GEOCHEMICAL CAPROCK INTEGRITY AND METAL MOBILITY FROM THE OPECHE FORMATION: A RED BED CAPROCK FOR CO2 STORAGE

The Wyoming CarbonSAFE project, sponsored by the Department of Energy, seeks to determine the feasibility of storing over 50 million metric tons of CO₂ from industrial sources over the course of 30 years in the Powder River Basin. The Permian Opeche Formation is the primary seal to the target CO₂ reservoir, the Pennsylvanian/Permian Minnelusa Formation. Since CO₂-water-rock driven mineral dissolution has the potential to alter the sealing ability of the caprock, this project aims to constrain potential metal release and evaluate geochemical caprock integrity of the Opeche Formation upon interaction with CO₂-saturated Minnelusa Formation water. The Opeche Formation is predominantly composed of red beds containing argillaceous and evaporite minerals cemented by hematite; the base of the formation is a gray chert and dolomite conglomerate that unconformably overlies the Minnelusa Formation. Preliminary XRD data indicates that red beds located 15 ft. (4.6 m) above the basal unconformity contain ~36% quartz, 35% dolomite, 13% feldspar, and 16% clay with abundant hematite cement and opaque minerals. The gray conglomerate located 4 ft. (1.2 m) above the unconformity contains ~88% quartz, 1% dolomite, 9% feldspar, and 2% clay with lesser amounts of hematite cement and opaque minerals. Trace metals tend to be more abundant in the red beds compared to the gray conglomerate including Pb (18 vs 6 ppm), U (17 vs 5 ppm), V (104 vs 81 ppm), and Zn (70 vs 17 ppm).

Four hydrothermal experiments will be performed at reservoir conditions (85°C and 26 MPa) for ~60 days; these experiments will target the red beds and gray conglomerate of the Opeche Formation. Two control experiments will react Opeche Formation with Minnelusa Formation water (pH 6, I = 0.9 M); two injection experiments will react Opeche Formation with CO₂-saturated Minnelusa Formation water. This experimental design and the results will constrain potential metal mobility as well as assess and reveal potential differences in geochemical caprock integrity within the Opeche Formation.