The researchers used molecular dynamics simulations to model asphaltene aggregate behavior in crude oil. They developed a simulation model using the MDynaMix program to model aggregates of 6 asphaltene molecules and 8 resin molecules in hexane solvent. Simulations of 2-32 nanoseconds found the asphaltene aggregates form stacked sheets with a distance of 3.74 angstroms between sheets. The aggregates had a density of 0.69 g/cm3 and limited mobility, diffusing around 3 times slower than hexane. The simulations provide insight into asphaltene aggregation and behavior in crude oil recovery.

1. Computer Simulation of Asphaltene Aggregate Molecules in Crude Oil
David Woo, Aleksey Vishnyakov
Department of Chemical and Biochemical Engineering, Rutgers University
Introduction
Asphaltene Molecules
 Oil fraction soluble in toluene, insoluble in heptane
 Primarily consists of polyaromatic material
 Most problematic fraction in crude oils, such as plugging
 Unconventional energy source and can be an important factor in enhanced oil recovery
Objectives of Research:
 To improve the overall understanding of asphaltene behavior in crude oil using coarse-grained models &
simulation tools
 Develop a simulation model for asphaltene aggregates using MDynaMix program
MDynaMix: Molecular Dynamics Program
 General purpose molecular dynamics program for simulation of either rigid or flexible molecules
 Uses standard molecular-mechanics force field including electrostatic and Lennard-Jones potentials
 Utilizes terms describing angles, dihedral angles, and covalent bonds
Force Field
The general form of the force field implemented in the MDynaMix Program:
𝑼 = 𝑼 𝐋𝐉 + 𝑼 𝐞𝐥 + 𝑼 𝐛𝐨𝐧𝐝 + 𝑼 𝐚𝐧𝐠 + 𝑼𝐭𝐨𝐫𝐬 + 𝑼𝐢𝐦𝐩𝐫
Lennard-Jones potential plot
Bonds/Angles
MDynaMix Simulation Methodology
Model Simulation
Sum of Lennard-Jones potential taken over non-bonded
atom pairs
Sum of electrostatic interactions between non-bonded
atom pairs
 Harmonic bonds & harmonic covalent angles
 Amber-type and MM3-type torsion angles
 “Improper torsions” to enforce planar structure of polyaromatics
k = force constant ϴ0 = equilibrium angle M = multiplicity factor
y = 1.4201x + 25.586
0
500
1000
1500
2000
2500
3000
3500
4000
0 500 1000 1500 2000 2500 3000
MSD(A)
Time (ps)
MSD vs Time (Resins)
0
500
1000
1500
2000
2500
0 1000 2000 3000 4000 5000 6000
MSD(A)
Time (ps)
MSD vs Time (asphaltene aggregate)
Insufficient statistics,
no linear region
Structure of molecules
Hexane (solvent)
Asphaltene (MW 600-2000Da,
low H/C ratio, polyaromatic,
most polar oil fraction)
Resin (polyaromatic+aliphatic,
higher H/C)
Molecule name Molecular Weight H/C Ratio
Asphaltene 773 Da 0.946
Resin 353 Da 1.54
Hexane 86 Da 2.33
Visualizations in Visual Molecular Dynamics
Simulations of individual molecules:
 MDynaMix program employed
 Lennard Jones parameters for aromatic carbons and alkanes from literature papers to create
the molecules for simulation.
 Bonds between carbon atoms of 1.4 Å and 120˚ angles
 Timestep of 0.01 fs, total 1200 steps.
Aggregate/Micelle Simulation
Simulation procedure
A micelle/aggregate of 6 asphaltenes, and 8 resin
molecules was created in periodic box and
compacted to experimental densities at P=100atm
and 300K
The aggregate was placed in a spherical void in bulk
solvent
Three NPT MD simulations were performed at T =
300K and P =1 atm
Timestep of 2 fs
Total simulation length 2 to 32 ns.
Box size about 60 x 60 x 60 Å3
A density of 0.69 g/cm3 is calculated after the
simulation, somewhat higher than the density of
hexane.
-20
0
20
40
60
80
100
120
140
160
0 2 4 6 8 10 12 14
RDF
R
Intermolecular RDF for asphaltene core
carbons
Conclusion
 Developed a simulated model for asphaltene aggregate system using MDynaMix program
 Density of 0.69 g/cm^3 calculated while distance between asphaltene sheets is 3.74 Å
 Mobility of asphaltene aggregate is not very fast because it is a random process
 Simulation models significantly impact the search for advanced techniques for asphaltene oil recovery and precipitation
prevention
asphaltene aggregate (solvent not shown)
 “stacking” of polyaromatic compounds
 Aggregate is kept together by dispersion/π-π interactions and
h-bonds
 Resins partially dissolved in hexane, partly “stacked” with
asphaltenes
Determining the distance between the stacks
Asphaltene in transport pipe plugging Fractal images of asphaltene
Distance
between
sheets
(3.74Å)
Acknowledgements and References
 TraPPE-UA force field used for hydrocarbons treated as pseudo-atoms
 Lennard-Jones parameters determined from single component VLCC
United-atom torsion potential for alkanes
𝑢tors = 𝑐0 + 𝑐1 1 + cos𝜑 + 𝑐2 1 − cos 2𝜑 + 𝑐3[1 + cos 3𝜑 ]
Pseudo-atom 𝝈 (Å) 𝜺/𝒌 𝐁 (K)
CH3 (sp3) 3.75 98.0
CH2 (sp2) 3.95 46.0
CH (aromatic) 3.695 50.5
Torsion 𝒄 𝟎
𝒌 𝐁
[K]
𝒄 𝟏
𝒌 𝐁
[K]
𝒄 𝟐
𝒌 𝐁
[K]
𝒄 𝟑
𝒌 𝐁
[K]
𝐶𝐻 𝑥 − (𝐶𝐻2) − (𝐶𝐻2)
− 𝐶𝐻 𝑦
0 335.03 -68.19 791.32
(1)Martin, M. G.; Siepmann, J. I. Novel Configurational-Bias Monte Carlo Method For Branched Molecules. Transferable Potentials for Phase Equilibria. 2. United-Atom Description of Branched
Alkanes. The Journal of Physical Chemistry B J. Phys. Chem. B. 1999, 103, 4508–4517.
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(4)Tanaka, R.; Sato, E.; Hunt, J. E.; Winans, R. E.; Sato, S.; Takanohashi, T. Characterization Of Asphaltene Aggregates Using X-Ray Diffraction and Small-Angle X-Ray Scattering. Energy & Fuels
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(5)Harris, K. R. Temperature And Density Dependence of the Self-Diffusion Coefficient of n-Hexane from 223 to 333 K and up to 400 MPa. Journal of the Chemical Society, Faraday Transactions 1:
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(6)Zhigilei, L. Diffusion, http://people.virginia.edu/~lz2n/mse627/notes/diffusion.pdf.
RDF first
maximum
(4Å)
r/AA
Estimated distance between asphaltenes in stack agrees with experimental
data [3,4] 3.6-4AA
Diffusion of resins and mobility of the aggregate as a whole in
MD simulations can be described using MSD
Calculated from MSD using Einstein relationship
D=1.4201 × 10−9 𝑀2
𝑆
~3 times slower than hexane diffusion