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 γ and brine–CO2–rock surface water contact angles θ directly impact structural and residual trapping capacities. While γ is fairly well understood there is still large uncertainty associated with θ. We present here an investigation of γ and θ using a molecular approach based on molecular dynamics computer simulations. We consider a system consisting of CO2/water/NaCl and an α-quartz surface, covering a brine salinity range between 0 and 4 molal. The simulation models accurately reproduce the dependence of γ on pressure below the CO2 saturation pressure at 300 K, and over predict γ by ∼20% at higher pressures. In addition, in agreement with experimental observations, the simulations predict that γ increases slightly with temperature or salinity. We also demonstrate that for non-hydroxylated quartz surfaces, θ 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 θ only weakly depends on salinity for the systems investigated in this work.

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► Strong water contact angle increase with pressure increase.
► Strong water contact angle decrease with increasing temperature.
► Minor influence of salinity on the water contact angle.
► Brine–CO2 interfacial tension γ computed with high precision with molecular dynamics for subcritical CO2 pressures.