Study on the Asphaltene Precipitation in CO2 Flooding: A Perspective

Dec 26, 2017 - Asphaltene precipitation is a common phenomenon in the exploitation of crude oil and closely correlates with oil recovery, especially i...
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Study on the Asphaltene Precipitation in CO2 Flooding: A Perspective from Molecular Dynamics Simulation Timing Fang, Muhan Wang, Jiawei Li, Bing Liu, Yue Shen, Youguo Yan, and Jun Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03700 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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Study on the Asphaltene Precipitation in CO2 Flooding: A Perspective from Molecular Dynamics Simulation

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Timing Fang, Muhan Wang, Jiawei Li, Bing Liu, Yue Shen, Youguo Yan*, Jun Zhang*

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College of Science, China University of Petroleum, Qingdao 266580, Shandong, China

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Abstract: The asphaltene precipitation is a common phenomenon in the exploitation of crude

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oil and closely correlates with the oil recovery, especially in CO2 flooding. In this work,

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employing molecular dynamics simulations, the asphaltene precipitation process in CO2 was

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investigated. The simulation results reveal that the CO2 could step-by-step extract nonpolar

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and light polar components from asphaltene micelle, and a two-step asphaltene precipitation

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process was illustrated. In our eight asphaltene molecule system, first, four asphaltene dimers

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formed. Two dimers get together into one aggregation in bulk, the other two dimers get

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together and adsorbed onto the silica surface. After that, the surface aggregation further

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induces the adsorption of bulk aggregation onto it to complete asphaltene precipitation. In

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addition, we also studied the pressure effect on asphaltene precipitation. Our work provided a

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molecular level understanding of asphaltene precipitation phenomenon in CO2 flooding, and

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the results have significant promise for the oil exploitation.

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Keywords: CO2 flooding; Molecular dynamics simulation; Asphaltene precipitation

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1. Introduction

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Global warming has become one of the most important environmental problems. Carbon

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capture and storage (CCS) has been widely recognized as a potential alternative which can

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reduce carbon dioxide (CO2) and mitigate climate warming effectively [1, 2]. CCS can not

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only reduce CO2 emissions, but also enhance the oil recovery (EOR); this method has been

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widely used in oil field development [3, 4]. CO2 flooding has been employed as EOR

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approach since the 1970s [5-8]. CO2 can dissolve into the crude oil and lower its viscosity,

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improving the oil mobility [9-12]. However, CO2 would preferentially extract light oil

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components and result in the precipitation of heavy oil components, especially asphaltene.

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The precipitation would reduce the effective oil flow aperture and deteriorate the oil recovery

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[13, 14]. Thus, it is necessary to reveal the precipitation phenomenon, which is helpful to

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develop a strategy to solve the problem mentioned above.

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In the past decades, the precipitation behavior has attracted great attention, and abundant

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researches have been done meanwhile. For example, Hu et al. [15] investigated the CO2

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effects on asphaltene precipitations under the reservoir condition, revealing that asphaltene

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precipitation was detected when the operating pressure approached or exceeded the minimum

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miscibility pressure. Other studies of Verdier et al. [16] and Srivastava et al. [17] suggested

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that high temperature and low pressure facilitated the occurring of asphaltene precipitation,

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and the increase of injected CO2 volume fraction also enhanced the asphaltene precipitation.

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Carauta et al. [18] studied the stability of asphaltene dimers in a series of solvents by

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measuring the distance between the molecular cores. The researchers found that the most

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stable dimers were in heptane and the most unstable dimers were in toluene. Despite

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modeling studies, it is not yet clarified in detail because it is still a notable challenge to

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observe this microscopic process under reservoir conditions to reveal key insight into the

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asphaltene precipitation in CO2 flooding.

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The mechanism of molecular attraction or separation can be described by using molecular

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dynamics simulation, which could reveal the relationship between molecular distribution and

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environment changes. An understanding of the interaction between asphaltene molecules and

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mineral surfaces in CO2 flooding, is not only germane to clarify the mechanism of their

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deposition, but also gradually important for the energy demand.

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In this study, molecular dynamics (MD) simulation was performed to investigate the

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asphaltene precipitation process in CO2 flooding at the molecular level. The present study is

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divided into three parts: Firstly, the formation and structural features of asphaltene micelle

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were analyzed. Secondly, the extraction process of nonpolar and light oil components in CO2

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flooding was presented, besides, the asphaltene precipitation could be divided into two steps:

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aggregation and adsorption to the silica surface. Furthermore, the effect of pressure on

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asphaltene precipitation was also studied. This study provides considerable insights into the

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behaviors of asphaltene molecules in CO2 flooding.

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2. Methods

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2.1. Force field

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MD

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Condensed-Phase-Optimized

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(COMPASS) were used to consider interatomic interaction [19]. COMPASS, a widely used

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all-atom force field based on ab initio and optimization by the experimental data, has been

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validated to be capable of accurately predicting structural and thermophysical properties for a

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broad range of organic and inorganic substances including CO2, silica and oil accurately [20,

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21]. COMPASS includes the bonded and non-bonded potential. The bonded potential consists

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of bond stretching, angular bending, dihedral angle torsion, cross terms and out-of-plane

simulation

was

performed

by

Molecular

Materials Potential

Studio. for

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Atomistic

force Simulation

field

of

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interactions as shown; the non-bonded potential is composed of long-range electrostatic

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interaction and short-range van der Waals (vdW) interaction. The electrostatic interaction was

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revealed by Coulombic equation and the vdW interaction was represented by 9-6

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Lennard-Jones function, which are described by the following equation [22].

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 =  +  +    +  +   +  +  

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 =  2    − 3    "





 = #

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$% $&





!

'

(1) (2) (3)

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2.2. Model construction

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The microscopic structure of asphaltene will have some impacts on the asphaltene

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aggregation. There have been recent advances in the structural elucidation of asphaltene

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molecules [23] and explained the process of aggregation. Boek et al. [24] used a quantitative

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molecular representation (QMR) algorithm to generate a range of molecular representative

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asphaltene architectures, which showed that the consistency between MD simulation and

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experimental data. Based on the structure information, Hu et al. [25] revealed that pure CO2

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would enhance the hydrogen bond and the dipolar interaction in asphaltene aggregates. The

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π-π charge-transfer interaction will be greatly enhanced after CO2 injection. This indicates

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that the more aromatic clusters and heteroatoms the asphaltenes have, the greater the

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deposition of asphaltene will be after CO2 injection. Sedghi et al. [26] confirmed that the

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interactions between aromatic cores of asphaltene molecules are the major driving force for

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the association. Moreover, the length and number of aliphatic chains do not seem to have a

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noticeable effect on asphaltene dimerization. However, Costa et al. [27] revealed the

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unexpected simulation results, which showed that the π–π stacking interaction and van der

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Waals forces between aromatic rings did not result in significant aggregation, but functional

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group surrounding the aromatic core played a critical part in the molecular interaction. Thus,

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it is a long-standing debate about asphaltene molecular architecture and properties mentioned

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above.

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In general, there is some consensus that asphaltene can be divided into "continental" and

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"archipelago" types [28, 29]. Based on those two types, analysis of the asphaltene aggregation

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suggests that asphaltene molecules are generally spherical, with the archipelago model

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favoring longer prolate structures and the continental model tending toward oblate structures

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[30]. Different types may also coexist in the real asphaltene. However, in order to reduce the

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computational cost, the "average" asphaltene molecules are used to represent the system. In

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this study, the continental model (C54H65NO2S) is used, because it can form aggregates

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through the accumulation of aromatic regions and is widely used as the basis for molecular

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models [31, 32]. This simplification is feasible because recent studies have shown that a

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well-defined compound can be used to mimic the asphaltene aggregation process successfully

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[33]. The model resin is selected from the published reference [28] and saturated hydrocarbon

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is dodecane molecules. The molecules were constructed by Sketch Atom tool of the Materials

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Studio software and optimized by Forcite calculation/Geometry Optimization with COMPASS

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Forcefield.

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We know that oil samples are usually divided into four fractions, including asphaltene,

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resin, aromatic and saturated hydrocarbon [34]. However, to simplify the models, resin

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molecules was used instead of the aromatic component due to the role aromatic played is

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similar to the resin component [32]. Therefore, the oil film in this study contains asphaltene,

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resin and saturated hydrocarbon components as shown in Fig. 1, a similar approach was also

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used in some experiment research [35].

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Figure 1. Molecular structure of model asphaltene (a), resin (b) and hydrocarbon (c).

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2.3. Simulation details

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The constructed oil film contains 8 asphaltene molecules, 40 resin molecules and 32

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hydrocarbon molecules (C12) using amorphous method, which roughly corresponds to a mass

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ratio in the reported literature [36]. The dimensions of hydroxylated silica surface were 54.01

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Å × 32.27 Å × 13.71 Å. The silica surface was built by cleaving the α-quartz along the (100)

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crystallographic orientation [37, 38]. The oil film was placed on top of the hydroxylated silica

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surface. A 5 ns NVT (constant atoms number N, volume V a temperature T) simulation was

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performed to get an initial equilibrated adsorption configuration of the oil film on silica

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surface. In this simulation, the periodic boundary conditions were applied in the XY

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dimensions. Then, a 105 Å CO2 phase in thickness was placed onto the equilibrated oil layer.

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At the top of the bulk CO2 phase, a dense CO2 layer with a thickness of 5.2 Å was fixed to

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prevent the CO2 escaping and maintain the CO2 pressure [20]. An 18 Å vacuum slab in

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thickness was added outmost to eliminate the effect of periodic boundary condition. A 10 ns

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NVT simulation was implemented to observe the precipitation process. In this simulation, the

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periodic boundary conditions were applied in three dimensions. In two MD simulations, the

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temperature was kept at 373 K controlled by an Andersen thermostat [39, 40], a time step of 1

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fs was adopted, and every 1000 steps puts a frame for data analysis.

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3. Results and discussion

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3.1 Formation of asphaltene micelle

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First, we want to get the appropriate micelle structure, which is the base to research the

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asphaltene precipitation phenomenon. Fig. 2(a) gives the final equilibrium configurations, a

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micelle forms on the silica surface, and detailed observation indicates that eight asphaltene

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molecules form four dimers, as signed by four individual configures for clearness. The dimer

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formation could be ascribed to their strong π-π electron interaction [41], the further

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explanation could be found in Section 1 of Supporting Information. The micellar profile (Fig.

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2(b)) indicates the asphaltenes locate in the center, while the lighter components including

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aromatic and saturated hydrocarbons locate outside. For clarity, a structural schematic of

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micelle was presented.

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Furthermore, the radial distribution functions (RDFs) profiles (center-to-center of

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molecules) of asphaltene-asphaltene, asphaltene-resin and asphaltene-hydrocarbon were

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calculated to reveal the spatial distribution in Fig. 2(c). The RDFs could be calculated by

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equation [42]: g(r) = ρ(r)/ρo, where ρ(r) is the local number density and ρo is the bulk number

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density. The RDFs describes the probability of the particle B presents at the distance r from

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the target particle A. This probability is described by the ratio of local number density to the

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bulk number density of particle B, which reflects the spatial distribution of B around A [22].

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Here, the g(r)asphaltene-asphaltene is related to the probability of finding an asphaltene molecule at

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distance r from another asphaltene molecule as shown in Fig. 2(c). For g(r)asphaltene-resin and

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g(r)asphaltene-hydrocarbon, the peaks are further distances of approximately 4.85 and 6.15 Å

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respectively, while the peak of g(r)asphaltene-asphaltene is located at approximately 3.65 Å. Thus,

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these results confirm micellar structure characteristics as shown in Fig. 2(b). Our obtained

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micelle and interlayer distance are well consistent with reported simulation [43] and

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experiment result which is observed by X-ray diffraction [44].

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Figure 2. (a) Equilibrium conformations of the sorption system after 5 ns MD simulation. Four

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configurations were presented here to give four asphaltene dimers for clearness, (b) a cross-section view of

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the micelle and a structural schemetic. Color codes: asphaltene, blue; resin, red; alkane, green. (c) RDFs of

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different components in micelle.

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3.2 Asphaltene precipitation phenomenon

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Fig. 3(a) exhibits the dynamical precipitation process, as seen, the hydrocarbon components

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first extracted by the CO2, and then the resin molecules gradually dissolve into the CO2 phase.

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The density distribution of resin and hydrocarbon at 2.5 ns and 5.5 ns was depicted in Fig.

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3(b), which reveals the process of step-by-step dissolution. In this extracting process, the

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asphaltene molecules merge together to complete precipitation. Furthermore, RDFs of the

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molecular mass center of various oil components were calculated to quantitatively display the

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precipitation process in Fig. 3(c-e). As described above, the asphaltene cores are surrounded

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by the resin and hydrocarbon components in the equilibrated micelle. Along with the

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simulation, the RDF peak of asphaltene-asphaltene gradually rises, indicating that the

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asphaltene molecule approaching together in the simulation. While the RDF peaks of

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asphaltene-resin and asphaltene-hydrocarbon gradually decrease. With these changes, the ACS Paragon Plus Environment

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separation between molecules could be reflected. The RDF changes clearly illustrate the

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asphaltene precipitation and resin (hydrocarbon) dissolution.

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Figure 3. (a) Side views for the evolutions of CO2 extracting oil components from 1 ns to 10 ns. (b)

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Density distribution of resin and hydrocarbon at 2.5 ns and 5.5 ns. Radial distribution functions for

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molecular mass centers between asphaltene and resin, hydrocarbon at (c) asphaltene-asphaltene, (d)

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asphaltene-resin and (e) asphaltene- hydrocarbon.

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Detailed observation gives a clear asphaltene precipitation process, as shown in Figure 4.

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For clarity of presentation, only asphaltene molecules and silica surface are present.

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Meanwhile, we found the initial aggregation of asphaltene molecules happens in bulk and

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silica surface simultaneously, they are displayed in Fig. 4(a) and Fig. 4(b), respectively. In this

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process, two asphaltene dimers first form and then merge into large aggregation. And then,

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these two aggregations further fuse and the bulk one adsorbed onto the silica one to complete

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the precipitation process (Fig. 4(c)). Overall, as displayed in Fig. 4(a-c), the asphaltene

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precipitation could be divided into three sequential steps including initial formation of dimer,

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the fusion of dimer, and last precipitation.

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Figure 4. The snapshots of asphaltenes aggregation process (a) happened in bulk (particle 1) and (b)

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happened on silica surface (particle 2), and (c) coalescence process of two particles.

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3.3 Asphaltene precipitation mechanism

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The interaction energies between various components were calculated to probe physical

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forces governing asphaltene precipitation as shown in Fig. 5. For instance, EHydrocarbon/CO2

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could be calculated by equation (4) [45, 46],

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()  ⁄*+, = *+,.()   − /*+, + ()   0

(4)

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Where EHydrocarbon/CO2 is the interaction energy between hydrocarbon and CO2, ECO2+Hydrocarbon

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is the total potential energy of hydrocarbon and CO2. EHydrocarbon and ECO2 represent the

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potential energy of hydrocarbon molecules and CO2 molecules. Increasingly negative energy

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represents the increasing interaction between the two components.

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Here, from Fig. 5 it can be observed that: (1) EHydrocarbon/CO2 is the major driver of dissolved

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hydrocarbon; (2) the driving force of resin dissolution comes from EResin/CO2, in addition, the

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weakened interaction between resin and asphaltene molecules can be observed from the

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change of EResin/asphaltene, but partial resin molecules still adsorbed on asphaltene aggregation;

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(3) the values of EAsphaltene/asphaltene and EAsphaltene/silica surface increased and held steady at the end

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of simulation, which promote the asphaltene precipitation. Furthermore, solubility parameters

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are also analyzed to reveal the reason of different solubility of oil components in section 2 of

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Supporting Information.

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Figure 5. Dynamic interaction energy between various components including the silica surface during

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asphaltene precipitation. (a) The interaction energy between hydrocarbon with others, (b) The interaction

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energy between resin with others, (c) the interaction energy between asphaltene with others.

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In addition, we performed umbrella sampling simulations to calculate the potential of mean

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force (PMF) [47], as shown in Fig. 6. PMF between two particles provides a quantitative

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description of the strength of aggregation. The difference in energy between the minimum of

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the PMF and the value of the PMF at long distances can be interpreted as the free energy of

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aggregation formation. For example, the value of free energy of dimer formation for

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asphaltene molecules is 31.03 kJ/mol. A higher value of free energy corresponds to a more

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stable system. Thus, the PMF comparison indicates that asphaltene molecules favor being the

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larger asphaltene aggregation in CO2 flooding.

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Potential of mean force kJ/mol

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PMFs of asphaltenes monomer-monomer 0 monomer-dimer monomer-tetramer -10

-20

31.03

-30

32.12 35.87

-40 0

4

8

12

16 。

20

com-com distance / A

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Figure 6. Potential of mean force curves for asphaltene in CO2 flooding between two monomers (black),

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between a monomer and a dimer (red) and between a monomer and a tetramer (blue).

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3.4 Pressure effect on Asphaltene precipitation

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Based on previous studies [48-50], the asphaltene precipitation is sensitive to pressure

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changes, high pressure is unfavorable to asphaltene precipitation. Here, we investigated this

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phenomenon to reveal the underlying mechanism. In order to illustrate these progresses, side

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views for the evolution of asphaltene molecules movement are shown in Fig. 7(a). Due to the

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penetration of CO2 molecules and swelling of the oil phase, the asphaltene aggregation

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became loose and disordered. This phenomenon became obvious with the increasing pressure,

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which led to weakening the interaction between asphaltene molecules. Fig. 7(b) exhibits

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PMFs of asphaltene-asphaltene at 20 MPa, 25 MPa and 40 MPa. With the increase of pressure,

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it can be seen that the value of free energy of dimer formation for asphaltene molecules

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decreased from 34.47 kJ/mol to 25.41 kJ/mol. In addition, Fig. 7(c) exhibits PMFs of

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asphaltene-silica surface at 20 MPa, 25 MPa and 40 MPa. With the increase of pressure, the

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value of free energy of the asphaltene molecules adsorbed on silica surface also became

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smaller. Therefore, high pressure helps to weaken the adsorption strength of asphaltene

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Figure 7. (a) Side views for the evolutions of asphaltene molecules movement versus the injection pressure

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(20 MPa (the density of CO2: 0.48 g/ml), 25 MPa (CO2: 0.59 g/ml) and 40 MPa (CO2: 0.76 g/ml)) of CO2

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flooding. Potential of mean force curves between two asphaltene monomers (b) and between asphaltene

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and silica surface (c).

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4. Conclusions

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In this study, the asphaltene precipitation process in CO2 was investigated by adopting

249

molecular dynamic simulations. The dynamical simulation process identifies that the CO2

250

could extract nonpolar and light polar components step-by-step, and a two-step precipitation

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process was found. First, the asphaltene dimers form and then grow up into large aggregation

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in two modes, one happens in bulk, the other happens on the silica surface. Besides, the

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surface aggregation inducing the further adsorption of bulk aggregation to complete

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precipitation was investigated. Potential of mean force calculations show that further

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asphaltene aggregation in size becomes more favored in CO2 flooding, and the aggregation in

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size and adsorption on silica surface become less favored at high pressures. Our work

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provided the understanding of asphaltene precipitation at a molecular level, and the results

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have significant promise for the oil exploitation and might show some application prospect in

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the future.

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ASSOCIATED CONTENT

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Supporting Information. Section 1: Spatial distribution of asphaltene molecules. Section 2:

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Cohesive energy density and solubility parameter for different components.

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AUTHOR INFORMATION

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Corresponding author

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*Tel.: +86 0532-86983366 E-mail: [email protected] (Jun Zhang)

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENT

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This work is financially supported by the National Basic Research Program of China

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(2014CB239204, 2015CB250904), the National Natural Science Foundation of China

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(U1262202), the Fundamental Research Funds for the Central Universities (15CX08003A,

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14CX05022A, 15CX05049A, YCX2017067).

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