Geochemical and Mineralogical Evaluation of CO2-Brine-Rock Experiments: Characterizing Porosity and Permeability Variations in the Cambrian Mount Simon Sandstone

The Cambrian Mount Simon Sandstone has been targeted as a major reservoir for carbon dioxide storage in the Illinois Basin. The Mount Simon Sandstone's geologic setting, mature quartz to arkosic composition, reservoir thickness, and overlying Eau Claire Formation seal make it an attractive candidate for long-term storage potential of carbon dioxide. Injection of carbon dioxide has been shown to cause a range of chemical alterations that causes dissolution of existing minerals and precipitation of secondary phases that can alter the porosity and permeability of the reservoir. This study focuses on using detailed microscopic analysis of two compositionally and texturally different Mount Simon Sandstone samples from the Illinois Basin that were experimentally exposed to CO2-rich brines for 6 months at the NETL in collaboration with the Indiana Geological Survey. Our objective was to examine the experimental samples to determine how post-experiment mineralogical and geochemical alterations relate to porosity and permeability variations. Gazzi-Dickinson point counting of Vermillion County samples adjacent to experimental sample depths (5805 ft) show that the sample contains an average of 78% quartz, 15% feldspar, 2% lithics, and 5% porosity. Point count data of Knox County samples from 8642.5, 8542, and 8642.2 show the experimental sample has an average of 70% quartz, 22% feldspar, 4% lithics, and 3.9% porosity. Both samples were submerged in carbon dioxide-saturated brine synthesized to match the measured geochemistry of Mount Simon Sandstone pore water for six months at 24MPa and 90 degrees to replicate sequestration conditions. The results of the experiment for the Vermillion County sample revealed a significant decrease in permeability and porosity. However the Knox County sample had a minor increase in permeability and porosity. Geochemical analyses (IC, ICP-MS, and ICP-OES) of brine geochemistry before and after the experiment show a decrease in pH and an increase in major ion and trace element concentration suggesting that dissolution of soluble phases occurred during the experiment. 174 precisely placed SEM/EDX measurements of overall elemental composition and 144 pores from the interior of the samples (presumably not in contact with the CO2 and brine) and the exterior edges of the samples (in direct contact with the CO2 and brine) show variations in cation concentration and pore-filling textures. In general, the sample interiors have more open pores and the edges have higher cation concentrations and pores filled with secondary minerals. Current results indicate carbon dioxide injection into samples from the same formation but dissimilar textural and mineral compositions can have variable affects on overall permeability and porosity.