CO2 and NOx reactions with CO2 storage reservoir core: NOx dissolution products and mineral reactions

The geological storage of CO2 is a key component in the energy transition to net zero emissions. Industrial CO2 streams from combustion, or hard to abate industries such as lime, cement, and steel production contain gases such as NOx, SOx, N2, H2S, NH3, or O2. These gases can impact the gas stream properties, and when dissolved some can form acids inducing rock reactions, or affect the system redox. The behaviour of NOx and its impacts on reservoir rock during CO2 storage, however, has received little attention. Experimental reactions were performed with supercritical CO2 containing 50 ppm NO and reservoir sandstones and mudstone. The solution pH decreased to between 3.2 and 4.8, with an initial increase in dissolved Si, Al, K, Ca, Fe, Mg, Na, and S concentrations. Si and Fe concentrations continued to increase from quartz rich sandstones from trace chlorite reaction during exper-iments, with Ca, K, and Si stabilising. Dissolved Ca, K, S, Si, Mg, Mn, and Fe continued to increase during the reaction of the mudstone from the reaction of mainly chlorite, ankerite, muscovite, and Ca-sulphate. Corrosion of sulphate minerals and chlorite, movement of kaolinite fines, and Fe-oxide precipitation were observed. A higher dissolved concentration of NO3? than NO2?was measured during CO2/NO experiments, indicating mainly HNO3 formation over HNO2. The concentration of NO2? displayed a first order type decline over time and was almost completely removed from solution. Dissolved Fe2+ potentially enabled reduction of NO2? forming Fe-oxides. Geochemical simulations predicted Fe-Mg-chlorite, K-feldspar, and ankerite were the main minerals reacting during experiments, with Fe-oxide precipitation. This study is broadly applicable to industrial CO2 storage in other sandstone reservoirs where NOx may be co-injected with CO2, however future studies are needed to broaden the understands for more reactive rock types, gas mixtures, and different storage conditions. CO2 geological storage studies are also applicable to improve prospects for underground hydrogen, gas, or com-pressed air storage for renewable energy and energy security.

Automated summary

Geological storage of CO2 is crucial for achieving the energy transition to net zero emissions. However, the behavior of NOx and its impact on reservoir rock during CO2 storage has been little studied. Experimental reactions show that NOx can affect the pH and induce rock reactions and precipitation when dissolved in CO2. Further studies are needed to understand the reaction mechanisms in different rock types, gas mixtures, and storage conditions, which is important for industrial CO2 storage and other renewable energy storage technologies.