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Journal of Molecular Liquids Volume 263, Pages 268–273, August 1, 2018

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Journal of Molecular Liquids Volume 263, Pages 268–273, August 1, 2018 Journal of Molecular Liquids Volume 263, Pages 268–273, August 1, 2018 DOI: https://doi.org/10.1016/j.molliq.2018.05.009 Molecular insights on the interfacial and transport properties of supercritical CO2/brine/crude oil ternary system a,⁎ b Sohaib Mohammed ,G.Ali Mansoori a Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA b Department of Bio and Chemical Engineering & Physics, University of Illinois at Chicago, M/C 063, Chicago, IL 60607-7052, USA abstract In this study, we conducted a series of molecular dynamics simulations to investigate the effect of supercritical carbon dioxide (sc-CO2) on the interfacial and transport properties of brine/crude oil at the reservoir conditions. We also studied the interfacial behavior of asphaltenes in presence of CO2. Crude oil was resembled by several hydrocarbons which are hexane, heptane, octane, nonane, cyclohexane, cycloheptane, benzene and toluene. The results showed that CO2, aromatics and asphaltenes accumulate at the interface at low CO2 mole fraction, however, as CO2 mole fraction increases, the relative density, the ratio of the density at the interface to the bulk density, decreases for both CO2 and aromatics. The decrease in CO2 relative density is due to the amount of CO2 dissolved in the oil bulk, which increases as CO2 mole fraction increase. It also found that CO2 displaces the oil molecules away from the interface, thus the relative density of aromatics decreases. Interestingly, it was found that as CO2 mole fraction increase, it enhances the face-to-face stacking between asphaltene molecules as noticed from the radial distribution function calculations. CO2 also force some asphaltene molecules to leave the interface and being dissolved and aggregated in the oil bulk. It also found that as CO2 mole fraction increased in the system, it dilutes the interface, penetrates to the water phase, forms hydrogen bonds with water and due to these effects, it reduces the interfacial tension of brine/crude oil system. The diffusivity of supercritical CO2/brine/crude oil system was also increased as a function of CO2 mole fraction. This study provides insights of the under-lying interfacial properties from molecular level for a more realistic system of brine/crude oil. 1. Introduction the underlying details of the immiscible fluids interfaces. Limited stud- ies (both experimental and simulation) have performed on the effect of One of the practiced techniques to increase the oil production from sc-CO2 on the interfacial properties of water/oil system. It was found ex- depleted oil reservoirs is the gas injection which is known as enhanced perimentally that the interfacial tension (IFT) of water/n-decane de- oil recovery (EOR) [1–2]. CO2 is widely used in EOR and it has been prac- creases as a CO2 content increase in the system [12]. It was also ticed since the 1970s [3–4]. It provides the advantage of mitigating the experimentally revealed that the IFT of brine/crude oil in the presence gas emissions as well as increasing the production of the energy re- of CO2 decrease as the pressure increase while it increases as the tem- sources from depleted oil reservoirs. Injecting CO2 into oil reservoir perature increase [13]. Molecular dynamics simulations were also causes many changes in the reservoir fluids such as heavy hydrocarbons employed to investigate the role of sc-CO2 in the interfacial properties aggregation and affecting the interfacial properties of water/oil emul- of water/oil systems. CO2 was found to reduce the IFT of brine/hexane sion [5–10]. Interfacial properties are of great importance in oil produc- system and it has amphiphilic feature toward the interface [14]. An ac- tion because of their impact on fluids flow in the shale nanopores. Thus, cumulation of sc-CO2 at the water-decane interface was detected which it is important to investigate the influence of CO2 on the interfacial and leads to a reduction in the IFT [15]. The amount of accumulated CO2 de- transport properties of brine/crude oil. pends on the nature of the oil whereas it is higher in paraffinic than ar- Studying the interfacial properties of two immiscible fluids experi- omatic oil [16–17]. Although the mentioned studies provided insights mentally are still challenging [11], especially under high pressures, into the role of sc-CO2 in water/oil system, it is still insufficient to due to the scale of the interface which is about few nanometers in completely comprehend such effect. The reason is that the MD investi- width. Molecular simulation provides an efficient alternative to explore gations were performed on pure hydrocarbons and binary systems to resemble oil phase which is not very realistic because crude oil is much more complex than one or two hydrocarbons. Thus, it is essential ⁎ Corresponding author. to investigate the effect of sc-CO2 on brine/crude oil systems to provide insight into the behavior of this ternary system. E-mail addresses: S. Mohammed: [email protected]; [email protected], G.A. Mansoori: [email protected]; [email protected]. S. Mohammed and G.A. Mansoori Molecular insights on the interfacial and transport properties of supercritical CO2/brine/crude oil ternary system J. Molecular Liquids 263: 268–273, 2018 269 In this study, we chose 0.5 M NaCl solution to represent the brine where nCO2 is the number of CO2 molecules, xCO2 is the required CO2 phase. The oil phase was introduced using several hydrocarbons that mole fraction, and nt is number of oil and CO2 molecules. were suggested to represent a light crude oil [18–19]. It is composed The energy of the initial configuration was minimized using of hexane, heptane, octane, nonane, cyclohexane, cycloheptane, toluene “Steepest decent” method for 50,000 steps. Each system was equili- and benzene. CO2 was added to the system to satisfy the following for- brated at the required temperature using canonical ensemble (NVT) mula: for 100 ps using Berendsen thermostat [33]. The production of the NVT step was simulated under the isobaric, isothermal and iso- interfacial area ensemble (NP AT) simulation for 15 ns. The proper- H O þ ½ðÞ1−x Oil þ xCO ð1Þ normal 2 2 ties and the governed data were averaged over the last 5 ns of the sim- ulation. The area of the interface (X and Y components) was kept where x is the mole fraction and equal to 0, 0.2, 0.4, 0.6 and 0.8. constant while the axis normal on the interface (Z direction) changes Asphaltenes are also known to stabilize the water/oil emulsion using a semiisotropic coupling. The temperature and pressure were [20–26]. Thus, we also modified the oil phase by adding 7 wt% controlled using Berendsen thermostat and Parrinello-Rahman barostat asphaltene to the system to investigate the asphaltene interfacial be- [34], respectively. Periodic boundary conditions were used in all direc- tions. The IFT was calculated using Gibbs formulation as follows: havior under different CO2 mole fractions. The systems were simulated under 100 bar and 350 K. This study provides an insight of the interfacial 1 Px þ Py behavior of sc-CO2 and asphaltenes and their effect on brine/crude oil γ ¼ Pz− Lz ð3Þ interface. n 2 We studied the density profiles of the components and further in- vestigated the constituents that accumulated at the interface which where n is the number of the interfaces formed in the system, Px, Py and P are the diagonal elements of the pressure tensor and L is the length of are CO2, aromatics, and asphaltenes at different CO2 mole fractions. In- z z terfacial tension, interfacial structure, intermolecular interactions, hy- the simulation box in Z direction. VMD was used for the visualization drogen bonds, and diffusion coefficients were also investigated. and image processing [35]. As shown in Fig. 1, the simulated asphaltene model contains an aro- 2. Simulation details matic core, aliphatic chain, and heteroatoms (sulfur and nitrogen). This model was proposed by Zajac et al. [36]. – A series of classical MD simulations were conducted using The system was validated in previous studies [16 17]. GROMACS 5.1.2 package [27]. The operating conditions of all simula- The hydrogen bonds (H-bonds) between water and CO2 molecules tions were 100 bar and 350 K. Lennard Jones and Coulomb's models were calculated by setting the OH group in water as a donor and O in were employed to account van der Waals interaction and electrostatic CO2 as an acceptor. The cutoff for the distance of donor-acceptor used interactions with a cutoff of 1.4 nm. Long-range electrostatic interac- is 3.5 Å. tions were treated using particle mesh Ewald (PME) summation method [28]. The bonded potential was considered for bond stretching, 3. Results and discussions angle bending, and dihedrals. The optimized potentials for liquid fi simulations-all atoms (OPLS-AA) [29], EPM2 [30], simple point charge/ 3.1. Density pro le extended (SPC/E) [31] were used to model hydrocarbons, CO2 and fi water molecules, respectively. Virtual sites were employed to maintain The density pro les of the molecules were calculated at CO2 mole fraction of 0.4 along with the axis normal to the interface (z-axis) as the rigid shape of CO2 [8,10,32]. The initial configuration was created by placing water and oil in a shown in Fig. 2. The system forms two distinct phases, brine and oil phases. Brine phase consists of water molecules and the contributed rectangular cell of dimensions Lx =Ly = 6 nm and Lz = 20 nm. 6000 water molecules were located on the left side of the box to render ions (Na and Cl). All the hydrocarbons became miscible with each water bulk properties while the oil phase on the right side.
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