping — In solubility trapping, a portion of the injected CO 2 will dissolve into the brine water that is present in the pore spaces within the rock. Diagram depicting the CO 2 interacting with the brine water, leading to solubility trapping. Over extended periods, this weak acid can react with the minerals in the surrounding rock to form solid carbonate minerals, permanently trapping and storing that portion of the injected CO 2.
Diagram depicting the formation of minerals on the surface of a rock grain bottom right of image as it reacts with the dissolved CO 2 in the brine water. Reality: There are four main mechanisms that help trap CO 2 in the subsurface and prevent it from migrating to the surface. When assessing a storage site, some of the reservoir characteristics that are studied for long-term carbon dioxide CO 2 storage include storage resource, injectivity, integrity, and depth. The term "subsurface storage complex" refers to the geologic storage site that is targeted to safely and permanently store injected CO 2 underground.
It includes a storage formation with at least one, or usually multiple, regionally continuous sealing formations called caprocks or seals. All of these characteristics are examined in order to determine if a potential storage complex has adequate conditions for CO 2 storage.
Image depicting the features of different types of carbon storage complexes including saline formations, oil and natural gas reservoirs, unmineable coal areas, organic-rich shales, and basalt formations.
All of the complexes include: 1 a confining zone that includes a thick or several sealing layer s above the storage zone, separating the stored CO 2 from drinking water sources and the surface; 2 adequate integrity within the storage formation and sealing layers; 3 sufficient porosity and permeability to store large amounts of CO 2 ; and 4 are at supercritical depth to allow for concentrated storage.
Myth: Any location that has an injection well can be used to inject and store carbon. These characteristics are determined through a rigorous characterization process that includes assessing potential storage risks and meeting the regulations under the U. These methods are controlled by natural physical processes and can take hundreds or even thousands of years to trap CO2, Alcalde says:.
Each scenario assumes that CO2 storage begins in and ends by , although the model runs 10, years into the future. The charts below show the expected leakage from each baseline scenario in model simulations. On the charts, the black lines show the total CO2 injected, the grey lines show the when the injection ceases, the red shading shows the proportion of CO2 that is leaked, the blue lines show the proportion of CO2 that is permanently trapped, and the green dashed lines show the total proportion of CO2 that is retained underground.
Expected CO2 leakage from an offshore scenario left , a well-regulated onshore scenario middle and a poorly-regulated offshore right. Black lines show the total CO2 injected, grey lines show the when the injection ceases, red shading shows the proportion of CO2 that is leaked, blue lines show the proportion of CO2 that is permanently trapped, and green dashed lines show the total proportion of CO2 that is retained underground.
The results show that the offshore baseline scenario is expected to experience the smallest amount of leakage 0. The charts below show the spread of the results from 10, different simulations in the model. Each simulation made slightly different assumptions about the model parameters, including the fraction of CO2 injection wells that would leak, the number of abandoned wells that whose whereabouts are unknown and the leakage rate from natural pathways.
Grey bars show the number of model runs reaching different proportions of CO2 leakage, while the red line corresponds to the baseline scenario as above. The distribution of model results for the offshore scenario left , well-regulated onshore scenario middle and poorly-regulated offshore right. Grey bars show the number of model runs reaching different proportions of CO2 leakage, while the red line corresponds to the baseline scenario.
The amount of leakage remains relatively low under poor regulation as a proportion of CO2 becomes trapped due to physical processes, such as mineralisation, the researchers say. However, other researchers are less confident about the potential of CCS. He tells Carbon Brief:. Unlike nuclear waste, CO2 becomes safer and more secure the longer it stays in the ground due to a range of physical processes, such as mineralisation. Alcalde, J. Get a Daily or Weekly round-up of all the important articles and papers selected by Carbon Brief by email.
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