Shale-brine-CO2 interactions and the long-term stability of carbonate-rich shale caprock

Abstract The success of geological carbon storage (GCS) depends on the sealing properties of caprocks, typically mudrocks, and their laminated variety – shales. In this study, we examined mineralogical changes in carbonate-rich Mancos Shale and corresponding changes in micro-mechanical properties following the reaction with carbon dioxide (CO 2 ). Mineralogical changes of Mancos Shale depended on the pressure of CO 2 during its exposure to the CO 2 -brine mixtures for up to 8 weeks. Dedolomitization was observed in the reactors pressurized with 100 psi of CO 2 , combined with the precipitation of gypsum. In the reactor pressurized with 2500 psi of CO 2 , the complete dissolution of calcite, partial dissolution of dolomite, and precipitation of magnesite and anhydrite were observed. Localized mechanical weakening was observed only for dolomite-muscovite-illite-rich laminae following whole shale puck alteration at 2500 psi of CO 2 , and a decrease of up to 50 ± 20% in scratch toughness was observed. The quartz-calcite-rich laminae did not exhibit a measurable difference in scratch toughness before and after reaction in CO 2 -rich brine. The predicted changes in mineralogy, porosity, density, and hardness of Mancos Shale are limited, according to the geochemical models describing alteration of shale by CO 2 -rich brine lasting for 5000 years. This study illustrates a coupled and localized chemical-mechanical response of caprock to the injection of CO 2 .