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Molecular simulation of equal density temperature in CCS under geological sequestration conditions

Storing carbon dioxide (CO2) in geological reservoirs, principally saline aquifers, is one of the main methods proposed to address the issue of anthropogenic CO2 emissions. With the aim of ensuring long‐term CO2 storage in geological reservoirs, the density of the CO2–brine solution is an important property that controls brine buoyancy. In this study, molecular dynamics simulations were performed to investigate the density of CO2–water solutions at temperatures in the range 60–240°C, pressures in the range 10–100 MPa, varying CO2 mass fractions (1%, 2%, and 3%), and varying CO2–NaCl–water solution salinities (2 and 4 mol/kg), all of which are crucial parameters for CO2 geological sequestration. The optimal combination model of CO2(TraPPE)–NaCl(OPLS)–water(TIP4P/2005) was determined by comparing experimental values with simulate density under geological conditions. The simulation results reproduced the trends of the experimental values. The equal density temperature (Te) indicated that solution density decreases upon CO2 dissolution in water when the temperature increases to a certain point. According to the simulation results, Te increases with increasing pressure and decreases with increasing salinity. Further, the results of analysis of the structural properties of the solution to determine the causes of Te showed that, with increasing temperature, the distance between CO2 molecules and adjacent atoms increases significantly, which reduces the local density around the CO2 molecules. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.