Coupling tough2 and SEAWAT variable-density models to understand potential geologic carbon sequestration impacts on groundwater resources

The Mt. Simon Formation, the basal sandstone reservoir in the Illinois Basin, is the storage reservoir for a geologic carbon sequestration (GCS) project in central Illinois (USA). The Mt. Simon is a highly saline reservoir at the injection location (Macon County), but its salinity decreases to the north, becoming a source of drinking water in Wisconsin. In the current study, an improved understanding of the possible e ffects of potential future commercial-scale GCS projects in the south on underground drinking water sources in the Illinois Basin is sought by coupling two codes. TOUGH2 is suitable for simulating multiphase and variable density flow problems such as GCS, while SEAWAT is suitable for variable density, groundwater flow problems. Rather than using a single TOUGH2 model, coupling SEAWAT and TOUGH2 lowers the computational cost of modeling this system, allowing the impacts of the GCS project (increasing formation pressure) and groundwater pumping (reducing formation pressure) to be analyzed more e fficiently. The use of the SEAWAT model also allows us to incorporate current and projected freshwater pumping data developed for a calibrated MODFLOW model. The migration of native brine and its impact on freshwater drinking sources is investigated by passing pressure and salt concentration data between the models at specific time steps. Results show the method to be successful with pressure impacts of the simulated GCS activity reaching the Chicago, IL region, but rather limited brine migration. Results indicate that no impacts on the Mt. Simon aquifer at the Illinois-Wisconsin border should be expected at the injection rates examined in this work