Impact of CO2 Impurities on Storage Performance and Assurance -Report on Task 5 (Integration of Flow and Geochemistry) and Final Report Prepared for: CO2 Capture Project (Phase III)

Abstract

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 work) can circumvent the capture step thanks to their CO2-rich composition (CO2 > 90%), but at the expense of CO2 stream purity. In addition to CO2, two non-reactive gases make up the bulk of impurities: N2 and Ar. O2 is the most commonly cited reactive gas, with molar concentrations ranging from <1% to >5%. Other minor species may also be present.

Relatively high purity levels must be achieved to avoid compression and complications in pipeline transportation (two-phase flow) and, potentially, subsurface impacts. This work investigates the latter, which, in turn, informs 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 due to density and viscosity differences and (2) undermine reservoir and top seal integrity due to reactions with reactive species (O2, CO, SOx).

To address the first issue, we conducted a desktop study using a numerical modeling tool and performed laboratory experiments to determine the actual viscosity and density of the mixtures. Information on the 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 study results was that the viscosity and density of the mixtures considered 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 a plume of pure CO2, possibly enhancing dissolution primarily because it is exposed to more rock/brine volume. However, a larger plume means that a larger area must be defined and monitored for leakage pathways, such as faults and wells, but the faster trapping translates into 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 proportionally reduced with depth.

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