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The Effect of CO2 on the Interfacial and Transport Properties of Water/Binary and Asphaltenic Oil: Insights from Molecular Dynamics Sohaib Mohammed, and G. Ali Mansoori Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00488 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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The Effect of CO2 on the Interfacial and Transport Properties of Water/Binary and Asphaltenic Oil: Insights from Molecular Dynamics Sohaib Mohammed a, * and G.Ali Mansoori b a

Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA. b

Departments of Bio- and Chemical Engineering, University of Illinois at Chicago, (M/C 063) Chicago, IL 60607-7052, USA. *Corresponding author

Email addresses: [email protected] [email protected] (G.A. Mansoori)

(S.

Mohammed),

[email protected];

Abstract We conducted molecular dynamics (MD) simulations to investigate the effect of supercritical (sc-CO2) on the interfacial and transport properties of water-oil systems. The oil phase was resembled by employing different binary hydrocarbons (paraffin + aromatic), namely benzene + hexane, benzene + octane, xylene + hexane, and xylene + octane. Furthermore, we added asphaltenes to the system that composed of xylene and hexane to study the interfacial behavior of the heaviest fraction of oil (asphaltene) in the presence of CO2. The simulations were performed under the operating conditions of 100 bar and 350 K. The results showed that aromatics, CO2, and asphaltenes accumulated at the interface at low CO2 mole fractions (xCO2). However, when xCO2 increased, it displaced the aromatics away from the interface and towards the bulk. At very high xCO2, the aromatics accumulated at the oil bulk. Similarly, asphaltene molecules stacked at the interface at low xCO2, and as xCO2 increased, some of the asphaltene molecules dissolved and aggregated in the oil bulk. CO2 forms a film between water and oil phases and as the thickness of the film increases, it displaces the hydrocarbons away from the interface. Sc-CO2 diluted the interface, formed hydrogen bonds (H-bonds) with water which stabilize the CO2 film, and reduced the interfacial tension (IFT) in all systems. Furthermore, the addition of sc-CO2 increased the diffusivity of the oil phase in all systems. However, it significantly affected the diffusivity of systems that have less polar aromatics.

Introduction The ability of compressed carbon dioxide (CO2) injection to maintain underground petroleum reservoir pressure, in addition to its high solubility in oil, has made it a possible option for enhancing oil recovery from depleted reservoirs. In CO2 enhanced oil recovery (CO2-EOR) processes, the interfacial properties of oil-water system change significantly with CO2 injection. 1 ACS Paragon Plus Environment

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Thus, it is of practical and fundamental importance to study the effect of CO2 on the interfacial properties of the oil-water system 1. Few experimental investigations were performed to explore the effect of the injected gas on interfacial properties of the oil-water system. It was found that interfacial tension (IFT) of oilwater in the presence of CO2 decreases as the pressure increases 2. It was also demonstrated that the increase of CO2 content in the decane-water two-phase system reduces the IFT between oil and water phases 3. Molecular dynamics simulation was employed to study systems composed of pure hydrocarbons systems. A study on hexane-brine showed that the interfacial tension between oil and water phases decrease and the interfacial roughness increases as CO2 concentration increases 4 . It was also found that CO2 reduces the IFT between decane and water phases and increases the diffusion coefficient for all water-decane-CO2 system5. A detailed study was conducted to investigate sc-CO2 effect on water and various pure hydrocarbons 6. It was observed that CO2 accumulates at the interface due to the difference between IFT of water/CO2 and water/hydrocarbons. It was also found that as sc-CO2 mole fraction increases, the diffusivity of the hydrocarbons increases. The magnitude of the diffusivity increase was determined by the molecular weight and the hydrocarbon polarity. Asphaltenes are known to stabilize the oil-water emulsion during the petroleum processing 7-11. The emulsion stabilization by asphaltenes may lead to serious effects such as fouling on the pipelines, corrosion in the plant equipment and increase operating costs. Jian et al. 12 performed an experimental and simulation study to investigate the effect of asphaltene on the interfacial tension of water/oil system. The results indicated that the reduction of IFT as a response to asphaltene addition is due to the surface concentration of the asphaltene rather than its bulk concentration. They also proposed a mechanism of the reduction in the interfacial tension by analyzing the hydrogen bonding which are one the driving forces of the IFT reduction in oil/water systems. Mikami et al. 13 studied the behavior of asphaltenes in two-phase systems composed of water-toluene and water-heptane using MD simulation. The findings showed that asphaltenes prefer to accumulate at the interface in case of water-heptane, while it is distributed in the oil phase in the case of water-toluene. The study also found that the IFT is slightly reduced when small asphaltene concentrations are present in the system, while it is reduced significantly when a large amount of asphaltenes added to the system. Liu et al. 14 studied the aggregation and orientation of asphaltenes at various water surfaces and found that asphaltenes make an ordered aggregate at the water/toluene interface. Ervik et al. 15 conducted a multiscale simulation of a water drop covered with asphaltenes and presented novel methods that help to comprehend the systems of two-phase flows with complex interfaces. They calculated the interfacial properties of their system using coarse grained molecular dynamics simulations and employed the governed properties in a macroscale simulation in the case of flow to consider the complex interfaces behavior. In the current study, we intend to provide a detailed investigation of the effect of sc-CO2 on the interfacial and transport properties of water/oil interface using series of molecular dynamics simulations. For this purpose, we chose the oil phase using binary mixtures of (an 2 ACS Paragon Plus Environment

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aromatic + a paraffin) hydrocarbon systems. Four systems composed of benzene-hexane, benzene-octane, xylene-hexane, and xylene-octane were brought into contact with water in the presence of sc-CO2. Another system was prepared by adding 7 wt.% asphaltenes 16-20 to the oil phase composed of xylene and hexane. The presence of asphaltene provides insights into the effect of sc-CO2 on the interfacial behavior of the heaviest oil fraction, which is of great practical importance. Each oil phase composed of equal wt.% of aromatic and paraffin. The overall systems are of the formula H2O + [(1-x) oil + x CO2) where x = 0, 0.2, 0.4, 0.6 and 0.8 mole fraction. In the rest of the paper, we will refer to the systems composed of water and benzenehexane, benzene-octane, xylene-hexane, xylene-octane and xylene-hexane-asphaltene as system A, B, C, D, and E, respectively, as illustrated in Table 1. For each system, we studied the density profile, interfacial properties and structures, intermolecular interactions, and diffusion of the oil phase.

Simulation details A series of classical MD simulations were performed using the GROMACS 5.1.2 package 21. Bonded and nonbonded interactions were considered in these simulations. Bonded interactions were accounted for bond stretching, angle bending, and dihedrals. Nonbonded interactions were accounted for van der Waals attractions, steric repulsions, and electrostatic interactions. A cutoff of 1.4 nm was determined to calculate the vdW interaction. Long-range electrostatic interactions were treated using a particle mesh Ewald (PME) summation method 22. A time step of 1 fs was used. The optimized potentials for liquid simulations-all atoms (OPLS-AA) 23, EPM2 24, and simple point charge/extended (SPC/E) 25 were used to model hydrocarbons, CO2, and water molecules, respectively. The simulation cell for the CO2-water-oil mixture was initially set as a rectangular box with dimensions Lx = Ly = 6 nm and Lz = 20 nm. 7000 water molecules with random placement and rotation were located on the left side of the box and the oil phase was located on the right side. Then, CO2 molecules were added to the system to satisfy the required mole fraction. The details of the number of molecules are shown in Table 1.

Table 1. Descriptions and number of the constituent’s hydrocarbon molecules in contact with water of the simulated systems.

System A B C

#Oil molecules Benzene (726) Hexane (658) Benzene (726) Octane (497) Xylene (534) Hexane (658)

0 0

CO2 mole fraction #CO2 molecules 0.2 0.4 0.6 346 923 2076

0.8 5536

0

306

815

1835

4892

0

298

795

1788

4768

3 ACS Paragon Plus Environment

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D

E

Xylene (534) Octane (497) Xylene (497) Hexane (612) Asphaltene (10)

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0

258

687

1547

4124

0

280

746

1679

4476

An energy minimization was performed on the initial configurations using the “steepest descent” method to remove high energy structures. Canonical ensemble (NVT) was employed for 100 ps to equilibrate the system at the required temperature using Berendsen thermostat with a time constant of 0.1 ps 26. We performed isobaric, isothermal, and iso-interfacial area ensembles (NpnormalAT) on the output of NVT step for 15 ns. X and Y components (the area of the interface) were kept constant while Z (the component normal to the interface) changes using a semiisotrpic coupling. The temperature was controlled at 350 K using the Berendsen thermostat, and the pressure was kept at 100 bar using the Parrinello-Rahman barostat with a time constant of 2 ps 27. Periodic boundary conditions were used in all directions. The IFT was calculated using the Gibbs formulation in term of pressure which describes the planner interfaces between the immiscible liquids 28. The IFT can be extracted as follows:

1

γ =  Pz − n 

Px + Py   Lz 2 

(1)

Where n is the number of the interfaces formed in the system (two identical interfaces in our simulations due to the periodic boundary conditions), Pz is the normal pressure on the P + Py is the tangential pressure and Lz is the length of the system in the Z direction. interface, x 2 VMD was used for the visualization and image processing 29. As shown in Figure 1, the simulated asphaltene model contains an aromatic core, aliphatic chain, and heteroatoms (sulfur and nitrogen). This model was proposed by Zajac et al. 30. The system was validated in a previous study 6. The asphaltene concentration in oil phase (7 wt.%) represents the upper limit of the expected concentration and typical of the heavy crude.

Figure 1.The simulated asphaltene structure.


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Results and Discussions 1. Density profile The density profiles of the components in systems C and E with xCO2 of 0.2 were measured along the axis normal to the interface, as shown in Figure 2. Two phases were created in the system, the water phase with a low concentration of CO2 and the oil phase with high CO2 concentration dissolved into it. Xylene was accumulated at the interface due to the weak hydrogen bonding between the aromatic ring and water proton 31. In general, all aromatic compounds (i.e. benzene in systems A and B and Xylene in systems C, D and E) exhibited an accumulation at the interface at low xCO2 as well as in the absence of CO2. Similarly, CO2 was also accumulated at the interface in all systems. In system E, all asphaltene molecules stacked at the interface at low CO2 concentrations.

Figure 2. Density profiles of components in o-xylene-hexane [system C (left side)] and o-xylenehexane-asphaltene [system E (right side)] vs. Z*=Z/Lz (dimensionless axis normal to the interface). To study CO2 accumulations, we calculated the relative density, the ratio of CO2 density at the interface to the bulk density, as a function of xCO2, as illustrated in Figure 3. The mentioned ratio decreased as xCO2 increased, which means that more CO2 prefer to be dissolved in the bulk than that accumulated at the interface. Furthermore, when xCO2 is low, it can be observed that the accumulation in system B is more than that in system A. These trends suggest that for a system composed of an aromatic compound with different paraffins, the accumulated amount of CO2 proportional directly to the molecular weight of the paraffin constituent. The driving force for CO2 accumulation is the difference between the IFT of water/CO2 and water/oil. The amount of accumulated CO2 proportion to the that IFT difference magnitude. The accumulation increases as the difference IFT difference between water/CO2 and water/oil increases.


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Figure 3. The ratio of CO2 density at the interface to CO2 bulk density (in oil phase) at a xCO2 of 0.2 (left) and 0.6 (right) vs. Z*=Z/Lz (dimensionless axis normal to the interface). Aromatic compounds also exhibited an accumulation at the interface in the absence and low levels of xCO2, as shown in Figure 4. The relative density is about 1.5 for all systems. At xCO2 = 0.4, the value of ρ/ρbulk reduced to about 1 for all systems. Interestingly, at xCO2 = 0.8, the accumulations reversed compared to the case of when xCO2 is low. In other words, the aromatic compounds accumulated at the bulk region rather than at the interface. Aromatics accumulate at the interface because they have interfacial interaction with water which leads to a lower IFT with water comparing with paraffins. However, as CO2 mole fraction increases, it displaces oil molecules away from the interface toward the oil bulk. CO2 forms a film between the two phases (i.e. water and oil) and as the thickness of this film increases, it displaces more oil molecules toward the bulk region.


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a

b

c

Figure 4. Aromatics densities normalized by the bulk density at xCO2 of (a) 0.2, (b) 0.4 and (c) 0.8 vs. Z*=Z/Lz (dimensionless axis normal to the interface). The addition of CO2 to the systems affected the behavior of asphaltenes in the system as shown in Figure 5. The driving force for asphaltenes stacking at the interface is the formation of hydrogen bonds with water due to the presence of heteroatoms in the asphaltene structure. Moreover, the attraction between the aromatic core and water also plays a significant role in the asphaltene interfacial behavior. To investigate the interaction between asphaltene molecules and their interfacial behavior, the center-to-center of asphaltene molecules radial distribution function (RDF) were calculated as follows: g (r ) =

ρ( r ) ρo

(2)

Where ρ ( r ) is the local number density and ρ o is the bulk number density. RDF describes the probability of finding asphaltene molecule at a distance r from another asphaltene molecule.


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It is shown that the peaks of the RDF located at a distance slightly lower than 1 nm. The peak of the RDF increases as CO2 mole fraction increases. In the absence of CO2, all asphaltene molecules stacked and distributed at the interface. When the amount of CO2 increased in the system, some of the asphaltene molecules dissolved in the oil phase. The dissolved molecules aggregated with each other, forming dimers. Thus, the peak of the radial distribution function (RDF) increases as a xCO2 increase. The aggregation of asphaltene molecules was due to the aromatic-core stacking. In the case of high xCO2, most of the asphaltene molecules dissolved in the oil phase.

Figure 5. Radial distribution function (RDF) of asphaltene-asphaltene in different xCO2.

2. Interfacial tension The governed IFT for the simulated systems were plotted as a function of xCO2, as shown in Figure 6. In the absence of CO2, the values of interfacial tensions of the systems follow the order B > A > C > D > E. This order owed to the nature of the systems’ compositions which have different molecular weights and polarities. We can compare the interfacial tensions of the systems A-D based on each system compositions. For the systems composed of the same paraffin with different aromatics, the interfacial tension increases as the polarity of the aromatic constituent decreases (xylene is more polar than benzene) because the components with higher polarity has higher affinity to accumulate at the interface and thus reduce the interfacial tension. The interfacial tension of the simulated systems decreased as CO2 content increased. The magnitude of that decrease was about 10.9, 12.1, 15.5, 12.8, and 8.5 for systems A, B, C, D, and 8 ACS Paragon Plus Environment

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E, respectively. If we excluded system E from comparison due to the presence of asphaltene which a surface-active component, we found that for the same paraffin, sc-CO2 has a higher impact in the presence of the more polar aromatic compound. In other words, the effect of scCO2 on the IFT of system A > system C and on system B > system D. This observation can be explained by the difference in the interfacial interaction of the different components as will be shown in the next sections. For system E, in the absence of sc-CO2, asphaltenes cause a reduction in the IFT. However, once CO2 added to the system, asphaltene role diminished and CO2 became the major factor in altering the interfacial properties. The variation in the interfacial tension of water/oil systems due to the addition of sc-CO2 is a result of several alterations such as an increase in the interfacial width, formation of CO2 film between the two phases, formation of hydrogen bonds between the interfacial CO2 and water molecules and the intermolecular interactions.

Figure 6. IFT as a function of xCO2.

3. Interfacial structure and properties To explore the effect of CO2 on the structure and properties of the interface, we calculated the interfacial width and studied the interaction energies which is the reason behind the alteration in IFT.


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As shown in Table 2, we calculated the interfacial width using the Gibbs Dividing Surface (GDS) principle, which states that the interfacial boundaries are located where the water density in the oil phase is approximately equal to the oil density in the water phase 4-5, 32. For all the simulated systems, the interfacial width increases as xCO2 increases, which means that the addition of CO2 dilutes the interface. Table 2. The interfacial width (nm) between water and hydrocarbon phases as a function of xCO2 present in the system. xCO2 System A B C D E

0

0.2

0.4

0.6

0.8

0.46 0.48 0.50 0.49 0.48

0.52 0.51 0.54 0.54 0.53

0.56 0.62 0.61 0.60 0.62

0.68 0.70 0.70 0.71 0.67

0.71 0.74 0.85 0.80 0.84

We proposed here that CO2 forms a thin film between the two phases (i.e. water and oil phases). This film connected with a water phase by H-bonds and with the oil phase by the interaction with the polar component in the system. The thickness of this film increases as the xCO2 as shown in Figure 7. The film is stabilized by creating H-bonds between CO2 and water molecules6.

a

b

c

Figure 7. Snapshots of interface region of system A at xCO2 of (a) 0.0, (b) 0.4 and (c) 0.8. The molecules were represented by CPK mode using VMD package. red, white and cyan colors represent oxygen, hydrogen and carbon molecules, respectively. CO2 molecules have been hidden for clarity. 10 ACS Paragon Plus Environment

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CO2 film is stabilized with water phase by creating hydrogen bonds. The average number of H-bonds were calculated for the last 5 ns of the simulation. The average number of H-bonds between water and CO2 increases as sc-CO2 mole fraction increases in the system as shown in Table 3. These H-bonds also contribute to the interaction between water and CO2 in addition to vdW and ES interactions.

Table 3. The average number of H-bonds as a function of CO2 mole fraction. xCO2

0.2

0.4

0.6

0.8

47 44 37 38 33

86 86 80 78 77

117 122 125 116 118

130 142 169 161 174

System A B C D E 4. Intermolecular interactions As shown in Figure 8, the intermolecular interactions between the constituents were calculated as a function of xCO2. The calculation of the intermolecular interactions helps to explain the accumulation of CO2 at the interface and as a result the alterations in the interfacial properties since these changes are caused by unbalanced interactions between the two phases. At low CO2 content, the difference between water-CO2 and hydrocarbon-CO2 is small, therefore most of CO2 accumulated at the interface to balance these interactions. As the xCO2 increases in the system, the hydrocarbon-CO2 interaction increases more dramatically than water-CO2, and thus more CO2 dissolved in the oil phase. The interaction of Hydrocarbon-CO2 comes mainly from the interaction of CO2 with the aromatic compound in each system. Furthermore, as the polarity of the aromatic component increase, the interaction of HydrocarbonCO2 increases. For example, the hydrocarbon-CO2 interaction of system C is higher than in system A because CO2 interaction with xylene is higher than the interaction with benzene.


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b

a

c

d

e

Figure 8. The intermolecular interaction between water, hydrocarbons, and CO2 as a function of xCO2 for systems (a) A, (b) B, (c) C, (d) D, and (e) E. Water-Hydrocarbon interaction is the summation of intermolecular interaction between water-paraffin and water-aromatic in each system. CO2-Hydrocarbon interaction is the summation of the interaction between CO2-paraffin and CO2-aromatic in each system.


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5. Diffusivity We calculated the diffusion coefficients of the oil as a function of xCO2, as shown in Figure 9. The diffusion coefficients were calculated using Einstein’s formulation as follows: D=

1 d n 2 lim ∑i Ri (t ) − Ri (0) t → ∞ 6 dt

(3)

The diffusivity of all the simulated systems was increased as a xCO2 increase in the system, however, this increase is not identical in all systems. It could be observed that the increase in the diffusivity has the following order: system A > system B > system C > system D. This suggests that there are two determinants control the oil diffusivity due to the addition of CO2 which are the molecular weight and the system polarity. The diffusivity of the heavier and more polar systems affected less significantly than the lighter and less polar systems. Also, sc-CO2 also affected the asphaltenes slightly. The diffusivity of asphaltene is an important parameter which could be used to achieve crucial details such as the asphaltene molecular size as well as the aggregation state. MD simulation provides the potential to calculate the diffusion coefficient of asphaltene in different solvents and combine these information with the asphaltene behavior. The experimental studies predicted that the asphaltene diffusion coefficient is of order 10-10 m2/sec in a media of heptol (heptane and toluene) 33. This value is in a good agreement with our result in the absence of CO2 in the system (about 0.2 x 10-5 cm2/sec). The addition of CO2 to the system cause an increase in the diffusivity of asphaltene in the system. This increase supports our earlier result which states that CO2 releases the some of the interfacial asphaltenes molecules and displace it to the oil bulk.


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a

b

c

d

e

Figure 9. The diffusion coefficients of the hydrocarbons in systems (a) A, (b) B, (c) C, (d) D, and (e) E as a function of xCO2.

Conclusions We performed a series of MD simulations to investigate the effect of CO2 addition to different water/oil systems. The oil phase composed of a binary mixture of paraffinic and aromatic 14 ACS Paragon Plus Environment

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components. We also introduced asphaltenes to the systems since it stabilizes the water/oil emulsion. The main findings of this study can be listed as follows: I.

II.

III.

IV.

V.

CO2 accumulates at the interface for all the simulated systems. At low xCO2, the accumulation at the interface is higher in the systems containing higher paraffin molecular weight for the same aromatic compound. In other words, the accumulation in the system composed of benzene and octane is higher than the accumulation in the case of benzene hexane, similarly, the accumulation in the case of xylene-octane is higher than that in xylene-hexane. Furthermore, when xCO2 increases, more CO2 prefer to be dissolved in the oil bulk than accumulated at the interface. The driving force of CO2 accumulation is the difference in the IFT between water/CO2 and water/hydrocarbons. Aromatic compounds prefer to accumulate at the interface when xCO2 is low in the system. However, as CO2 content increases, it displaces the hydrocarbons away from the interface and the accumulation of the aromatic compounds vanish. At very high xCO2 (i.e. 0.8), the hydrocarbons accumulated in the center of oil bulk. The accumulation of the aromatics components at low xCO2 is derived by the weak hydrogen bonding between aromatic ring and water proton. At low xCO2, all asphaltene molecules stacked at the interface due to the formation of H-bonds and the interaction of aromatic core with water molecules. However, as xCO2 increases, asphaltenes begin to dissolve in the oil bulk. Additionally, the asphaltenes dissolved in the oil bulk aggregates with each other, forming dimers. Asphaltenes also reduced the interfacial tension in the absence of CO2 by about 6 mN/m but once CO2 added to the system, this effect has been diminished and the systems behaved as there are no asphaltenes in term of interfacial properties. The addition of CO2 dilutes the interface and increases the interfacial width. It also increases the interfacial roughness due to the penetration of CO2 molecules from the oil phase into the water phase. It also forms a film between the two phases and this film stabilized by hydrogen bonds between CO2 and oil molecules. These factors contribute negatively to the interfacial tension. The diffusivity of the oil phase increased as the amount of CO2 increased in all systems. There are two determinants of the impact of sc-CO2 on the diffusivity, which are the molecular weight and the system polarity. The heavier and more polar systems affected less significantly than the lighter and less polar systems.

This study is important in order to understand the effect of CO2 on different oil components. It describes the alteration in interfacial and transport properties of different systems composed of different components in term of polarity and molecular weight.

Author Information Corresponding Author Email for Sohaib Mohammed: [email protected]; [email protected] 15 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest.

Acknowledgement This research is supported, in part, by Higher Committee for Education Development in Iraq (HCED)/Prime Minister office.

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