Understanding CO2 mineralization and associated storage space changes in illite using molecular dynamics simulation and experiments

Clay minerals have the potential to capture anthropogenic CO2 emissions permanently and safely. Understanding the kinetics of cation leaching and carbonate formation, as well as changes in clay structure, has resource and environmental implications. However, metadynamics mechanism and relevant structural changes in representative clay minerals exposed to supercritical carbon dioxide (scCO2) are rarely studied in current research. In this work, ReaxFF molecular dynamics simulation in combination with scCO2‒H2O‒illite experiments at 10 MPa and 333 K were carried out to investigate the mechanisms of mineralization and structure alteration under geological conditions. The results show that interlayer K+ cations were leached out due to surface non-bridging oxygen protonation, subsequently bonding with HCO3− and forming K2CO3 molecules at the surface. Upon analyzing the chemical and structural results of experiments, carbonate precipitation and accumulation reduce storage space and modify the composition of illite, but the octahedral and tetrahedral sheets of the illite are structurally stable. The efficiency of mineralization is typically dominated by the exposed surface, where sufficient cations can be provided to enhance interactions at the illite/liquid interface. In comparison to a decrease in plane porosity of 27.3%, the mineralization degree with values between 15.22% and 33.12% is comparable and acceptable. These findings present a non-structural mechanism in clay minerals that might have critical influence on CO2 geo-sequestration in shale gas reservoirs.