Molecular dynamics simulations are carried out to study the two-phase behavior of the n-decane+water system in the presence of methane, carbon dioxide, and their mixture at reservoir conditions. The simulation studies were complemented by theoretical modeling using the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) and density gradient theory. Our results show that the presence of methane and carbon dioxide decreases the interfacial tension (IFT) of the decane+water system. In general, the IFT increases with increasing pressure and decreasing temperature for the methane+decane+water and carbon dioxide+decane+water systems, similar to what has been found for the corresponding decane+water system. The most important finding of this study is that the presence of carbon dioxide decreases the IFT of the methane+decane+water system. The atomic density profiles provide evidence of the local accumulation of methane and carbon dioxide at the interface, in most of the studied systems. The results of this study show the preferential dissolution in the water-rich phase and enrichment at the interface for carbon dioxide in the methane+carbon dioxide+decane+water system. This indicates the preferential interaction of water with carbon dioxide relative to methane and decane. Notably, there is an enrichment of the interface by decane at high mole fractions of methane in the methane/decane-rich or methane/carbon dioxide/decane-rich phase. Overall, the solubility of methane and carbon dioxide in the water-rich phase increases with increasing pressure and temperature.Additionally, we find that the overall performance of the PC-SAFT EoS and the cubic-plus-association EoS is similar with respect to the calculation of bulk and interfacial properties of these systems.

We perform molecular dynamics simulation study of CO, methane, and their mixture in the presence of brine over a broad range of temperature (311-473 K), pressure (up to about 100 MPa), and NaCl concentration (up to about 14 wt %). The general decrease in the interfacial tension (IFT) values of the CH-brine system with pressure and temperature is similar to that obtained for the corresponding CH-water system. The IFT of methane and brine is a linearly increasing function of salt concentration, and the resulting slopes are dependent on the pressure. A similar behavior as methane is observed for such systems containing CO and CO-CH mixture. The IFT of CO and brine increases linearly with increasing salt content; however, the resulting slopes are independent of pressure. The simulations show that the presence of CO decreases the IFT values of the CH-water and CH-brine systems, but the degree of reduction depends on the amount of CO in each sample, which is consistent with experimental evidence. These IFT values show a linear correlation with the amount of CO, and the resulting slopes are dependent on the temperature and pressure. Furthermore, our results for the mole fractions of the different species in the CO-CH-water system at 323 K and 9 MPa are in agreement with those of experiments. The mole fractions of methane and CO in the water-rich phase decrease with increasing salt concentration, whereas that of HO in the methane- or CO-rich phases remains almost unaffected in all of the studied cases. Our results could be useful because of the importance of carbon dioxide sequestration and shale gas production.

Molecular dynamics simulations and theoretical analysis were carried out to study the bulk and interfacial properties of carbon dioxide−methane−water and carbon dioxide−methane−brine systems under geological conditions. The density gradient theory with the bulk phase properties estimated using the cubicplus-association (CPA) equation of state (EoS) can well describe the increase in the interfacial tension (IFT) of the CO 2 −water system in the presence of methane. The theoretical estimates of species mole fractions in the carbon dioxide−methane−water system are in good quantitative agreement with the experimental results. Furthermore, simulations of carbon dioxide−methane−brine system show that the IFT of the CaCl 2 case is generally higher than that of the NaCl case. This is probably due to the stronger hydration of Ca 2+ ions and their stronger repulsion from the interface as compared to Na +. While the overall shape of the ionic profiles is not much affected by the ion type, the water profiles show a local enrichment at the interface in the system with CaCl 2. In contrast to the case of NaCl, the slopes of the plots of IFT vs CaCl 2 concentration are dependent on temperature. Species mole fractions in the carbon dioxide−methane−brine system predicted by combining the CPA EoS with the Debye-Hückel electrostatic term are in good agreement with simulation results. 2

Grand canonical Monte Carlo and molecular dynamics simulations were applied to understand the molecular mechanism of ion and water transport in montmorillonite clays as a function of relative humidity (RH). The variation of basal spacings of montmorillonite as a function of RH predicted based on the swelling free energy profiles was consistent with X-ray data. The hydration of the montmorillonite shows the following well-known order: Mg 2+ > Ca 2+ > Sr 2+ > Li + > Na + > K +. The relative contribution of water on external surfaces only becomes significant close to the saturation pressure. However, this behavior for K-montmorillonite starts to occur well below the saturation pressure due to the clay-swelling inhibition by potassium ions. The diffusion of water and ions generally increases with RH in all samples. However, for samples with weakly hydrated ions, the water mobility in thin films adsorbed on external basal surfaces of clay can be higher than that in the water-saturated mesopores. For a given RH, mobility of interlayer species is typically lower than that from the external surfaces. The results of the simulations were applied to interpret recent laboratory measurements of ion mobility with changing RH. We also assess the effect of layer charge distribution on such sorption and diffusion processes.

We perform Monte Carlo (MC) and molecular dynamics (MD) simulations to study the adsorption and diffusion properties of methane and CO2 in cross-linked polyethylene in the temperature range 300–600 K. A hybrid MC/MD approach was used to incorporate the effects of framework flexibility and polymer swelling on the gas adsorption. The polymers show negligible swelling at the studied conditions. A nonmonotonic behavior of gas adsorption as a function of the cross-linking degree was obtained. Notably, a similar behavior was observed for the void fraction and pore diameters. This shows a direct correlation between gas adsorption and the pore characteristics of the cross-linked polymer network. Mobility of methane and carbon dioxide in the polymer matrix increases with temperature. Also, gas mobility decreases with increasing cross-linking degree, consistent with experiments. These results can be explained by the fact that the waiting time for a gas molecule in a cavity before the jump increases with decreasing temperature and increasing cross-linking degree. Interestingly, the activation energy for gas diffusion generally decreases with increasing cross-linking. This is possibly due to the fact that increasing the cross-linking degree leads to smaller pore sizes especially at high temperatures. Such a molecular-level understanding of adsorption and diffusion of gases in cross-linked polyethylene is important in improving the performance of polymer networks for potential applications in gas separation, barrier technology, and food packaging.