Sequestration of CO2 in Geological Formations as Carbonate Minerals.

Atmospheric carbon dioxide concentrations have been steadily increasing over the past century causing detrimental effects on the earth’s climate.  In addition to efforts to decreased future carbon emissions, the capture and storage of current CO2 in the atmosphere is an important component of a long-term solution to for reducing CO2 concentrations.  One method proposed for this is geological CO2 storage.  This is a process in which CO2 emissions are pumped into geological formations instead of into the earth’s atmosphere.  Since the CO2 being inserted in to the rock is buoyant, when compared to the rock and surrounding water, there are different trapping mechanisms to insure that the CO2 remains at depth and does not resurface to be released into the air.  The focus of this paper by Matter and Kelemen (2009) is “mineral tapping” in which dissolved CO2 reacts with water and the minerals of the surrounding rock to form solid carbonate that will remain in place.  This is a long-term storage solution for large quantities of CO2.  The success of this solution, however, depends on the type of physical and chemical properties of the location chosen for injection.—Anna Fiastro
Matter, J., Kelemen, P., 2009. Permanent storage of carbon dioxide in geological reservoirs by mineral carbonation. Nature geoscience. doi:10.1038/NGEO683.
The emissions are pumped to depths of over 800 meters where the combination of temperature, pressure, and salinity in addition to the pH of the location induced a fluid-rock reaction that causes carbonate mineral formation.  Early studies examined deep aquifers in sedimentary rock because of the porous nature of the rock.  It was thought that space was a necessary characteristic of the host rock because it offered a place to deposit the carbonate mineral product.  Sedimentary rock includes sandstone, siltstone, shale, and limestone, however these types of rock have very low mineral trapping potential.  This is seen in prediction models run in various labs, and in field observations of natural CO2 reservoirs leeching into rock.
          The field of research then looked towards aquifers containing ‘basic’ silicate minerals, such as olivine, serpentine, pyroxenes, plagioclase, and basaltic glass. It was found that silicate minerals buffer the pH in these reactions making them essential for enhancing mineral storage. It has been shown in laboratory experiments and in natural analogues that these types of rock react rapidly to form carbonate minerals.  These types of rock are also commonly found all around the world and on every continent.  This means that their capacity for CO2 storage in carbonate is enormous.
The original concern with mineral trapping was the need for space.  The reactions are often self-limiting because they fill in empty space and can create boundaries between the unreacted CO2 and fluid.  As was mentioned earlier, this was the advantage of sedimentary rock originally being examined.  The porous nature of the rock is important to ensure ample room for product creation. It was thought that optimal rock containing silicate minerals would not be porous enough to have a continued reaction and convert all of the CO2 to carbonate.  As a solution to this, it is hypothesized that the crystallization can fracture the rock to increase permeability.  This has been proven to occur in both laboratory simulations as well as naturally occurring systems.  The fracturing that occurs creates more space for the carbonate product to be deposited and allowed the reactants to continually come in contact with each other forming more carbonate.
Another aspect that makes this process a favorable solution to CO2 capture is the self-heating cycle that occurs.  Heat is given off from the initial reaction and remains to speed up the continued reaction of more and more CO2, increasing the overall reaction rate.  With continued reactions taking place, the elevated temperature is maintained and so is the speedy reaction rate.  This results in more and more CO2 being sequestered.
The combination of silicate minerals, fracturing and excess heat allow for large quantities of carbon dioxide to be captured and deposited in underground aquifers as carbonate minerals.  This is a solution to increased CO2 levels that is being examined further.

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