Review of CO2 storage in basalts – new technical review from IEAGHG on the potential of using basalts and other magnesium rich rocks to store CO2.
Conventional CO2 storage relies on injection into a reservoir in sedimentary rock which has an impermeable caprock. It is also possible to trap CO2 in igneous rock formations with high magnesium, iron and calcium contents. Minerals with these metal cations react with CO2 especially if water is present. New carbonate minerals then form permanently locking the CO2 in the subsurface. Because this process is relatively rapid potential leakage is minimised.
Basalts are volcanic in origin and consequently they form rocks with a fine grained mineral matrix when they solidify often with vesicles that can form layers with high porosity and permeability. CO2 injected into these layers is then trapped by carbonation reactions. Two high profile sites, CarbFix in Iceland and the Wallula project in Washington State have both injected and monitored CO2 storage in basalts since 2012. Evidence from both sites shows that injected CO2 reacts relatively rapidly to form carbonate minerals. One potential limitation of this form of carbon sequestration is the large quantity of water required. Further tests are required to demonstrate the process at larger scale.
Some other igneous rocks with high magnesium (>12% by weight) contents are also known to react with atmospheric CO2. Naturally occurring carbonate minerals can be observed where these ultramafic rocks outcrop, for example in Oman. Such rock formations are comparatively rare compared to basalts and do not form layers with permeable zones which limits their carbon sequestration potential. However, ultramafic rocks are mined where they contain valuable metals particularly platinum and chromium. After extraction the crushed rock tailings are dumped in large quantities. One of the largest producers of platinum from ultramafic rocks is South Africa, a country which has evaluated the potential of using mine tailings for CO2 sequestration.
Further details about CO2 storage in basalts, and the potential that ultramafic rocks could offer, are explain in this latest technical review from IEAGHG.