COUPLING SEAWAT AND TOUGH2 TO ANALYZE POTENTIAL EFFECTS OF GEOLOGIC CARBON SEQUESTRATION ON UNDERGROUND DRINKING WATER SOURCES

Potential commercial-scale Geologic Carbon Sequestration (GCS) efforts utilizing the basal formation of the Illinois Basin, the Mt. Simon Sandstone, are modeled to analyze possible effects on underground sources of drinking water (USDWs) within the basin. While current GCS efforts in the Illinois Basin are located in central Illinois, USA, a very saline portion of the Mt. Simon Sandstone, future commercial-scale GCS operations could potentially cause pressure increases and brine migration in fresher portions of the Mt. Simon, tens of kilometers away. Numerical models have been developed to predict the fluid flow and brine migration as a result of potential commercial-scale projects. A regional TOUGH2-PC model, simulating multi-phase, variable density fluid flow in the Illinois Basin deep system has been developed to simulate carbon dioxide (CO₂) injection. A SEAWAT model, which can handle variable density fluid flow in porous media, has been adapted from a calibrated regional MODFLOW model of Northeastern Illinois (obtained from the Illinois State Water Survey) to model the northern portion of the Illinois Basin deep system where the CO₂ plume is not expected to reach. The spatial domain of these two models overlap and simulated fluid pressure and salt concentration values are passed between the two models. This method allows for the basin scale processes to be modeled while reducing the computational costs associated with solving multi-phase problems. This work may provide decision makers with a basis for monitoring strategies of USDWs, and can guide in focusing future data collection efforts to reduce model uncertainties.