The behavior of dissolved organic carbon (DOC) at geological sequestration sites



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Geologic carbon sequestration has been proposed as a means of mitigating anthropogenic greenhouse gas emissions. At depth, supercritical CO₂ may rise above the surrounding fluid. Detecting leaks from CO₂ storage reservoirs is important to evaluate the effectiveness of carbon sequestration and address public concern for negative environmental impacts. Other attempts have been made to detect leaks, such as changes in pH, pressure and direct observation of CO₂ in the AZMI (Above Zone Monitoring Interval). Each has limitations and here we investigate dissolved organic carbon (DOC) as a potential indicator for fugitive CO₂. This study uses a series of batch experiments to evaluate the interaction between dissolved CO₂ and DOC. The batches consist of homogenized and sieved 250 micron to 425 micron matrix samples of varying mass and type, 2mL of DI water and a headspace of pure carbon dioxide or air. The three different rock samples analyzed are Buffalo River Sediment, illite and Barnett Shale. A pure CO₂ headspace results in lower amount of DOC in solution than an air headspace. All matrix samples demonstrated this effect. The proposed mechanism to describe the observed results is that a lowered pH shifts speciation of weak organic acids and protonated humic substances causing decreased solubility and increasing the adsorption of these compounds. These results suggest that a decrease in DOC concentrations could be used to detect CO₂ leakage and that CO₂ leakage would not deteriorate water quality by releasing DOC.