HYDROLOGY OF THE FOREST CITY BASIN, MID-CONTINENT, USA: IMPLICATIONS FOR CO2 SEQUESTRATION IN THE ST. PETER SANDSTONE

The Paleozoic Forest City intracratonic basin lies near several major coal-burning power plants in the central U.S. states of Kansas, Nebraska, Iowa, and Missouri. Despite the basin’s relatively shallow maximum depth of ~1600 m, stratigraphic modeling carried out in the present study using published drill hole logs indicates that much of the Ordovician St. Peter Sandstone aquifer within the basin lies below the ~750 m depth required for supercritical CO₂ sequestration. Depending on uncertainties in porosity and the CO₂ storage efficiency factor, the St. Peter Sandstone is estimated to have a CO₂ storage capacity of 3 to 45 million tons within the Forest City basin, a mass that is sufficient to warrant further investigation into the basin’s CO₂ sequestration potential. A further step taken in this investigation so far has consisted of constructing a coupled variable-density groundwater flow and heat transport model for the basin using the MODFLOW, MT3DMS, and SEAWAT software packages in order to provide a baseline hydrologic characterization for more detailed future work. Several hydrologic features were consistently predicted throughout the model sensitivity analysis. The hydraulic head gradient over the western part of the basin was predicted to be from west to east, whereas over the northern part of the basin the head gradient was from north to south. The intersection of these two head gradients near the basin’s depocenter creates a stagnation zone that could help to isolate any injected CO₂ in the deeper parts of the basin. CO₂ sequestration in the St. Peter Sandstone would be aided further by the fact that little vertical flow was predicted to occur in the lower hydrostratigraphic units. Heat transport throughout the basin was found to occur primarily by steady-state conduction, consistently leading to temperatures in excess of 31° C in the St. Peter where it lies below a depth of 750 m, thus allowing CO₂ to exist in a supercritical state. Thus, based on its available pore volume, depth, and hydrology, the St. Peter Sandstone in the Forest City basin may be capable of long-term sequestration of modest but significant amounts of CO₂.