Molecular Dynamics Simulations Study on the Shear Viscosity, Density, and Equilibrium Interfacial Tensions of CO₂ + Brines and Brines + CO₂ + n-Decane Systems

Abstract

The shear viscosity, density, and interfacial tensions (IFT) of two systems, namely, brine and brine/n-decane, blended with carbon dioxide (CO2) were investigated via molecular dynamics simulations over broad ranges of temperature, pressure, CO2 mole fraction, and brine concentration. The operating conditions for the molecular simulations to be studied are similar to the CO2 geological storage processes. The effects of temperature, pressure, and concentrations on the viscosity and IFT have been investigated and analyzed. All four influencing parameters affect the shear viscosity and IFT. The pressures and temperatures up to 1000 bar and 573 K, respectively, were used for predicting the viscosity and IFT by considering intermolecular interactions, while salinities up to 32 000 ppm and CO2 mole fractions between 0 and 0.5 were used in the simulations. Comparisons were made between simulated values and the predicted results of an empirical correlation, both against experimental data. Both monovalent and divalent ions and their mixtures were used in the simulations, and the results showed that monovalent ions impose stronger interactions in the solution than divalents. The results have revealed that the supercritical CO2’s capability to reduce the IFT of the brine/n-decane interface is remarkable, which makes it a promising agent for underground geological injection for enhanced oil recovery. Also, viscosity and density ratio analysis have confirmed the viability of CO2 storage in deep saline aquifers, where harsh geothermal conditions of high salinities limit the extent of the experiments. The molecular simulation results are in good qualitative agreement with the experimental data available in the literature for the viscosity, density, and IFT.

Interfacial behavior of the decane + brine + surfactant system in the presence of carbon dioxide, methane, and their mixture

“…Past studies have shown that the IFT of the alkane + water + CH 4 /CO 2 systems is more similar to that of the corresponding alkane + water system alone. 25,35,36,[38][39][40][41][42][43]45 Also, the IFT of the decane + brine + CH 4 / CO 2 systems was more similar to that of the corresponding decane + brine system alone. 26,29,39,44,45 The IFTs of all these systems decreased with temperature.…”

“…25,35,36,[38][39][40][41][42][43]45 Also, the IFT of the decane + brine + CH 4 / CO 2 systems was more similar to that of the corresponding decane + brine system alone. 26,29,39,44,45 The IFTs of all these systems decreased with temperature. For instance, the simulation value of the IFT of the decane + water system (20 MPa) decreased from about 47.8 mN m À1 at 323 K to about 36.5 mN m À1 at 443 K. 39 It was found that the IFTs of these systems generally increased with pressure.…”

“…Experiments, [25][26][27][28][29][30][31][32][33][34][35][36][37] theory, [32][33][34][36][37][38][39] and simulations [38][39][40][41][42][43][44][45] have been successfully employed to understand the bulk and interfacial properties of alkane + water + CH 4 /CO 2 systems. These studies have reported the occurrence of, for example, a two-phase region at high pressures.…”

“…The IFT of the alkane + water + CH 4 /CO 2 two-phase systems was more similar to that of the corresponding alkane + water system alone. 25,35,36,[38][39][40][41][42][43]45 The IFTs of the alkane + water + CH 4 /CO 2 and alkane + brine + CH 4 /CO 2 systems increased with decreasing x CH 4 /x CO 2 (x CH 4 and x CO 2 are the mole fractions of CH 4 and CO 2 in the alkane-rich phase, respectively). This can be attributed to the fact that the interface was enriched with CH 4 and CO 2 molecules.…”

“…[38][39][40][41][42][43] The IFTs of the alkane + brine and alkane + brine + CH 4 /CO 2 systems were reported to increase with increasing salt concentration. 26,27,29,39,44,45 It is also known that the addition of surfactants such as sodium dodecyl sulfate (SDS) generally decreases the IFT. [46][47][48][49][50][51][52][53][54][55][56][57][58] Molecular simulations showed that, in the water + surfactant system, water molecules and sodium counterions are relatively near the SDS headgroups.…”