Geochemical impact of oxygen on siliciclastic carbon storage reservoirs

In planning for large-scale CO2 capture procedures, there exists an economic incentive to leave minor gas components in the captured CO2 stream for geological storage, because doing so would reduce the cost of the capture process. However, co-injection of reactive impurities (such as O2, CO, H2, NOx, and SOx) may have detrimental effects on well injectivity, wellbore integrity, seal performance, and gas-brine-rock interactions. In this study we investigated, using a series of autoclave experiments, the potential impact of O2 as an impurity on geochemical reactions. Three sandstones were tested: Miocene sandstone (Texas offshore, USA), lower Tuscaloosa sandstone (Cranfield field, Mississippi, USA) and Cardium sandstone (Pembina field, Alberta, Canada). Samples were subjected to reaction with 1.88 M NaCl solution and CO2, with and without the addition of a small volume of O2 in order to determine the geochemical effects of O2. A total of ten reaction experiments were conducted at 200 bar and 70 °C or 100 °C. Overall, for rock samples lacking reducing minerals, co-injection of a small volume of O2 with supercritical CO2 showed only a limited geochemical impact compared to the reactions with pure CO2. The presence of O2 did not accelerate the dissolution of carbonate and feldspar minerals, nor did it alter the reaction pathways. However, the small amount of pyrite (≈0.7%) contained in the Cardium sandstone had an important impact. As it was oxidized, the solution pH was further lowered because the Cardium sandstone contains little pH-buffering carbonate and feldspar minerals. Consequently, dissolution of carbonates and feldspars was enhanced by the more-acidic solution. Aqueous ferrous iron was largely converted to iron oxyhydroxides and precipitated on mineral surfaces. The minor cations that showed the largest increases in aqueous concentration (such as Mn, Sr, and Ba) were mostly mobilized from carbonates and were not significantly enhanced by O2 for the low-pyrite sandstones. Releases of trace metals, with the exception of Zn and Pb, were not enhanced by the presence of O2. In fact, concentrations of As, V, Mo, and other oxyanions were lower in the reactions involving O2 than in the pure CO2 reactions, because these metals sorb on the newly precipitated iron oxyhydroxides.