Carbon Storage

HOW IS CO2 TRAPPED IN THE SUBSURFACE?

Trapping refers to the way in which the CO2 remains underground in the location where it was injected. There are four main mechanisms that trap the injected CO2 in the subsurface. Each of these mechanisms play a role in how the CO2 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 CO2 in the rock and is the mechanism that traps the greatest amount of CO2. The rock layers, fractures, and faults within and above the storage zone where the CO2 is injected act as seals, preventing CO2 from moving out of the storage zone. Once injected, the supercritical CO2 can be more buoyant than other liquids present in the surrounding pore space. Therefore, the CO2 will migrate upwards through the porous rocks until it reaches (and is trapped by) an impermeable layer of seal rock.

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Diagram depicting two examples of structural trapping. The top image shows the CO2 being trapped beneath a dome, preventing it from migrating laterally or vertically. The bottom image shows that CO2 is prevented from migrating vertically by the overlying seal rock and a fault to the right of the CO2.

Residual Trapping – Residual Trapping mechanism refers to the CO2 that remains trapped in the pore space between the rock grains as the CO2 plume migrates through the rock. The existing porous rock acts like a rigid sponge. When supercritical CO2 is injected into the formation it displaces the existing fluid as it moves through the porous rock. As the CO2 continues to move, small-portions of the CO2 can be left behind as disconnected, or residual, droplets in the pore spaces which are essentially immobile, just like water in a sponge.

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Diagram depicting the pockets of residually trapped CO2 in the pore space between the rock grains as the CO2 migrates to the right through the openings in the rock.

Solubility Trapping – In Solubility Trapping, a portion of the injected CO2 will dissolve into the brine water that is present in the pore spaces within the rock.

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Diagram depicting the CO2 interacting with the brine water, leading to solubility trapping. At the CO2/brine water interface, some of the CO2 molecules dissolve into the brine water within the rock’s pore space. Some of that dissolved CO2 then combines with available hydrogen atoms to form HCO3.

Mineral Trapping – Mineral Trapping refers to a reaction that can occur when the CO2 dissolved in the rock’s bine water reacts with the minerals in the rock. When CO2 dissolves in water it forms a weak carbonic acid (H2CO3) and eventually bicarbonate (HCO3-). 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 CO2.

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Diagram depicting the formation of minerals on the surface of a rock grain (bottom right of image) as it reacts with the dissolved CO2 in the brine water. The magnesium in the rock grain combines with the CO3 in the water to produce the mineral MgCO3 on the grain’s surface.

Myth: There is nothing preventing injected CO2 from migrating to the Earth’s surface through the overlying rock, making CO2 leakage inevitable.
Reality: There are four main mechanisms that help trap CO2 in the subsurface and prevent it from migrating to the surface.

What characteristics are studied to determine if a location is suitable for carbon storage?