BASIN-SCALE ENVIRONMENTAL IMPACT OF GEOLOGICAL CARBON SEQUESTRATION IN THE ILLINOIS BASIN

The Illinois Basin was used to model the environmental impact of future multiple injection sites in the thick, extensive Mount Simon Formation. The basin-scale model domain of 241,000 km2 covered a core injection area, a near-field area, and a far-field area. The model adequately captures the characteristics of the Mount Simon Formation in the core injection area, which include (1) an overall thickness of 300 to 680 m, (2) an upper unit of sandstone and shale tidally influenced and deposited, (3) a thick middle unit of relatively clean sandstone of relatively high permeability, and (4) a lower arkosic unit of higher permeability with an average thickness of 90 m. The model assumed that there were twenty sequestration sites (spaced 30 km apart) within the core injection area. At each site, five million tonnes of CO2 was injected annually for 50 years into the lower arkosic unit. A three-dimensional unstructured mesh of 1.25 million gridblocks was used. Both the two-phase CO2-brine flow within the twenty CO2 plumes and the single-phase brine flow away from the plumes were simulated using the parallel TOUGH2/ECO2N simulator. Simulation results indicate that (1) the maximum pressure buildup in the core injection area is 45.7 bar at the end of the 50 years of injection, less than 28% of the hydrostatic pre-injection pressure and far less than the maximum pressure gradients regulated for waste injection; this indicate the geomechanical integrity of the caprock can be ensured; (2) various conditions are favorable for safe storage of large volume of CO2 in the Mt Simon Sandstone. The simulation results also indicate that (1) significant pressure buildup (as high as 2 bar) in the northern Illinois (where groundwater resources are important) is observed, and such pressure buildup may have impacts on groundwater supply; (2) salinity change along the updip Mt Simon is negligible; (3) brine leakage through the Eau Claire Formation, simulated using a regional-scale permeability representative of all localized leaky faults and multi-aquifer or abandoned wells, during and after the CO2 injection may have negative impact on the shallower Ironton-Galesville freshwater aquifers, and this negative impact on water quality will be addressed in the future.