The impact of CO2 impurities (SO2, NOx, O2) on mineral dissolution/precipitation processes in a CO2 storage reservoir – Field-based results from the CO2CRC Otway Project site

Abstract:

CO₂ storage with impurities such as O₂, NO_x and SO₂ is increasingly considered, because the complete removal of impurities is costly. Theoretical and laboratory-based studies have mostly focused on the formation of sulfuric and nitric acid from SO₂ and NO_x, respectively, while redox reactions induced by the co-injection of O₂ have received little attention. Here we present results from the first field-based experiment pointing to an important role of O₂.

At the CO2CRC Otway Project site, CO₂ and CO₂ enriched with impurities (54 ppm SO₂, 9 ppm NO_x and 6150 ppm O₂) were mixed at depth with water leading to CO₂ saturated water. The CO₂ saturated water was then allowed to react with the reservoir at a depth of 1400 m (Paaratte Formation, Otway Basin) over a total of 23 days. Reservoir water at in situ pressure conditions was taken for analyses using the U-tube sampling system. Back-produced water showed no evidence of silica or carbonate mineral dissolution, which can be explained by the level of alkalinity effectively buffering the additional acidity (as the result of the co-injection of SO₂ and NO_x). Independently, co-injected O₂ caused oxidation of pyrite, resulting in rapid increase in SO₄^2- concentration during the first three days of the subsurface incubation. Subsequently, the SO₄^2- concentration dropped in the back-produced water, which may be explained by SO₄^2- adsorption on the surface of minerals like iron (oxyhydr)oxides and kaolinite or by reduced pyrite oxidation at distance from the well bore. Reactive transport modeling is currently underway to better understand the observations.