Geochemical impact of O2 impurity in CO2 stream on carbonate carbon-storage reservoirs

Abstract

Carbon capture and storage (CCS) is regarded as an effective, large-scale mitigation technology for reducing CO2 atmospheric emissions from the use of fossil fuels. One of the major barriers for widespread deployment of the technology is the high cost of CO2 capture from the flue gas of fossil-fuel power plants and other industrial emitters. In general, the purer the desired captured CO2 stream, the more expensive the capture process. Coinjecting some of these impurities with CO2 can reduce the operational cost of CCS. Potential detrimental effects, if any, of impurities on storage formations need to be identified and evaluated. Previous studies on the effects of impurities have focused on sandstone reservoir rocks and mudstone caprock, but little work has been done to evaluate potential impacts on carbonate reservoirs, which are major storage candidates. We conducted a series of autoclave experiments on Redwater Leduc limestone (Alberta, Canada) and SACROC dolostone (Texas, United States) to investigate the effect of O2 impurity on CO2–brine–rock interactions. A total of eight reaction experiments were conducted with or without O2 for ∼3 weeks each at 200bar and 70°C or 100°C. Chemical analyses of the reaction fluids show that carbonate dissolution is the major mineral reaction caused by injection of CO2. The addition of 3.5% O2 had no significant impact on the limestone, whereas it led to precipitation of iron hydroxides in dolostone experiments that contained ankerite and a small amount of siderite. Porosity and permeability increased when CO2 was added, but the addition of O2 did not lead to notable changes. The results suggest that the addition of O2 impurity into the CO2 stream will not cause significant damage to carbonate formations, which potentially allows significant cost reduction by retaining a small O2 content in the injection gas stream.