AN EXPERIMENTAL AND GEOCHEMICAL MODELING STUDY OF CO2-BRINE-SHALE INTERACTION

The late Paleozoic Black Warrior Basin (BWB) has the potential for long-term subsurface CO₂ storage in the Gulf Coast region. In virtue of high adsorption capacity and trace element contents of shale, knowledge of potential CO₂-brine-shale interactions is critical for the success of carbon geo-sequestration. In this research, potential mineral precipitation and dissolution reactions and trace element mobilization from the shale caprock via pH-controlled desorption process will be explored thoroughly by laboratory experiments and geochemical modeling. Laboratory experiments will be conducted under 20 MPa and 80°C. Mineralogy and element composition of powder and bulk shale samples will be analyzed before and after the laboratory experiment by using XRD, XRF, ICP-MS, and electron microprobe. Changes in the geochemical composition of brine will be examined using ICP-MS. Geochemical modeling techniques will be used to investigate mineral precipitation-dissolution mechanisms, speciation, and adsorption/desorption of trace elements under varying pH conditions. The Conasauga, Neal/Floyd, and Chattanooga Shales from the Black Warrior Basin will be used in the experiments. These shales have different mineralogical and organic matter contents. Carbonate minerals are abundant in the Conasauga Shale while the Neal/Floyd Shale contains more organic carbon and sulfide minerals. As carbonate minerals serve as major buffers for pH, CO₂–brine–shale interaction is expected to vary in different samples. Of particular interest is how CO₂ partitioning into formation fluids may reduce brine pH and affect carbonate mineral dissolution. The changing pH conditions would also affect the adsorption and desorption of trace elements on oxides, sulfides, and clay minerals.