A Novel Approach to Experimental Studies of Mineral Dissolution Kinetics

Currently, DOE is conducting pilot CO{sub 2} injection tests to evaluate the concept of geological sequestration. One strategy that potentially enhances CO{sub 2} solubility and reduces the risk of CO{sub 2} leak back to the surface is dissolution of indigenous minerals in the geological formation and precipitation of secondary carbonate phases, which increases the brine pH and immobilizes CO{sub 2}. Clearly, the rates at which these dissolution and precipitation reactions occur directly determine the efficiency of this strategy. However, one of the fundamental problems in modern geochemistry is the persistent two to five orders of magnitude discrepancy between laboratory-measured and field derived feldspar dissolution rates. To date, there is no real guidance as to how to predict silicate reaction rates for use in quantitative models. Current models for assessment of geological carbon sequestration have generally opted to use laboratory rates, in spite of the dearth of such data for compositionally complex systems, and the persistent disconnect between lab and field applications. Therefore, a firm scientific basis for predicting silicate reaction kinetics in CO{sub 2} injected geological formations is urgently needed to assure the reliability of the geochemical models used for the assessments of carbon sequestration strategies. The funded experimental and theoretical study attempts to resolve this outstanding scientific issue by novel experimental design and theoretical interpretation to measure silicate dissolution rates and iron carbonate precipitation rates at conditions pertinent to geological carbon sequestration. In the first year of the project, we have successfully developed a sample preparation method and completed three batch feldspar dissolution experiments at 200 C and 300 bars. The changes of solution chemistry as dissolution experiments progressed were monitored with on-line sampling of the aqueous phase at the constant temperature and pressure. These data allow calculating overall apparent feldspar dissolution rates and secondary mineral precipitation rates as a function of saturation states. State-of-the-art atomic resolution transmission electron microscopy (TEM), scanning electron microscopy, and electron microprobe was used to characterize the reactants (feldspars before experiments). We experimented with different sample preparation methods for TEM study, and found excellent images and chemical resolution with reactants, which shows promise of the technology and establishes the baseline for comparison with products (feldspars after the experiments). Preliminary electron microscopic characterization shows that the reacted feldspars have etch pits and are covered with secondary sheet silicate phases. Reaction-path geochemical modeling is used to interpret the experimental results. We have established the software and database, and are making great progress. Also during the first year, our education goal of graduate student training has been achieved. A Ph. D. student at Indiana University is progressing well in the degree program and has taken geochemical modeling, SEM, and TEM courses, which will facilitate research in the second and third year. A Ph. D. student at University of Minnesota is progressing well in conducting the experiments, and is near graduation. With the success of training of graduate students and excellent experimental data in the first year, we anticipate a more fruitful year in the second year.