Molecular Dynamics Simulations of the CO2-Water-silica Interfacial Systems

AbstractCO2 injection and migration in porous media are dependent of interfacial tension (IFT), wettability, capillarity and mass transfer. The interfacial property at high pressure and high temperature (HPHT) condition and various water salinities must be investigated for application of CO2 sequestration in deep saline aquifer. Molecular Dynamics (MD) simulation can deal with multi-components system, such as brine (water)/CO2 interface, and easily control pressure, temperature and ratio of ions. In addition, most of experiments have employed the pendant droplet method to determine the interfacial tension based on a general Laplace-Young equation. This method requires a very accurate density measurement in situ, which is, however, not so easy to be instrumented. Consequently, the reported data are controversial and some of them are problematic. MD calculations can avoid this type of problem encountered by experimental measurement, as it uses a different way to determine the IFT. In our study, IFT for brine/CO2 system and wettability for water/CO2/Silica were calculated using MD. We have discussed how pressure, temperature and water salinity have an influence on IFT and wettability. At first, IFT sharply decreases with pressure below the critical pressure (i.e. in the presence of gaseous CO2), while IFT stays constant with pressure at high pressure when CO2 density is constant (i.e. in the presence of liquid or supercritical CO2). Secondly, IFT decreases with temperature at high pressure, while IFT is less affected by temperature at low pressure however a maximum is discernible. At last, water salinity builds IFT up with no strong relation to pressure. Contact angle calculations showed that wettability for water/CO2/hydrophilic Silica is independent of pressure change and water contact angle is about 20°