Geochemical modelling of CO2 interactions with shale: Kinetics of mineral dissolution and precipitation on geological time scales

The research on Carbon Capture and Storage (CCS) has become fruitful as energy-intensive industries are working towards transitioning to low carbon energy industry. Shale gas reservoirs have been recently considered as suitable geological targets for carbon dioxide (CO2) storage. However, due to the high reactivity of shales to CO2, the mineralogical changes after CO2/brine/shale interactions play a decisive role in defining the sealing properties of shales at geological time scales. Up to date, this issue is rarely investigated; therefore, in this study, a simplified 1-D reactive transport model was constructed based on the properties obtained from Eagle Ford and Mancos shales. PHREEQC software was utilized to simulate equilibrium and kinetic behavior and evaluate the alterations in minerals at 177 atm and 70 °C. The equilibrium model indicated that calcite and clay minerals dissolved in CO2-saturated brine, while quartz grains precipitated, due to the formation of carbonic acid. This behavior confirmed the high reactivity of shales to the injection of CO2-saturated brines. The kinetic model indicated that the geological time scale for CO2/brine/shale interaction can be divided into three phases. Primary minerals alterations occurred during the first 10 years, however, the main alteration in mineralogy occurred between 10 and 100 years, whereas the reactants continued to dissolve in low portions until the equilibrium state was reached beyond 100 years. The model showed that carbonate and clay minerals dissolved during the CO2/brine/shale interaction, which could provide the potential for mineral trapping as an effective sealing mechanism in the middle phases of the storage lifetime in shales, confirming the high potential of shales for CO2 containment. The main observations and conclusions obtained from this work can be easily extrapolated to other shale formations with similar mineral compositions.