GEOCHEMICAL CHARACTERIZATION OF THREE POTENTIAL RECEIVING FORMATIONS FOR GEOLOGIC CARBON DIOXIDE SEQUESTRATION IN SOUTHWESTERN WYOMING, USA

This study documents baseline geochemistry of three deep saline aquifers that have been identified as potential receiving formations for CO₂ sequestration projects proposed in the Green River Basin of southwestern Wyoming. The EPA proposed rule for Class VI wells used for sequestration requires that owners and operators submit baseline geochemical data on receiving formation fluids in order to 1) prevent degradation of underground sources of drinking water by injecting only into formations with greater than 10,000 mg/L TDS, 2) identify potential chemical reactions between CO₂ and formation fluids, and 3) monitor subsequent changes in geochemistry during injection as an indication of the extent of the CO₂ plume. Geochemical data in the public domain (USGS, WOGCC and WRDS) has been compiled for three saline formations; the Mississippian Madison Limestone, Pennsylvanian Tensleep/Weber Sandstone and Jurassic Nugget Sandstone. Geochemical spatial variation maps have been prepared for all three formations, showing the chemical variation in the Moxa Arch anticline and Rock Springs uplift. The saline aquifers lie at depths of greater than 12,500 feet, at which CO₂ will be supercritical and contain brine with >10,000 mg/L TDS. The storage units appear to be overlain by a series of sealing lithologies that ensure that CO₂ will be retained. These geologic sites are adjacent to several significant sources of anthropogenic CO₂, including PacifiCorp’s Jim Bridger power plant and ExxonMobil’s Shute Creek natural gas processing facility. Preliminary characterization of these carbonate and sandstone formations at the Rock Springs Uplift and Moxa Arch indicates that they have appropriate thickness, reservoir properties, overlying low-permeability lithofacies, and structural integrity to be good candidates for CO₂ storage. Based upon limited data, the waters of the Madison limestone are predominantly Na-Cl type with few Ca-Cl-SO4 and Na-SO4 types. The waters of the Tensleep-Weber sandstone are dominantly Na-Cl and Na-SO₄ type. The Madison and Tensleep both have greater than 10,000 mg/L TDS in the basin. The TDS of Nugget Sandstone waters are highly variable, ranging from 100 to >100,000 mg/L, and are of the Na-Cl or Ca-Cl type. On the basis of this data it appears that the Paleozoic formations best meet Class VI well requirements in the study site.