Subsurface geochemical fate and effects of impurities contained in a CO2 stream injected into a deep saline aquifer: What is known

The presence of inert or non-condensable impurities in the injected CO2 stream, such as Ar, N2 and CH4, will have no or negligible geochemical effects in the subsurface, notwithstanding a reduction in CO2 storage capacity. The presence of acid gases such as SOx (and NOx) will likely have a negative effect particularly in the near-well region due to their tendency to accumulate in acidic forms in the water (and mineral phases) around the injection well. Other impurities that may be present in the CO2 stream, such as O2 and H2S, may or may not have a negative effect, depending on aquifer's mineral composition. The severity of these effects should be evaluated through appropriate modelling of the impure-CO2/water/rock system. In extreme cases this may impose more severe limitations than those imposed by transportation systems on the type and amount of certain impurities present in the CO2 stream. In general, further development of the work performed to date will continue to yield valuable results dealing with impure-CO2/ aquifer interactions. Future research will benefit from closer integration between the various research communities to better define the myriad chemical environments which may develop due to the injection of impure CO2 streams. Exploration of the post-injection interactions in the vicinity of injection wells is one avenue which should be explored more vigorously.