TRACING THE EFFECTS OF FUGITIVE CO2 ON DISSOLVED ORGANIC CARBON AT CARBON SEQUESTRATION SITES

The possibility exists that natural imperfections in the cap-rock (fracture zones or faults) will be encountered by injected CO₂ which may result in the leakage ofCO₂. At depth, supercritical CO2 may rise above the surrounding fluid due to buoyancy caused by CO₂ and the surround fluid being immiscible. Due to the reactive nature of CO₂ and the complications of measuring an atmospheric gas at surface, other indicators of unintended CO₂ migration into shallow reservoirs, such as changes in dissolved organic carbon (DOC) may provide insights to fluid migration. CO₂ will interact and dissolve into groundwater causing an increase in pCO₂ and a decrease in pH. A change in groundwater chemistry will change the amount of dissolved organic carbon in solution. Consequently any changes to pH from injected CO₂ should be seen in DOC concentrations. Changes in groundwater DOC concentration is a concern for groundwater quality.

This study uses a series of batch experiments to evaluate the interaction between dissolved CO₂ and DOC. The batches consist of homogenized and milled rock samples of varying mass, 2mL of DI water and a headspace of pure carbon dioxide or atmosphere. Diffusion of the headspace gas into solution causes samples with pure CO₂ in the headspace have a pH of 4.3 while the atmospheric samples have a pH of 7.5. Two different rock samples were analyzed, Buffalo River Sediment which has a TOC of 3.4% and Illite (Green Shale) which has a TOC of 0.3. The amount of carbon that dissolves can be expressed as a function of Y mg of C/L= .004*(X mg) + 2.03; where X and Y are defined as the amount of sediment added to the batch experiment and the amount of dissolved organic carbon in solution. The results indicate a pure CO₂ headspace causes less DOC to desorb than an atmosphere headspace. Both rocks demonstrate this effect of CO₂ with a greater difference between the samples for the Buffalo River Sediment.

Changes in DOC are potentially a groundwater quality issue. These results indicate that CO₂ decreases the amount of dissolved organic carbon in solution. DOC forms strong complexes with metals, which enhances metal solubility. Therefore understanding the relationship between CO₂ and DOC may have implications for metal solubility and transport.