Molecular dynamics computations of brine-CO2 interfacial tensions and brine-CO2-quartz contact angles and their effects on structural and residual trapping mechanisms in carbon geo-sequestration

In the context of carbon geo-sequestration projects, brine-CO2 interfacial tension gamma and brine-CO2-rock surface water contact angles 0 directly impact structural and residual trapping capacities. While gamma is fairly well understood there is still large uncertainty associated with 0. We present here an investigation of gamma and 0 using a molecular approach based on molecular dynamics computer simulations. We consider a system consisting of CO2/water/NaCl and an alpha-quartz surface, covering a brine salinity range between 0 and 4 molal. The simulation models accurately reproduce the dependence of gamma on pressure below the CO2 saturation pressure at 300 K, and over predict gamma by similar to 20% at higher pressures. In addition, in agreement with experimental observations, the simulations predict that gamma increases slightly with temperature or salinity. We also demonstrate that for non-hydroxylated quartz surfaces, 0 strongly increases with pressure at subcritical and supercritical conditions. An increase in temperature significantly reduces the contact angle, especially at low-intermediate pressures (1-10 MPa), this effect is mitigated at higher pressures, 20 MPa. We also found that theta only weakly depends on salinity for the systems investigated in this work. 2012 Elsevier Inc. All rights reserved.