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Novel Dispersant for Formation Damage Prevention in CO2: A Molecular Dynamics Study Evan Lowry, Mohammad Sedghi, and Lamia Goual Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01512 • Publication Date (Web): 30 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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Novel Dispersant for Formation Damage Prevention in CO2: A Molecular Dynamics Study Evan Lowry, Mohammad Sedghi*, Lamia Goual Department of Chemical and Petroleum Engineering, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming 82071, United States

ABSTRACT The injection of supercritical carbon dioxide (sc-CO2) into depleted or overproduced oil reservoirs has become a popular practice for enhancing oil recovery over the last several decades due to the favorable mobility characteristics of miscible CO2 floods. Unfortunately, dissolved CO2 often causes mass precipitation of heavy-end hydrocarbon fractions such as asphaltenes, leading to formation damage. The prevention of formation damage ensuing from asphaltene deposition in porous rocks often requires the use of chemical additives such as solvents or inhibitors. Effective additives are able to disperse asphaltene aggregates by curbing their growth in the bulk phase. In particular, CO2-soluble polymers represent a promising type of asphaltene dispersants since they can eliminate the need for large amounts of hydrocarbon solvent additives to injection mixtures. However, most of these polymers have been relatively untested and their interactions with asphaltenes in sc-CO2 are still unclear. In this study, a siloxane-based polymer was introduced as a potential candidate for an effective, soluble, and environmentally friendly polymeric dispersant.1 Molecular Dynamics (MD) simulations and calculations based on the Flory-Huggins polymer solution theory were conducted to demonstrate the superior low-pressure solubility of this new polymer as compared to two other commercially available polymers, polyvinyl acetate and polydimethylsiloxane. The new polymer was also shown to effectively maintain the dispersity of three different types of model asphaltenes in bulk sc-CO2 by maintaining low aggregation numbers and reducing the interaction energy between the asphaltene molecules. In addition, the polymer was effective at preventing the adsorption of polar asphaltenes onto calcite surfaces in sc-CO2 via hydrogen bonding.

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1. INTRODUCTION Enhanced oil recovery (EOR) through carbon dioxide injection is becoming a very common procedure in oil reservoirs that have already been produced extensively via primary recovery mechanisms. CO2-EOR has been shown to increase production and enable up to 4-12% of original oil in place (OOIP) in extra production.2 CO2 is often miscible or nearly miscible with the produced fluid enabling more favorable mobility and enhanced displacement due to swelling of the oil phase.3 However, compositional changes induced by the introduction of sc-CO2 to the reservoir fluid may lead to the precipitation of heavy organic compounds, such as asphaltenes, blocking the near-wellbore pore spaces and depositing in production facilities, resulting in significant and costly production losses.4 Asphaltenes are heavy polyaromatic macromolecules that can become unstable as pressure and oil composition are varied.5 They often contain heteroatoms (such as nitrogen, oxygen, and sulfur) in their structure, which may enhance their van der Waals interactions through pi-pi stacking of their aromatic cores, leading to the formation of nanoaggregates.6 Upon destabilization, nanoaggregates have a high propensity to grow into clusters that constitute the building blocks of flocculates and precipitates.7 Several studies have investigated CO2-induced asphaltene precipitation and deposition. Kalantari-Dahaghi and others showed that bulk phase separation of asphaltenes occurred by addition of 40-60 mol% sc-CO2.8 Deo and Parra reported an onset of asphaltene precipitation near 20-30 mol% sc-CO2.4 Ibrahim and Idem investigated the precipitation tendency of asphaltenes in three different oil samples mixed with varying amounts of CO2 and nonylphenol as a dispersant.9 They found that the precipitation onset point, as a function of CO2 fraction, decreased as the asphaltene aggregation tendency increased while the precipitation onset point increased when the paraffin fraction of the asphaltene was high. In addition, it was noted that asphaltenes with higher heteroatom content showed less precipitation in the presence of sc-CO2. In an early study, Wolcott and others used core flooding and microscopy to study CO2 induced asphaltene deposition.10 The results showed that calcite and clay mineral sites were responsible for the bulk of asphaltene deposition during the CO2 injection. Takahashi and others used sandstone and carbonate cores along with compositional simulations to study asphaltene deposition on specific minerals when exposed to CO2 flooding.11 The study showed that large 2 ACS Paragon Plus Environment

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amounts of deposition occurred when CO2 was in excess of 50 mol%. Asphaltene deposition occurred more within the carbonate cores as compared with the sandstone cores. A number of different types of chemical treatments have been suggested for preventing the aggregation and precipitation of asphaltenes from sc-CO2 during flooding operations. Successful attempts have been made to chemically inhibit the precipitation of asphaltenes by mixing various proprietary additives with the CO2 injection stream in a study by Yin et al.12 Hu and Guo further investigated the effects of various alkylbenzene-derived amphiphiles on asphaltene precipitation. Using a high-pressure mixing cell, the asphaltene precipitation in sc-CO2 with various additives was measured. The experiments determined that certain alkylbenzene sulfonic acids were effective at reducing the amount of precipitated asphaltenes.13 Comparatively few molecular level investigations of sc-CO2 induced asphaltene precipitation have been carried out. Headen and Boek suggested that d-limonene is an effective inhibitor of asphaltene aggregation when added to sc-CO2 at 50 mol%.14 Using the OPLS-AA force field, molecular dynamics (MD) simulations were carried out showing that a one-to-one mixture of sc-CO2 and d-limonene reduced the aggregation of 5-6 asphaltene molecules by nearly 70% compared to just CO2. Recently, polymeric dispersants have shown better results at maintaining the polydispersity of asphaltenes when compared with alkylphenols.15 Sedghi and Goual showed the effectiveness of d-limonene and polyvinyl acetate (PVAc) at significantly inhibiting the aggregate growth of two types of model asphaltenes.16 PVAc was chosen due to its uniquely high solubility in sc-CO2 as compared with most common hydrocarbon polymers. Two different model asphaltenes were used, one containing a hydrogen bonding OH group as well as nitrogen and sulfur heteroatoms and the other containing only nitrogen and sulfur heteroatoms. It was determined that PVAc was effective primarily due to its hydrogen bonding interactions with asphaltenes through the vinyl functional groups. The results indicated that PVAc was effective at reducing the aggregation of asphaltenes in CO2, however large amounts of limonene were needed in order to prevent formation of a separate polymer-asphaltene phase.16 In an application setting, the addition of large amounts of solvent becomes expensive. Due to this fact, it is desired to find a single polymeric additive that maintains solubility in sc-CO2 while effectively dispersing asphaltenes without the aid of a solvent such as d-limonene or BTX (benzene, toluene, xylene).


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One major problem with many polymers is their relatively low solubility in sc-CO2. CO2 is a weak solvent due to its non-polar bond configuration and short-range quadrupole interactions.17 In the past, most CO2-philic polymers have been fluorocarbons, which are not environmentally friendly18, or silicone based polymers such as polydimethylsiloxane (PDMS).19 PDMS is both environmentally friendly and quite soluble in sc-CO2 in its lower molecular weight oligomers. Due to the lack of present literature covering polymeric dispersants for sc-CO2, we propose a possible polymer dispersant based on a silicone backbone that shows promise for exhibiting high solubility in sc-CO2 and containing hydrogen bonding functional groups to enhance the dispersity of asphaltenes. The structure of this paper is as follows. In section 2, the molecular dynamics methods are introduced along with three polymers and three asphaltenes that are presented for testing in simulation. A method for calculating the solubility parameter of polymers using MD is demonstrated along with the Flory-Huggins polymer solution theory that is used to provide qualitative predictions and compare the solubility of the three polymers. Bulk phase as well as adsorption simulations were used to determine the effect of each polymer type. Section 3 presents the results of the simulations and an ensuing discussion of the implications. Finally, section 4 provides the major conclusions from the simulation results and summarizes the possibilities for future work.

2. METHODS All MD simulations were carried out using GROMACS 5.1.0 software. In order to maintain consistency with previous literature, the transferable potentials for phase equilibria force field (TRAPPE-FF) with virtual sites were used to model sc-CO2 and the OPLS-AA Force Field20 was used to parameterize the additional molecules needed for the simulations.16,14 As was shown by Sedghi & Goual, the TRAPPE-FF can accurately reproduce experimental densities for supercritical CO2.16 Two different asphaltene structures were used for the simulations and were based on those used in several previous studies.6,7,21 The model asphaltenes were chosen in order to investigate the effects of polar heteroatoms and functional groups within the structure. Figure 1 shows the structures of all the molecules used in the simulations. Three different polymers were selected for use in the simulations. Polydimethylsiloxane (PDMS) is a silicone-based polymer that exhibits very high solubility in sc-CO2.22 PDMS was chosen 4 ACS Paragon Plus Environment

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since it has been widely used as a silicone stabilizer for CO2 phase polymerizations and due to its exceptional solubility and gelling properties.23 Polyvinyl acetate (PVAc) is a common industrial polymer that has the benefit of being a hydrocarbon-based polymer, which significantly reduces cost and environmental concerns. PVAc exhibits an unusually low glass transition temperature and is available in low molecular weight chains, leading to relatively high solubility in scCO2.19,24 In addition, PVAc has been shown to be effective at aiding asphaltene dispersity.16 In an effort to increase the asphaltene dispersion capability of PDMS while maintaining its superior solubility characteristics, a third polymer was introduced by copolymerizing dimethyl siloxane with propyl acetate methyl siloxane to create PDMS-g-Propyl Acetate. Variations of this polymer have been previously synthesized and shown to possess superior solubility in sc-CO2 by Fink et al.1 Table 1 shows the number of repeat polymer units and the overall molecular weight for each polymer used. 2.1 Solubility of PDMS-g-Propyl Acetate Since there was no available data on the solubility of the chosen variant of PDMS-g-Propyl Acetate (hereafter referred to as PPAc) polymer, simulations to obtain an estimate of the Hildebrand solubility parameter were conducted using a 10  10  10 nm3 periodic box containing the polymer. The box was equilibrated for 10 ns to a pressure of 500 bar while undergoing a temperature annealing sequence up to 800 K and back down to 300 K in order to cause the polymer to achieve a semi-solid state configuration. After the initial equilibration sequence, 10 ns simulations were carried out at pressures of 100, 150, 200, 250 and 300 bar to gather data points for averaging. After simulation, the enthalpy of vaporization was calculated by considering the difference in enthalpy of a single polymer simulated in the gas phase and the equilibrated, compressed polymer at the desired pressure. This methodology used for calculating the solubility parameter is very similar to that outlined by Chen et al.25 From the enthalpy of vaporization and the molar volume, the total solubility parameter can be calculated easily via Eq. (1) and an average value taken over the pressure range. 𝛿ℎ = √

Δ𝐻𝑣𝑎𝑝 −𝑅𝑇

(1)

𝑉𝑚


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The same method was used to calculate the solubility parameter for PVAc in order to verify the simulation results by comparing with available literature values. The literature value for the solubility parameter for PVAc was taken from Barton26 and data for PDMS was obtained from Lee et al.27 CO2 solubility parameters were calculated based on data from Linstrom & Mallard28 and correlations presented by Barton26 presented here as Eq. (2). 𝛿=

1/2

1.25𝑃𝑐 𝜌𝑟 𝜌𝑟 (𝑙𝑖𝑞𝑢𝑖𝑑)

(2)

In Eq. (2), 𝜌𝑟 is the reduced density of the supercritical CO2 and 𝜌𝑟 (𝑙𝑖𝑞𝑢𝑖𝑑) is the reduced density of the fluid in its liquid state, which has been previously estimated at value of around 2.7.29 Given that the solubility parameter of polymers is only a very weak function of pressure, the values did not change considerably with pressure whereas the solubility parameter of CO 2 is highly sensitive to pressure due to large changes in density in the locality of the critical point. The calculated solubility parameters served to provide an estimate of the solubility behavior of the PPAc in CO2, which can be modeled using the Flory-Huggins polymer solution theory. The Flory-Huggins theory has been previously used to model polymer solubility behavior and has been specifically used to model chemical solubility in sc-CO2.30,18 The Flory-Huggins interaction parameter often provides a decent qualitative first approximation of polymer solubility in dilute solutions and depends directly on the Hildebrand interaction parameters of both solvent and solute denoted as components 1 and 2, respectively in Eq. (3).31 𝜈

𝜒 = 𝑅𝑇1 (𝛿1 − 𝛿2 )2

(3)

For the limiting case of large polymeric chain lengths and small solvent molecular size, an interaction parameter above 0.5 is generally indicative of poor solubility.26 In most cases the Flory-Huggins interaction parameter, χ, is not a function of pressure. However due to the large compressibility of CO2, its solubility parameter, and as a result the interaction parameter, is consequently sensitive to pressure due to the extreme variations in density around the critical point.29 In addition to calculating the Flory-Huggins interaction parameter, simulations were conducted with each of the three polymers in CO2 at 150 and 300 bar to visually confirm the predictions made by the Flory-Huggins theory. To quantify the relative solubility trends of each polymer at 6 ACS Paragon Plus Environment

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pressures of 150 bar and 300 bar, the number average mass was calculated for each simulation. In order to calculate the number average, the simulation box was discretized into 0.75 nm3 grid blocks and the respective mass was calculated for each discrete box. After the CO2 was removed from the simulation box, these calculations were used to create a mass distribution of the polymer from which the total number average was calculated based on the frequency using Eq. (4). In this case, a low number average was an indicator of a more disperse polymer solution and consequently higher solubility. 𝑚 ̅𝑛 =

∑𝑖 𝑓𝑖 𝑚𝑖

(4)

∑𝑖 𝑓𝑖

2.2 Bulk Phase Simulations In order to determine the impact of polymer type on aggregation number of asphaltenes, simulations were conducted in bulk phase supercritical CO2 at 300 bar. To investigate the specific effects of asphaltene polarity and structural groups, in particular hydrogen bonding functional groups, all simulation trials were carried out using each type of asphaltene separately. The initial simulation set-up involved placing 200 asphaltenes in a 20  20  20 nm3 periodic box. The concentration of asphaltenes was set at 4 wt% for the purposes of calculating solvent quantities. Polymers were tested at concentrations of 5 and 10 wt%, by weight of asphaltenes. Initially, a 5 ns equilibration simulation was conducted at NPT conditions for each polymer and asphaltene combination in order to reach the appropriate pressure and temperature. The Berendsen thermostat and pressure-coupling algorithm were employed during the equilibrium simulations. After initial equilibration, 80 ns of NPT simulation were conducted to examine the effects of the polymer on asphaltene aggregation. The Nose-Hoover thermostat was used to regulate the system temperature at 308 K due to its ability to produce a thermodynamically accurate ensemble. The Parrinello-Rahman pressure-coupling algorithm was used for all production simulations. After simulation, the z-average aggregation number was calculated for the asphaltenes over the entire 80 ns simulation time.6 The interaction energies were also calculated between the asphaltenes and the solvent in order to determine the mechanisms of action for each polymer. 2.3 Adsorption Simulations 7 ACS Paragon Plus Environment

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Following the bulk phase simulations, the effect of PPAc polymer at preventing deposition of asphaltenes on the calcite surface was investigated. The calcite surface was modeled using parameters from Raiteri et al. and the non-bonded parameters for calcium and carbonate were obtained from the CHARMM 36 force field.33 An aggregate from the bulk simulations containing ANO asphaltenes with the PPAc polymer was extracted and placed 1 nm above the calcite surface. The aggregate was placed close to the surface in order to reduce the amount of simulation time required to observe a collision event between the surface and the aggregate. In a second simulation, the same initial configuration was used except that PPAc polymer was removed from the aggregate. Both simulations were carried out at NVT conditions for 100 ns to allow the aggregate to reach equilibrium near the surface. The pressure was modulated at 300 bar through the use of a hard-sphere piston placed above the CO2 solvent and imparted with a constant downward acceleration.16,35 After the initial 100 ns equilibration simulation at 300 bar for each system, the pressure was reduced to 150 bar and the temperature was annealed from 308 K to 390 K before returning to 308 K again at 100 ns.36 The annealing sequence was conducted in order to accelerate the kinetics of the asphaltene molecules, allowing the system to evolve to a more mature state. The pressure was reduced to determine the effects on asphaltene adsorption. At the end of the simulation, the number of hydrogen bonds and the interaction energies between asphaltenes and calcite surface were calculated.

3. RESULTS AND DISCUSSION 3.1 Solubility of PDMS-g-Propyl Acetate The initial solubility parameter calculations were used to compare the solubility of PVAc, PPAc and PDMS in sc-CO2 at different pressures. Figure 2 shows the Flory-Huggins interaction parameters calculated for PVAc, PPAc and PDMS with respect to sc-CO2. As can be noted, the interaction parameter predicts better solubility for PPAc at lower pressures when compared with PVAc. This is also in good agreement with the results obtained by Fink et al. who showed that PDMS-g-Propyl acetate possesses good solubility at low pressures which provided a reduction in cloud point pressure of 2500 psi at 22 °C when compared to analogous alkyl functionalized PDMS polymer.1


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To quantify the phase separation of the PVAc polymer and compare to the solubility of the PPAc polymer, the number average mass was calculated for the simulations with 5 wt% polymer in CO2 at the end-point pressures of 150 bar and 300 bar.32 These simulations were intended to verify the qualitative trend predictions obtained from the calculated solubility parameters and the Flory-Huggins interaction parameter. The results of this analysis are shown in Table 2 for all three polymers. The number average of PPAc polymer shows an increasing trend, indicating lower solubility at high pressures, which agrees with the Flory-Huggins prediction. Conversely, PVAc shows a decreasing number average with increasing pressure, which again follows from the previous calculations. As expected, PDMS shows no change in number average, indicating excellent solubility throughout the pressure range. This analysis, along with the Flory-Huggins calculations, demonstrates that PVAc polymer does not maintain good solubility at low pressure as compared to PPAc and PDMS polymers. Figure 3 shows images of the polymers at both pressures at the end of simulation. The visual results presented in Figure 3 somewhat support the solubility arguments, showing the complete phase separation of PVAc at 150 bar. It is necessary to note that although the Flory-Huggins calculations predict roughly equal solubility of PVAc and PPAc at 150 bar, both the number averages in Table 2 and the images in Figure 3 show that PVAc is in fact significantly less soluble in sc-CO2. This is because the Flory-Huggins model contains very little information about the specific interactions between the solvent and the solute and contains numerous simplifying assumptions. As a result the model is only useful for predicting general solubility trends. For each polymer, it can be said that the results in Figure 2 accurately represent the trend of solubility with pressure but were not useful for predicting actual solubility. The primary conclusion from Figure 2 in relation to Figure 3 and Table 2 is that the Flory-Huggins model is a decent tool for obtaining a qualitative estimate of the solubility trend of polymers but further investigation is ultimately required to determine the actual solubility. We suggest that although the Flory-Huggins model was inadequate for predicting actual solubility, it may be useful as an easy first estimate for new polymers in determining general solubility trends with pressure due to its comparative lack of computational time. Due to the results of the different methods used to assess the solubility of the three polymers, a simulation pressure was chosen for the bulk simulations with asphaltenes. Upon examining the 9 ACS Paragon Plus Environment

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results of the Flory-Huggins analysis, it first appeared that PPAc polymer might be insoluble at 300 bar, however the polymer-CO2 simulations showed that PPAc still exhibited decent solubility at 300 bar. Although PPAc and PDMS were very soluble at 150 bar, PVAc was not (see Figure 3). For this reason, 300 bar was a better choice in order to observe the full effect of all the polymers on asphaltene aggregation and allow PVAc to exhibit the best solubility. 3.2 Effect of Polymers on Asphaltene Aggregation The z-average aggregation number of the asphaltenes was calculated for each simulation frame to determine the effectiveness of each polymer for maintaining asphaltene dispersity in bulk scCO2. As was noted previously, each polymer was tested with two different asphaltene types at 5 and 10 wt%. PDMS showed no advantage in reducing aggregation. This may be due in part to lack of interaction between the PDMS and the asphaltenes, as shown in Table 3. In addition, the PDMS does not possess any side chain branching or functional groups, which may also lead to its reduced effectiveness as an asphaltene dispersant. PVAc showed excellent reduction in the aggregation number but upon closer examination of the simulation frames, did not remain soluble in the CO2. Probably due to its hydrocarbon backbone, PVAc formed a separate asphaltene-polymer phase, thereby nullifying the de-aggregating effect of the vinyl ester side functionalities. Figure 4 shows images extracted after simulation with ANO asphaltenes and each polymer type. Figure 5 (a)-(c) shows the effect of each polymer at 10 wt% for each type of asphaltene. It can be noted that although PVAc polymer outperformed PPAc polymer in terms of producing lower aggregation numbers, PVAc formed a polymer-asphaltene phase for all asphaltene types rendering it ultimately ineffective. PPAc was the best polymer for balancing solubility in sc-CO2 with polymer-asphaltene interaction. The PPAc polymer successfully reduced the aggregation state of asphaltenes while maintaining its solubility in the CO2 phase, as can be seen in the simulation image in Figure 4 (d). As expected, PPAc polymer was most successful with the more polar ANO asphaltenes, reducing the aggregation number by up to 75%. This observation indicates that the reduction in aggregation number was a direct result of the polymer side chain interaction with the polar functional groups of the asphaltene molecules. Table 3 shows the


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interaction energies between asphaltenes and various polymers as well as the asphalteneasphaltene interactions. The energy calculations shown in Table 3 clarify the primary mechanisms of interaction for each polymer. From the table, it is clear that PDMS does not interact strongly with asphaltenes as compared to PPAc and PVAc. PPAc shows a larger coulomb interaction with ANO asphaltenes compared to PDMS, indicating that its mechanism of action is primarily through polar functional groups. PVAc shows the largest interaction energy with the asphaltenes, leading to the phase separation of the polymer with the asphaltenes. This large attraction between PVAc and asphaltenes could be due to the hydrocarbon backbone, which provides favorable Van der Waals interactions, allowing the phase separation to occur more readily. Finally, PPAc shows less hydrogen bonds to the ANO asphaltenes when compared to PVAc, probably due to the higher density of polar carbonyl groups in the PVAc chain. PPAc polymer was also able to effectively reduce the intermolecular interactions between the asphaltenes as compared with PDMS and the reference case (i.e., asphaltenes with no additives). As mentioned before, PVAc showed a large apparent reduction in the intermolecular asphaltene interaction energy due to the phase separation. 3.3 Simulations with Calcite Surface To determine the effects of PPAc polymer on the adsorption of asphaltenes on the calcite surface, an aggregate was removed from the bulk simulation with PPAc polymer and placed 1 nm above the calcite surface. After the initial 100 ns equilibration simulation, the aggregate without polymer had started to form hydrogen bonds with the surface. After the 150 bar annealing sequence, the aggregate with the polymer had migrated away from the surface and ceased to interact with the calcite. The asphaltenes without polymer formed hydrogen bonds to the calcite surface, as is shown in Table 4. PPAc polymer was able to prevent formation of any H-bonds between ANO asphaltenes and the calcite surface. Figure 6 shows snapshots from the end of the simulation process for ANO asphaltenes with and without PPAc polymer. The PPAc polymer was clearly more effective for asphaltenes possessing polar functional groups that may be able to coordinate with the calcite surface. By reducing the amount of hydrogen


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bonding groups available for adsorption, PPAc polymer may be able to prevent the strong binding of the asphaltenes to the calcite surface.

4. CONCLUSIONS In this work, a PDMS variant (PPAc), co-polymerized with Propyl Acetate side chain via the method described by Fink et al.,1 was suggested as a better solution for dispersing asphaltenes while maintaining solubility in sc-CO2. Bulk MD simulations showed that polydimethylsiloxane (PDMS) was ineffective at dispersing asphaltenes despite its excellent solubility in sc-CO2. Polyvinyl acetate (PVAc) lost all solubility in the sc-CO2 due to the dominating Van der Waals and electrostatic interactions between the polymers and the asphaltenes. Preliminary simulations calculating the solubility parameter indicated that the PPAc polymer is soluble even at low pressures in sc-CO2 and bulk simulations with asphaltenes showed favorable dispersion effects while maintaining good solubility. The number average mass was also introduced and shown to verify the predictions made by the Flory-Huggins theory. Simulations with calcite mineral surface revealed that the PPAc polymer effectively inhibited formation of hydrogen bonds between the polar asphaltenes and the surface, a critical step in the adsorption of the aggregate. Based on atomistic simulations; the PPAc polymer appears to be a very promising solution to asphaltene aggregation, precipitation and deposition in sc-CO2 mixtures. Further experimental studies will be needed to verify these findings.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support of the Enhanced Oil Recovery Institute at the University of Wyoming. The authors are also thankful to Dr. Mohammad Piri for the use of Loren computer cluster.

REFERENCES [1]

Fink, R.; Hancu, D.; Valentine, R.; Beckman, E. J. Toward the Development of “CO2Philic” Hydrocarbons. 1. Use of Side-Chain Functionalization to Lower the Miscibility


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Table 1 – Number of repeat units and molecular weight of each polymer used in the simulations Polymer

Repeat Units

Molecular Weight (g/mol)

PVAc

11

949

PDMS

38

2989

PDMS-g-Propyl Acetate (PPAc)

24

3102

Table 2 – Number averages (𝐠 × 𝟏𝟎−𝟐𝟒 ) calculated for each polymer type at 5 wt% in CO2 at the end of 80 ns simulation Polymer PVAc PPAc PDMS

𝑚 ̅𝑛 150 bar

300 bar

0.256 0.198 0.199

0.188 0.211 0.198


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 3 – Interaction energies (kJ/mol) for asphaltene-asphaltene interactions and asphaltene-additive interactions from the last 40 ns of simulation Van der Waals Energy

Solution

Coulomb Energy

H-Bonds

AB - Additive

+ 10 wt% PDMS + 10 wt% PVAc + 10 wt% PPAc

-920 -34,021 -5,648

19.8 -2,157 -722

-

AN - Additive


-681 -28,351 -2,443

13.8 -1,636 -426

-

ANO - Additive


-658 -34,074 -5,617

10.7 -4,594 -2,100

91 31

AB - AB

No Additive + 10 wt% PDMS + 10 wt% PVAc + 10 wt% PPAc

-33,906 -33,556 -16,097 -29,373

-740 -836 -479 -777

-

AN - AN


-35,245 -34,916 -21,706 -33,180

-822 -859 -726 -853

-

ANO - ANO


-36,301 -35,321 -17,459 -32,364

-2,212 -2,185 -797 -1,896

58 51 17 54


Energy & Fuels

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Table 4 – Average interaction energy (kJ/mol) and hydrogen bonds between calcite and ANO asphaltenes from the last 50 ns of simulation with and without PPAc polymer Additive

Coulomb Energy

Van der Waal’s Energy

H-Bonds

No Polymer PPAc

-77 -0.9

-30 -3.8

15 0


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

AN Asphaltene

ANO Asphaltene

AB Asphaltene

Polyvinyl-Acetate (PVAc)

PDMS-g-Propyl Acetate (PPAc)

Poly (dimethylsiloxane) (PDMS)

Figure 1. Structures of asphaltenes and polymers used in the simulations


Energy & Fuels

0.8 PVAc - CO2 0.7

Flory-Huggins Parameter, χ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

PPAc - CO2 PDMS - CO2

0.6

0.5

0.4

0.3

0.2

0.1

0 0

50

100

150

200

250

300

350

Pressure, bar Figure 2. Calculated Flory-Huggins interaction parameter versus pressure for PVAc, PPAc and PDMS


Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

150 bar

300 bar

Figure 3. Polymers in CO2 at 150 and 300 bar. From top to bottom: PVAc, PPAc and PDMS. CO2 is not shown for clarity.


Energy & Fuels

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(a)

(b)

(c)

(d)

Figure 4. Images from end of 80 ns simulation. (a) No polymer, (b) PDMS, (c) PVAc and (d) PPAc


Page 22 of 24

Page 23 of 24

Aggregation Number

250

(a)

200 150 100 50 0 250

Aggregation Number

(b) 200 150 100 50 0 250

(c)

Aggregation Number

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

200 150 100 50 0 0

10

20

30

40

50

60

70

Time, ns PPAc

PDMS

PVAc

No Polymer

Figure 5. Z-average aggregation number for (a) ANO, (b) AN and (c) AB asphaltenes


80

Energy & Fuels

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(a)

(b)

Figure 6. ANO Asphaltenes after 100 ns of annealed simulation on the calcite surface. Asphaltenes shown in green and PPAc polymer is shown in red


Page 24 of 24

A Molecular Dynamics Study - American Chemical Society

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