Molecular Dynamics Simulation of the Salinity Effect on the n-Decane/Water/Vapor Interfacial Equilibrium

Abstract

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.

How do ions contribute to brine-hydrophobic hydrocarbon Interfaces? An in silico study

“…S1 (see Supporting Information), in which water molecules were placed at middle of the cell while being enclosed by two identical stacks of hydrocarbon molecules. The biphasic systems were set up in accordance with the typical thickness of thin brine films found in the oil reservoirs (1-10 nm) and also configurations adopted by previous researchers [4,[17][18][19]. The number of organic molecules in each slab was set to nearly match the reported densities at ambient conditions (toluene: 0.862, n-heptane: 0.679, and heptol: 0.775 g.cm À3 ) [28].…”

“…In concentrated solutions, the strong electrostatic forces induced by ions encourages polar water molecules to interact preferably with the interior hydrophilic bulk, rather than meeting the hydrophobic compounds at the interface. In light of this conceptual explanation, prior studies pointed out that the interface width of both vapor/liquid and liquid/liquid systems is inversely proportional to the amount of dissolved salt [17,18].…”

“…They pointed out that the interfacial thickness decreases upon increasing salinity, resulting in higher IFTs. Later, Zhao et al expanded that work by focusing on diluted NaCl solutions and observed a nonmonotonic trend of IFT for n-decane/brine systems [18]. Khiabani et al utilized MD simulation to evaluate ion distributions in a CaCl 2 solution confined by pure n-hexane.…”

Molecular dynamics investigation of substrate wettability alteration and oil transport in a calcite nanopore

“…To further validate our system setup and the applied force field, more MD simulations for water/n-decane/vacuum equilibrium interactions have already been conducted and compared our calculated physical properties, i.e. interfacial tensions, contact angles, etc., with available experimental and simulation results [64]. To investigate the surface charge effect on the water/oil/mineral equilibrium interactions, the final-configuration snapshots of the salt-water/n-decane/calcite systems with neutral and charged pore wall surfaces (systems NS-0.50M and CS-0.50M) after 5 ns EMD simulation period are presented in Fig.…”

Interfacial and bulk properties of concentrated solutions of ammonium nitrate

“…Besides the thermodynamic models suggested by several investigators, [2][3][4][5] molecular simulations have contributed significantly to the present understanding of the surface tension of ionic solutions at the microscopic level, in particular, by describing the distribution of ions near liquid-vapour interfaces. [6][7][8][9][10][11][12] Normally, the presence of ions at the interface, which attract each other through electrostatic forces, and interact with the solvent through the solute-solvent attraction, increases the surface tension, as there is more resistance to stretch the surface. This makes the formation of new interfaces more energy intensive during the emulsification.…”