Carbon Mitigation Initiative
CMI

Eighth Year Report 2009
Carbon Storage: Geochemical Reactions in Saline Aquifers

Upscaling Reaction Rates

Catherine Peters is leading a project funded by the DOE Office of Science, titled “Up-Scaling Geochemical Reaction Rates for Carbon Dioxide (CO2) in Deep Saline Aquifers,” for which the Peters Group will collaborate with the Celia Group and W. Brent Lindquist (SUNY Stony Brook). The goal is to bridge the gap between our knowledge of small-scale geochemical reaction rates and rates meaningful for modeling reactive transport at core scales. The focus is on acid-driven mineral dissolution and precipitation relevant in the context of geological sequestration of CO2.

The major challenge with predicting reactive transport in consolidated media is accurate characterization of mineral surface areas. Existing methods of quantifying mineral surface areas for consolidated media at best are imprecise and at worst are not valid because they do not account for armoring clay minerals, matrix cementation, and grain inclusions. Through a combination of Backscattered Electron (BSE) scanning electron microscopy (SEM) and Energy Dispersive X-ray (EDX) analysis of thin sections, the group has developed a novel image analysis method to quantify the accessibility of minerals to formation fluids in sedimentary sandstones (Figure 14). For example, for one of the sandstones studied, the researchers concluded that if a mineral volume fraction is used as a proportional measure of accessible surface area in consolidated sandstones, the reaction rates are likely to be overestimated by three to five times.

Figure 14

Figure 14. Elements of a BSE-EDX mineral map. Reactivity in sandstones is notoriously difficult to characterize due to the high degree of consolidation and cementation. This novel image processing algorithm allows us to quantify the accessibility of minerals to carbonic acids, thereby determining mineral reactivity under acidic conditions.

This novel method is further being applied to provide insight on how uncertain the estimates of surface areas often used in reactive transport and reaction path models are in the context of deep sedimentary formations.

Impacts of SO2 Co-Injection

Peters group is now focussing on a new research question related to the effects of co-injection of SO2 in the context of CO2 geologic sequestration. To quantify the extent to which SO2 will dissolve into and acidify formation brines, the researchers have determined the solubility of SO2 in brine at high pressures, simulated the diffusive transport of SO2 from a supercritical CO2 phase, and modeled the formation of various sulfur-containing acids. Hydrolysis alone will produce sulfurous acid, which is somewhat stronger than carbonic acid. While fully oxidizing conditions are needed to convert all the SO2 to sulfuric acid, a very strong acid, simulations show that, even in the absence of strong oxidant the SO2, disproportionation reactions can generate both sulfuric acid and hydrogen sulfide. The sulfuric acid will lead to significant brine acidification, pH<2, if phase equilibrium between the injection plume and formation brine is reached. These findings indicate that SO2 contact with the brine is likely to be limited by diffusion through the supercritical CO2 plume, thereby causing a delayed and less severe impact on brine pH.

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