Interaction mechanism of supercritical CO2 with shales and a new quantitative storage capacity evaluation method

Geochemical interactions between shale and supercritical carbon dioxide (scCO2) may dominate CO2 storage in shale gas reservoirs by changing the brine chemistry and rock properties. Understanding the thermodynamic mechanism of scCO2 with shale is particularly essential for CO2 storage, as it provides an innovative strategy to evaluate the potential of shale gas reservoirs. In this study, ReaxFF molecular dynamics (MD) simulations in combination with scCO2-brine-shale experiments were conducted to gain insight into the storage mechanism. A series of laboratory tests and analyses were used to determine variations in physical and chemical properties. The DAM model was employed to calculate the dissolution, adsorption and mineralization capacities of shale samples. The MD trajectory shows that scCO2/H2O molecules rapidly diffuse to mineral surfaces within 1 ps to initiate adsorption and protonation processes. The interlayer cations that are released more slowly from illite (1.1 ns) and Ca-montmorillonite (1.5 ns) structures consume CO2 via mineralization, resulting in carbonate formation during the reaction. However, due to the release of CO32-during calcite decomposition, Ca2+ leaching at 1.6 ns cannot reduce the amount of CO2 molecules. As evidenced by reaction kinetics from MD and experiments, dissolution, adsorption and mineralization prosesses dominate the CO2 storage in shale gas reservoirs, accounting for 10.0-47.6%, 11.0-28.2% and 21.1-73.5%, respectively. The adsorption capacity is associated with space volume, and the mineralized amount is controlled by the illite and mixed-layer illite/smectite contents. Other factors, such as fluid property and space compressibility may also impact the storage capacity, resulting in a maximum adsorption capacity with a depth of around 800 m and an enhanced mineralized capacity with increasing depth. These findings demonstrate that adsorption and mineralization at storage conditions play a key role in CO2 geo-sequestration.