CO2 Mineralization in Reactive Rocks

The safest long-term geologic storage of CO2 is its mineralization. Such mineralization is most efficient via the injection of water-dissolved CO2 into mafic or ultramafic rocks. The injection of water dissolved CO2 is shown to be financially favorable. Notably the relative costs of the injection of pure CO2 is found to be similar to that of the dissolution of this CO2 into water as it is injected into the subsurface. This is because the injection of pure CO2 requires higher injection pressures to get the CO2 into subsurface rock formations, owing to its low density, and these higher injection pressures require more robust and costly wells. Such factors tend to balance out the cost of the larger number of wells required for the CO2-charged water injection and the water needed for the dissolution of this gas. This latter cost is reduced if one used seawater rather than freshwater. Once injected into the subsurface this CO2-charged water will react with the subsurface rocks forming both carbonate minerals for the permanent storage of CO2, but also a suite of other secondary mineral products including clay minerals. The choice of target rock formation for CO2-charged water will affect greatly the rate and efficiency of this carbonation process.

Western Saudi Arabia contains a large variety of mafic and ultramafic rock types including fresh and altered basalts, pyroxenites, anorthosites, and altered dunites. Among these altered ultramafic rocks containing substantial brucite will most rapidly carbonate leading to hydromagnesite or magnesite with minor clay mineral formation. In less mafic rocks the presence of aluminum leads to substantial Mg-clay formation, but most of the Ca present in the minerals will be available for mineral carbon storage through the formation of calcite or aragonite. A suite of calculations using a newly constructed mineral kinetic database has been applied to estimating the carbonation rates of subsurface rocks in response to water-dissolved CO2 injections. Results of these calculations provide insight into the relative potential and rates at which each type of rock for the permanent storage of CO2 within these rocks.