Molecular Dynamics Simulations of Interfacial Tensionsand Contact Angles of the Nitrogen+Oil+Brine+Rock System

Molecular dynamics (MD) simulations of the N₂+hexane+H₂O system (two-phase) were conducted at 343–443 K and 60–150 MPa. The MD results agreed reasonably well with the density gradient theory (DGT) results based on the CPA EoS. The interfacial tensions (IFTs) were found to increase with pressure and decrease with temperature. An important finding is that the IFTs only slightly decreased with increasing N₂ mole fraction in the N₂/hexane-rich phase (<i>x</i>_N₂). In general, N₂ shows a positive surface excess, and hexane shows a negative surface excess. The increase in the IFT with pressure indicates that the IFT behavior is dominated by the negative surface excess of hexane. MD simulations of the corresponding N₂+hexane+H₂O+silica (hydrophilic) system showed that the water contact angles (CAs) are not greatly affected by pressure or temperature. Importantly, the water CAs slightly decreased with increasing <i>x</i>_N₂, and the adhesion tensions increased with increasing <i>x</i>_N₂. MD simulations of the N₂+hexane+brine system were also conducted (salt concentration (<i>c</i>_s) up to 5.4 mol/kg NaCl). The MD results agreed reasonably well with the DGT results based on the CPA EoS with the Debye–Hückel contribution. Here, Na⁺ and Cl^– were excluded from the interfacial regions. The solubility of N₂ in the H₂O-rich phase decreased with increasing <i>c</i>_s, because of the salting-out effect. The IFTs increased linearly with increasing <i>c</i>_s. MD simulations of the corresponding N₂+hexane+brine+silica (hydrophilic) system showed that the water CAs increase with increasing <i>c</i>_s. Our previous studies showed that the CO₂+hexane and CO₂+hexane+silica (hydrophilic) systems in the presence of water or brine gave generally similar results. However, for example, the adhesion tensions of the CO₂+hexane+H₂O+silica (hydrophilic) system decreased with increasing <i>x</i>_CO₂.