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

Sohaib Mohammed a,⁎,G.Ali Mansoorib 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 studies (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 simulation. 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 investigated 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. We used other forming the oil phase and uniformly distributed in that phase – hexane (133), heptane (105), octane (108), nonane (111), cyclohexane which agrees with previous studies [37 38]. CO2 showed a low and (91), cycloheptane (126), toluene (134) and benzene (61) to represent high solubility in brine and oil phases, respectively. CO2, aromatic frac- the oil phase. We also added 7 wt% asphaltene of this mixture in sepa- tion, and asphaltenes exhibited an accumulation in the interfacial region rate simulations to study the interfacial behavior of the heaviest oil frac- at the simulated conditions. The accumulation of CO2, aromatics and asphaltene in the interface was predicted separately in different simulation in the presence of CO2.ThenCO2 molecules were added to the tions [15,18,37–40]. system to achieve the required mole fraction. CO2 molecules were calculated as follows: The accumulation of CO2 was further analyzed by calculating the relative density (the partial density divided by the bulk density) of the gas as shown in Fig. 3. The relative density decreases from about 2.7 to 1.3 as n ¼ x n ð2Þ CO2 CO2 t CO2 mole fraction increased from 0.2 to 0.8. The reduction in CO2

Fig. 1. The simulated asphaltene model. S. Mohammed and G.A. Mansoori Molecular insights on the interfacial and transport properties of supercritical CO2/brine/crude oil ternary system 270 J. Molecular Liquids 263: 268–273, 2018

relative density is due to the amount of CO2 dissolved in the bulk region which reduces the difference of the interfacial tensions between water-

CO2 and water-oil. CO2 accumulation at the interface can be theoreti- cally attributed to the difference in the interfacial tension between

water-CO2 and water-oil. This variation in the interfacial tension comes from the fact that CO2 is partially miscible with water due to the strong diffusibility which is a result of the active and strong bond di-

pole [41]. The accumulation of CO2 plays a signiﬁcant role in altering the interfacial properties of water-oil systems. It negatively contributes to the IFT. The driving force of the aromatic accumulation is the difference between the interfacial tension of water-aromatics and water-alkanes (both n- and cycloalkanes) due to the weak hydrogen bonding between water and aromatics compounds. The relative density of the aromatics

in the absence of CO2 is about 2.2 as shown in Fig. 4, however, it decreased with the increase of CO2 in the system. At high CO2 mole fraction (i.e. 0.8), the accumulation of the aromatics reversed. In other words, it accumulated in the bulk rather than in the interfacial region. Fig. 2. The density proﬁle of the components along with normalized z-axis at a CO2 mole fraction of 0.4. The x-axis represents z-coordinate divided by the length of the system in Moreover, as the polarity of the aromatic compound increased, it exhib- that direction. ited more accumulation in the bulk region. This conclusion can be cor- roborated by the difference in the relative density between benzene and toluene, whereas, toluene exhibited more accumulation in the bulk than benzene. The decrease in the relative density of aromatics

suggested that the addition of CO2 to water/oil system displaces the oil molecules away from the interface toward the oil phase. We have de- scribed the mechanism of such displacement previously [16]. We found

that CO2 forms a ﬁlm between oil and water phases and as the thickness of that ﬁlm increases, it displaces more oil away from the interface. This conclusion was adjusted by calculating the water and oil densities at the interfacial region. It was observed that the oil density decreased signif-

icantly as CO2 mole fraction increases while water density remained unchanged. The interfacial behavior of asphaltenes was also analyzed at different

CO2 mole fractions. The model asphaltene chosen in this study consists of a core of polyaromatic rings, long aliphatic chain, and heteroatoms (nitrogen and sulfur). Asphaltene molecules stacked at the interface due to the interaction between the aromatic core with water as well as the hydrogen bonding interaction between nitrogen and water molecules. As shown in Fig. 5, the radial distribution function (g(r)) of asphaltene-asphaltene, which is the distribution of the molecules away from a reference molecule, was calculated as a function of CO Fig. 3. Relative CO2 density at mole fractions of 0.2, 0.4, 0.6 and 0.8 along the normalized z- 2 axis. mole fraction.

Fig. 4. The relative density of aromatics along with the normalized z-axis at a CO2 mole fraction of 0.0, 0.4 and 0.8. 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 271

Fig. 5. Radial distribution function (RDF) for asphaltene-asphaltene in systems contain CO2 mole fraction of 0.0, 0.4 and 0.8. Oil and CO2 molecules were hidden from the snapshots to show the asphaltenes behavior more clearly.

Itisobviousthatthepeakofg(r)increasesasaCO2 content increase IFT generally showed a negative relation with the amount of CO2 in the system. The different peaks are a result of different stacking ways added to the system. (i.e. either face-to-face or T shape stacking). The dramatic increase in g The strength of water network due to the hydrogen bonds between (r) value in a distance about 0.5 nm refers to the face-to-face stacking the water molecules is responsible for the interfacial tension between model. As CO2 mole fraction increases in the system, the face-to-face oil and water. As CO2 mole fraction increased in the systems, H-bonds stacking being enhanced. The face-to-face stacking is due to the between water and CO2 were found to be increased, which means at polyaromatic cores interactions. RDF calculation is also showed that the same time a reduction in H-bonds between water molecules. Fur- face-to-face stacking is dominant in these systems. It also could be ob- thermore, as CO2 present in the interface, more CO2 molecules pene- served that in the absence of the CO2, all asphaltene molecules stacked trated to the water phase which leads to an increase in the interfacial at the interface. However, when CO2 mole fraction increases in the sys- roughness. The increase in the interfacial roughness contributes nega- tem, some of the asphaltenes molecules dissolved in the oil bulk. tively to the IFT. Moreover, CO2 diluted the interface and increased the interfacial width. The interfacial boundaries were determined using 3.2. Interfacial properties Gibbs dividing surface (GDS) which states that the interfacial region is located between the points where the density of water in the oil The interfacial tension of brine/oil was calculated as a function of phase is approximately equal to the density of oil in the water phase.

CO2 mole fraction as shown in Fig. 6. Previous MD studied represented The interfacial width was found to increase as CO2 mole fraction in- the oil phase as pure or binary organic solvents, however, oil is much crease in the system as shown in Fig. 7. more complicated than one or two components. Thus, the data The intermolecular interactions between water, oil, and CO2 were governed by this study reﬂects the behavior of more realistic system. also calculated as a function of CO2 mole fraction as shown in Fig. 8.

Fig. 7. The interfacial width (or GDS) and number of H-bonds between water and CO2 as a

Fig. 6. The interfacial tension of brine/water as a function of CO2 mole fraction. function of CO2 mole fraction. S. Mohammed and G.A. Mansoori Molecular insights on the interfacial and transport properties of supercritical CO2/brine/crude oil ternary system 272 J. Molecular Liquids 263: 268–273, 2018

Fig. 8. Intermolecular interaction between the components as a function of CO2 mole fraction.

Fig. 9. The diffusion coefﬁcient of the system as a function of CO2 mole fraction.

The alterations in the interfacial properties are due to the unbalanced interactions between the constituents. The general trends in the inter- 4. Conclusion actions showed that water-oil interactions decrease as CO2 mole fraction increased in the system while water-CO2 and oil-CO2 interactions The effect of sc-CO2 on the interfacial properties of brine/crude oil increased. Oil-CO2 interactions increased more dramatically than were studied using series of molecular dynamics simulations under water-CO2 which explains why the relative density of CO2 decreased 100 bar and 350 K. The brine and crude oil formed two distinct phases. as a function of CO2 mole fraction. CO2 exhibited low and high solubility in brine and oil phase, respec- At low CO2 mole fractions, the difference between the water-CO2 tively. It was found that CO2, aromatics, and asphaltenes accumulate and oil-CO2 interactions are small, thus CO2 molecules accumulated at the interface at low CO2 mole fractions. The relative density calcula- at the interface to balance the interaction difference. However, tions showed that the accumulation of CO2 and aromatics decrease as when CO2 mole fraction increase in the systems, the interaction oil- the CO2 mole fraction increase, which suggested as CO2 mole fraction in- CO2 become much higher than water-CO2 more CO2 dissolved in crease, it being dissolved more in the oil phase and displace the aro- the oil phase. matics away from the interface. The accumulation of CO2 and The decrease in water-oil interaction suggests that CO2 forms a ﬁlm aromatics attributed to the difference in the IFTs. Asphaltenes were dis- between the two phases and as the CO2 mole fraction increases in the tributed at the interface at low CO2 mole fraction, however, as CO2 mole systems, the thickness of that ﬁlm increases and displaces the oil mole- fraction increase, some of the asphaltene molecules were dissolved and cules away from the interface. aggregated at the oil bulk. Also as CO2 mole fraction increases, it enhances the face-to-face stacking between asphaltene molecules. The

change in the interfacial compositions due to the addition of CO2 caused 3.3. System diffusivity an alteration in the interfacial properties such as a decrease in the interfacial tension, an increase in the interfacial width and formation of hy-

Molecular dynamics simulation provides the advantage of drogen bonds between CO2 and water molecules. The calculations of fi predicting the diffusion coef cients for the contributed molecules the diffusion coefficient showed that the addition of CO2 caused an in- which is very challenging to be determined experimentally. It is even crease in the system diffusivity. This study provides important underly- fi fi more complicated to de ne the diffusion coef cients for a system com- ing details about the effect of sc-CO2 on the interfacial and transport posed of similar molecules such as a mixture of hydrocarbons using ex- properties that could be used to improve EOR. perimental techniques. The complex chemical and environmental conditions are another obstacles can be added to the difficulty of deter- Notes mining the diffusion coefficient experimentally. Thus, it is important to employ molecular dynamics simulation to study the effect of sc-CO ad- 2 The authors declare no competing financial interest. dition on the system diffusivity. We used Einstein formulation to calcu- late the diffusion coefficient of the systems as follows: Acknowledgements

1 1 XN MSDðÞ t This research is supported, in part, by Higher Committee for Educa- D ¼ lim ð4Þ 6 t→∞ N t tion Development in Iraq (HCED)/Ofﬁce of the Prime Minister. i

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