Once the CO2 is captured and liquefied we reach the last step in the CCS process chain, a permanent storage site.
There are four different mechanism that allows us to trap CO2 underground.
- Structural trapping: An impermeable layer traps CO2 within a reservoir and prevents CO2 mitigation to the surrounding. The physical trap of CO2 by a caprock securely works as a seal and ensure a secure and permanent storage.
- Residual trapping: Liquid CO2 is trapped in pore spaces of surrounding rock formations at the storage site as CO2 migrates through the rock.
- Dissolution: CO2 dissolves into the water in pore spaces in the surrounding rock formation forming bicarbonate (HCO3–) and carbonic acid (H2CO3). This chemical reaction increases the density and causes the water to sink.
- Mineral trapping: The week carbonic acid reacts with the mineral of the surrounding rock formation and form solid carbonate minerals which permanently traps the once injected liquid CO2.
Potential storage sites includes following formations:
Injection in oil fields, which is a well-adapted technique to increase the pressure in the reservoir and thus push remaining oil out in into the wells. The method is known as enhanced oil recovery (EOR) and while the primarily targets isn’t to permanently store CO2 the benefits of using depleted oil fields for large-scale CO2 is investigated in various projects.
Another possible storage method is injection of CO2 into saline formations consisting of large porous sedimentary basins filled with salty water. The CO2 dissolves into the salty water and eventually mineralized and permanently trapped.
While already mentioned storage sites are located subsea, there are possible storage sites located on land as well. By injecting CO2 directly into a basaltic bedrock, which is reactive towards CO2, the formation of carbonic minerals is enhanced and CO2 is trapped.