Effect of NO₂ in exhaust gas from an oxyfuel combustion system on the cap rock of a proposed CO₂ injection site

A laboratory geochemical study was conducted using a drill core sample of cap rock from the Surat Basin, Australia, to investigate the effect of NO₂ contained in the CO₂ gas exhausted from the oxyfuel combustion process (oxyfuel combustion CO₂) on the cap rock. A gas (CO₂ containing NO₂) was prepared to simulate the exhaust gas produced from the oxyfuel combustion process. Two types of gases (pure CO₂ and CO₂ containing SO₂) were also prepared as reference gases. The effect of NO₂ on cap rock was studied experimentally using these gases. No differences in the amounts of leached ions and pH changes for CO₂ containing NO₂ (36 ppmv), pure CO₂, and CO₂ containing SO₂ (35 ppmv) existed. The pH values decreased immediately after CO₂ gas injection but increased with time as a result of mineral buffering. Leaching of Fe, Mg, Ca, and K was suggested to have occurred as the result of dissolution of Fe-chlorite, prehnite and illite-smectite mixed layer clay in the shale sample. The amounts of Ca, Fe, and Mg leached with CO₂ containing NO₂ (318 ppmv) were higher than those for pure CO₂. For the mixture containing 318 ppmv NO₂, the pH increased more than that for the other gas conditions immediately after the pH fall at the start of the experiment, because oxidation-reduction reactions occurred between Fe²⁺ and NO₃ ^-. Moreover, the results indicated that some of the leached Ca and Fe were deposited on the shale sample because of the pH increase. Therefore, we concluded that the effects of NO₂ on mineral dissolution and pH changes of formation water are negligible when oxyfuel combustion CO₂ containing about 30 ppmv of NO₂ is injected into an underground aquifer. In addition, even if about 300 ppmv NO₂ is accidentally injected into the underground aquifer, mineral dissolution is suppressed due to the buffering of pH decrease after gas injection.