Variations in coal characteristics and their possible implications for CO2 sequestration: Tanquary injection site, southeastern Illinois, USA

As part of the U.S. Department of Energy's Regional Sequestration Partnership program, the potential for sequestering CO2 in the largest bituminous coal reserve in United States - the Illinois Basin - is being assessed at the Tanquary site in Wabash County, southeastern Illinois. To accomplish the main project objectives, which are to determine CO2 injection rates and storage capacity, we developed a detailed coal characterization program. The targeted Springfield Coal occurs at 274 m (900 ft) depth, is 2.1 m (7 ft) thick, and is of high volatile B bituminous rank, having an average vitrinite reflectance (Ro) of 0.63%. Desorbed Springfield Coal gas content in cores from four wells ~ 15 to ~ 30 m (50 to 100 ft) apart varies from 4.7-6.6 cm3/g (150 to 210 scf/ton, dmmf) and consists, generally, of > 92% CH4 with lesser amounts of N2 and then CO2. Adsorption isotherms indicate that at least three molecules of CO2 can be stored for each displaced CH4 molecule. Whole seam petrographic composition, which affects sequestration potential, averages 76.5% vitrinite, 4.2% liptinite, 11.6% inertinite, and 7.7% mineral matter. Sulfur content averages 1.59%. Well-developed coal cleats with 1 to 2 cm spacing contain partial calcite and/or kaolinite fillings that may decrease coal permeability. The shallow geophysical induction log curves show much higher resistivity in the lower part of the Springfield Coal than the medium or deep curves because of invasion by freshwater drilling fluid, possibly indicating higher permeability. Gamma-ray and bulk density vary, reflecting differences in maceral, ash, and pyrite content. Because coal properties vary across the basin, it is critical to characterize injection site coals to best predict the potential for CO2 injection and storage capacity.