Potential Subsurface Impacts of CO2 Stream Impurities on Geologic Carbon Storage

A key impediment to carbon capture and storage is the cost of CO2 capture, particularly for conventional power plants whose flue gas is dominated by gases other than CO2. Waste-gas streams from power plants that use novel technologies (such as oxyfuel, the focus of this paper) can circumvent the capture step thanks to their CO2-rich composition (CO2>90%), but at the expense of stream CO2 purity (N2, O2, Ar, and other minor species may be present). Relatively high purity levels must be achieved to avoid compression and complications in pipeline transportation (two-phase flow) and, potentially, subsurface impacts. The CO2 Capture Project Phase 3 (CCP3) has started investigating the latter, which, in turn, inform techno-economic assessments of capture and transportation economics. Subsurface impacts of an impure CO2 stream could be twofold: (1) complicate flow behavior and reduce static capacity because of density and viscosity differences and (2) undermine reservoir and top seal integrity due to reaction with reactive species (O2, CO, SOx). Using a range of potential oxyfuel waste-gas compositions, we approached the first issue through a desktop study using the numerical modeling tool. So that we could work with accurate flow parameters, we performed laboratory experiments in order to determine the actual viscosity and density of the mixtures. Information on solubility of these various mixture components in the aqueous phase under various pressure, temperature, and salinity conditions was also collected. An important observation controlling all results of the study was that viscosity and density of mixtures are lower than those of pure CO2 at the same temperatures and pressures. It follows that a plume of CO2 with impurities, moving updip with no barrier, will migrate farther from the point of injection but will be trapped through residual saturation sooner than will a plume of pure CO2. A larger plume means that a larger area must be inspected for leakage pathways, such as faults and wells, but faster trapping means a shorter monitoring period. Equally important is that contrasts of viscosity and density between pure CO2 and a CO2 mixture decrease with depth, suggesting that differences in flow behavior and storage capacity are similarly reduced with depth. Whereas flow behavior may impact the whole field, geochemical impacts are more likely to be restricted to the well-bore environment and the near field. Batch experiments conducted in high-pressure, high- temperature autoclaves with rocks immersed in synthetic brine and exposed to supercritical CO2 with and without admixed O2 suggest that O2 may change the geochemistry of subsurface systems in ways that the pure CO2 case does not. Results of the study, therefore, present the CO2 project developer with tradeoffs in capacity, pressure evolution, and monitoring scenarios, with additional costs likely more than offset by reduced capture costs.