Low-salinity water flooding of formation water in rock cores is, potentially, a promising technique for enhanced oil recovery (EOR), but details of the underlying mechanism remain unclear. The salinity effect on the interface between water and oil was investigated here using the Molecular Dynamics (MD) simulation method. n-Decane was selected as a representative oil component, SPC/E water and OPLS-AA force fields were used to describe the water/oil/ionic interactions for salt water and n-decane molecules. Equilibrium MD simulations were firstly conducted to study the n-decane/vapour and salt-water/vapour interface systems at six different NaCl concentrations (0 M, 0.05 M, 0.10 M, 0.20 M, 0.50 M and 1.00 M). The water/oil interface was then investigated by calculating bulk density distribution, radial distribution function, interface thickness and water/oil interfacial tension (IFT). Sufficiently long MD simulations of water/n-decane/vapour were performed, followed by an analysis of the effect of salinity on the water/oil/vapour interface. The IFT values for the water/vacuum interface, ndecane/vacuum interface and water/n-decane interface were obtained from the pressure tensor distribution after system equilibration, with values of 71.4, 20.5 and 65.3 mN/m, respectively, which agree well with experimental and numerical results reported in the literature. An optimal salinity of ~0.20 M was identified corresponding to a maximum interfacial thickness between water and oil phase, which results in a minimum water/oil IFT value and a maximum value for the oil/water contact angle, a condition beneficial for enhanced oil recovery.

Physical properties of polyelectrolytes have been shown to be significantly related to their chain conformations. Atomistic simulation has been used as an effective method for studying polymer chain structures, but few has focused on the effects of chain length and tacticity in the presence of monovalent salts. This paper investigated the microscopic conformation behaviours of polyacrylic acid (PAA) with different chain sizes, tacticity and sodium chloride concentrations. The hydrogen behaviours and corresponding radial distribution functions were obtained. The results showed that the increase of salt concentrations led to the collapse of PAA chains, especially for longer chains. It was found that the effects of salt were mainly attributed to the shielding screening effect by sodium ions rather than the hydrogen bonding effect. Two different structure were form by iso-PAA and syn-PAA, respectively, which due to the deprotonation patterns along the PAA chain.

No abstract

This is a repository copy of Molecular dynamics investigation of substrate wettability alteration and oil transport in a calcite nanopore.

In this study, classic Molecular Dynamics (MD) simulations with established force fields were first performed to investigate the salinity effects on the static contact angle of a n-decane droplet immersing in the water atmosphere within a calcite nanochannel to advance our microscopic understanding on low salinity flooding. By applying an external body force, dynamic contact angle of n-decane in the water phase was also studied in the presence of various salt concentrations based on Non-Equilibrium MD (NEMD) simulation. The predicted n-decane static contact angles are around 59.68º ± 0.26º, which agree well with experimental results in previous studies. A reduction of the static contact angle of the nanodrop is observed with the increase of salinity, which implies an enhancement of surface hydrophilicity. Under flow conditions, the deformation of nanodrop, as evidenced by the centre of mass analysis, becomes faster by increasing the salt concentration. The recovery/mobility of the n-decane 2 nanodrop is, however, still significantly restricted by the adsorption interaction between the substrate and n-decane phase, which may lead to droplet snapping off and/or breaking up into small droplets.