Dedolomitization, CO2 and Freshwater Aquifers
Role of Dedolomitization in the Detection of Anthropogenic CO2
in Freshwater Aquifers
GCCC Digital Publication Series #10-02
Katherine D. Romanak
Rebecca C. Smyth
Susan D. Hovorka
CO2-EOR (Enhanced oil recovery); Field study-SACROC-Scurry Co-TX; Modeling-Geochemical; Monitoring; Monitoring-groundwater-USDW; Rock-CO2-water interaction; Rock-water-CO2 reaction
Romanak, K. D., Smyth, R. C., Yang, C., Hovorka, S. D., and Lu, J., Role of dedolomitization in the detection of anthropogenic CO2 in freshwater aquifers: paper presented at the Water Rock Interaction 13 Conference, 16-20 August, 2010, Guanajuato, México. GCCC Digital Publication Series #10-02, pp. 1-4.
Our examination of groundwater geochemistry at a site where CO2 has been injected for decades shows that understanding water-rock interaction is critical to correct interpretations regarding CO2 storage evaluation. Whereas it is currently widely accepted that input of CO2 into an aquifer system will result in calcite dissolution leading to increases in HCO3 - and decreases in pH, we have found that these generalizations may not hold true in all hydrogeologic settings.
For example, our geochemical models show that the process of dedolomitization will mimic the effects of calcite dissolution by increasing Ca2+ and decreasing pH, but this reaction is in response to input of calcium ions, not CO2. The dedolomitization model also indicates that dissolved HCO3 - is consumed in the initial phases of water-rock interaction; a trend very different from the progression that results from reaction of CO2 with calcite. Under simulated circumstances of exogenous CO2 input, water-rock interactions continue to be driven by Ca2+ rather than CO2.
If these geochemical trends are observed in the absence of an understanding of the hydrogeologic system, erroneous interpretations regarding CO2 storage may result. Errors could be either at sites where CO2 leakage has occurred but is not detected or at sites where non-injection-induced changes are mistakenly assumed to indicate leakage. The implication for monitoring geologic sequestration sites is that characterization of the fundamental geochemical processes is necessary for correctly assessing the consequences of CO2 input.
Monitoring groundwater resources above geologic carbon sequestration reservoirs is proposed as a method for ensuring that potable water supplies have been protected and that CO2 is adequately sequestered with respect to the biosphere. Currently, the position of many researchers is that simple carbonate parameters can be used for monitoring. This hypothesis is based on the premise that a predictable consequence of CO2 in a dilute aquifer is an increase in dissolved inorganic carbon (DIC) predominantly as bicarbonate ion (HCO3-), a decrease in pH, and mobilization of ions such as calcium (Ca2+) and/or trace metals (Assayag et. al., 2009; Apps, et. al., 2009; Wang & Jaffe, 2004; Carroll et. al., 2009).
The results of a groundwater study at a site where CO2 has been injected for decades (the SACROC oil field) give strong evidence against these conclusions. At SACROC, natural and man-induced processes have created a geochemical system that is more complex and reacts differently to CO2 than the simple system put forth by previous researchers. The implications are that groundwater monitoring strategies at geologic sequestration sites may need to be site-specific to hydrogeologic setting and those that use natural constituents will have limited use given the variability of these environments.