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.

Using grand canonical Monte Carlo (GCMC) simulations, we study the adsorption behavior of CH 4 , CO , and their mixtures at 298.15 K and pressures up to 50 bar in Na-, Cs-, and Ca-montmorillonite clays in the presence of water. Montmorillonite clays in the presence of preadsorbed water, preferentially adsorb CO 2 over CH 4 during both pure component and mixture adsorption. The atomistic model we have used, gives good agreement with available single-component experimental adsorption isotherms, for CH 4 and CO 2 molecules adsorbed onto montmorillonite clays in the presence of water. We observe the general trend that the presence of increasing preadsorbed water content in the clay interlayers, reduced adsorption amounts of pure CH 4 and CO 2 molecules. With a relatively large basal spacing (d= 30Å), the favorability of adsorption of CO 2 by montmorillonite at relatively low pressures and intermediate water contents has been demonstrated using simulations. GCMC simulation is also used to assess the effect of water on the adsorption of N 2 /CH 4 , H 2 S/CH 4 , CO 2 /N 2 , and CO 2 /H 2 S binary mixtures in Na-montmorillonite clay. The ideal adsorbed solution theory is shown to agree well with the observed adsorption capacities and selectivities of mixtures in Na-montmorillonite clay.

Molecular dynamics simulations were performed to study the bulk and interfacial properties of methane + n-decane, carbon dioxide + n-decane, and methane + carbon dioxide + n-decane systems under geological conditions. In addition, theoretical calculations using the predictive Peng-Robinson equation of state and density gradient theory are carried out to compare with the simulation data. A key finding is the preferential dissolution in the decane-rich phase and adsorption at the interface for carbon dioxide from the methane/carbon dioxide mixture. In general, both the gas solubility and the swelling factor increase with increasing pressure and decreasing temperature. Interestingly, the methane solubility and the swelling of the methane + n-decane system are not strongly influenced by temperature. Our results also show that the presence of methane increases the interfacial tension (IFT) of the carbon dioxide + n-decane system. Typically, the IFT of the studied systems decreases with increasing pressure and temperature. The relatively higher surface excess of the carbon dioxide + n-decane system results in a steeper decrease in its IFT as a function of pressure. Such systematic investigations may help to understand the behavior of the carbon dioxide-oil system in the presence of impurities such as methane for the design and operation of carbon capture and storage and enhanced oil recovery processes.

We perform grand canonical Monte Carlo simulations to study the detailed molecular mechanism of intercalation behavior of CO2 in Na-, Ca-, and Mg-montmorillonite exposed to variably hydrated supercritical CO2 at 323.15 K and 90 bar. The simulations indicate that the intercalation of CO2 strongly depends on the relative humidity (RH). The intercalation of CO2 in the dehydrated interlayer is inhibited, followed by the swelling of the interlayer region due to uptake of water and CO2 as the RH increases. In all of the hydrated clay samples, the amount of the intercalated CO2 generally decreases as a function of increasing RH, which is attributed mainly to the weakening of the interaction between CO2 and clay. At low RH values, Ca- and Mg-montmorillonite are relatively more efficient in capturing CO2. The amount of CO2 trapped in all clay samples shows similar values above RH of ∼60%. Molecular dynamics simulations show that the diffusion coefficient of each species generally increases with increasing RH due to the associated expansion of the interlayer distance of the clay. For all the hydrated samples, the diffusion coefficients of CO2 and water in the interlayers are mostly comparable due to the fact that CO2 molecules are well solvated. The diffusion of CO2 in each hydration state is mostly independent of the type of cation in accordance with the fact that CO2 molecules hardly migrate into the first hydration shell of the interlayer cations.

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

Molecular dynamics simulations were carried out to study the structural and transport properties of carbon dioxide, methane, and their mixture at 298.15 K in Na-montmorillonite clay in the presence of water. The simulations show that, the self-diffusion coefficients of pure CO 2 and CH 4 molecules in the interlayers of Na-montmorillonite decrease as their loading increases, possibly because of steric hindrance. The diffusion of CO 2 in the interlayers of Na-montmorillonite, at constant loading of CO 2 , is not significantly affected by CH 4 for the investigated CO 2 /CH 4 mixture compositions. We attribute this to the preferential adsorption of CO 2 over CH 4 in Na-montmorillonite. While the presence of adsorbed CO 2 molecules, at constant loading of CH 4 , very significantly reduces the self-diffusion coefficients of CH 4 , and relatively larger decrease in those diffusion coefficients are obtained at higher loadings. The preferential adsorption of CO 2 molecules to the clay surface screens those possible attractive surface sites for CH 4 . The competition between screening and steric effects leads to a very slight decrease in the diffusion coefficients of CH 4 molecules at low CO 2 loadings. The steric hindrance effect, however, becomes much more significant at higher CO 2 loadings and the diffusion coefficients of methane molecules significantly decrease. Our simulations also indicate that, similar effects of water on both carbon dioxide and methane, increase with increasing water concentration, at constant loadings of CO 2 and CH 4 in the interlayers of Na-montmorillonite. Our results could be useful, because of the significance of shale gas exploitation and carbon dioxide storage.

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.