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FEASIBILITY OF EXPLOITING COMPUTATIONAL DESIGN
METHODS TO DEVELOP NOVEL ASPHALTENE INHIBITORS
Final Report
1
Summary
This project was a ROPA award for a feasibility study into the scope for exploiting molecular
modelling to understand asphaltene deposition and design better prevention strategies. Of
particular interest was to determine whether full-scale atomistic modelling of asphaltene
aggregation—including explicit solvent—was now viable. This was a 15 month study, and
was a major new research initiative.
The results of the study have been extremely promising. Fully atomistic molecular
dynamics simulations of the small aggregates that form as the first stage in asphaltene
deposition and immersed in realistic solvents have been shown to be viable. Most
importantly, the stability of these primary particles has been shown to be assessable from
multi-nanosecond simulations; such simulations can be performed in a reasonable time on a
modest, modern linux cluster. Further, the difference in stability induced by solvents such as
heptane (which fosters precipitation) and toluene (which promotes solubility) were correctly
reproduced. Thus this project has established that reliable simulations of asphaltene
aggregation / dispersion can be performed at a molecular level, and that the scale of the
simulations is such that they can readily be extended to consider the influence of natural
resins or potential inhibitors. This proof of concept has already attracted interest from the oil
industry.
2
Background/Context
The exploration, production and transportation of crude oils is beset with a number of
problems arising from the deposition of unwanted solids. Probably the most intractable of
these is the precipitation of asphaltenes, which can occur at any stage of oil production and
can be responsible for blocking pipes and well heads. The term asphaltenes refers to a highly
complex mixture of heavy organic residues composed of fused aromatic and heterocyclic
rings together with paraffinic side chains. They are usually strongly polar; contain a
significant percentage of hetero-atoms, particularly sulphur; and may contain traces of metals
such as vanadium and nickel.
Asphaltene precipitates usually form as either a tarry solid or a sludge, both of which
forms can seriously impair the flow of other fluids and can be very difficult to remove. As a
consequence it is desirable to be able to predict the likelihood of asphaltene deposition
occurring so that appropriate dispersants can be added in advance. Unfortunately, the theory
for asphaltene precipitation is not yet adequate. Nor is there a sufficient understanding of the
process to adopt a rational approach to developing more effective dispersants and inhibitors.
The major barrier to both of these goals is probably the lack of a molecular-level
characterisation of the secondary structure in asphaltenes. There is a reasonably good
knowledge of their chemical composition1—individual molecules can be characterised by
planar sheets of polar, aromatic groups, edged by aliphatic side chains—although any
particular asphaltene sample will be a heterogeneous mixture of compounds that fit this
general description. There is also some information about the nature of the aggregation
process. In particular, X-ray diffraction studies2 indicate that the aggregation is a two-step
process as illustrated in Figure 1.
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Figure 1: Schematic of the
deposition process. (Taken from
Brandt et al., J. Phys. Chem., 1995, 99,
10430)
However, there are still fundamental questions about the conformation of the individual
stacks (or primary particles), their form when in the oil phase, the nature of the forces that
lead to flocculation, and how these may arise from conformational changes induced by
variations in the temperature or composition of the oil. Such issues need to be addressed
before a reliable theory can be developed, and before a rational approach to developing better
dispersants can be adopted.
Although computer modelling has proved to be an ideal tool for providing exactly this
type of information in many other applications, only simplified studies of asphaltenes had
been performed prior to this project.3 There had been some energy minimisation calculations,
but applications of the more sophisticated statistical mechanical techniques were essentially
absent. Most importantly, there had been no serious attempt to include solvent effects; yet
solvent effects clearly make the difference between dissolution and deposition. Indeed, a
working definition of asphaltenes is: that component of a crude oil that precipitates when
heptane is added to a solution of the oil in toluene. The purpose of this project was,
therefore, to determine whether the considerable advances made in molecular simulation of
materials and biomolecules could now be extended to provide realistic molecular models of
asphaltene deposition as well. In particular, to assess whether molecular dynamics
simulations with explicit solvent were an appropriate tool for studying the early stages of
Figure 1.
3
Key Advances and Supporting Methodology
AIMS AND OBJECTIVES
As outlined in the original report, the aim of this project was to assess the feasibility of using
rigorous molecular simulation methods to study the precipitation of asphaltenes from oil.
Such simulations would provide invaluable information for developing effective asphaltene
prevention technologies, but only if the simulations could be made sufficiently realistic. This
project was funded to perform exploratory simulations using a fully atomistic model of
asphaltenes in heptane and toluene in order to quantify the resources needed for a full-scale
modelling study and indicate the type information that could be obtained.
These objectives have been realised in full during this project. Atomistic molecular
dynamics simulations have been performed on collections of asphaltene molecules in the
specified solvents. Our work has demonstrated that aggregation / dispersion of asphaltene
molecules is seen on the timescale accessible to such simulations (ca. 10–8 s using modern
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linux clusters) and that the solvent effects are correctly reproduced in at least a semiquantitative manner: small aggregates of 2 and 4 asphaltene molecules are found to be stable
in n-heptane, but to disperse in toluene at temperatures around 300 K. We have also
confirmed that such simulations can provide information about the energetics of aggregation,
the solvent structure and dynamics that facilitate it, and the fundamental intermolecular
interactions that drive it. In summary, this study has verified that a full scale simulation of
asphaltene deposition is now feasible, and can provide reliable information about the
molecular mechanisms of asphaltene aggregation that would be extremely useful in devising
inhibition strategies.
DETAILED RESULTS
Simulation Methodology
The asphaltene was constructed with a composition typical of those analyses published in the
literature: the polyaromatic sheet consisted of 10 fused phenyl rings and two aromatic
heterocycles, supplemented with two aliphatic cycles and 5 aliphatic side chains with
typically 4–6 heavy atoms; both sulphur and nitrogen were present (see Figure 2).
Stacks of both 2 and 4 asphaltene molecules were constructed within the
Quanta/CHARMm package by using both constrained molecular dynamics (in vacuum) and
manual docking. Initially, a number of potential dimer conformations were constructed. The
lowest energy of these were then subjected to extensive annealing to ensure that the stacks
would remain stable at 300 K in vacuum. Several different dimer conformations were then
selected for use in the solvated simulations. Asphaltene tetramers were constructed from two
dimers, with a somewhat less extensive annealing process.
The asphaltene stacks were solvated by embedding them in a solvent box obtained
from an MD simulation of the neat solvent, and removing solvent molecules that overlapped
with the asphaltenes. NPT MD simulations were then carried out at 0.02 MPa and 300 K,
with the asphaltene stack treated as a rigid body, in order to relax the solvent to an
appropriate density. About 200 ps simulations were required for this. Finally, about 2 ns
simulation time was accumulated in which the asphaltene was treated as a fully flexible
molecule. Solvating the dimers required 255 heptane molecules or 329 toluene molecules in a
simulation box of length ca. 35 Å. For the tetramer, 482 heptane or 484 toluene molecules
were used with the resulting simulation box being about 50 Å. Several different initial dimer
geometries and one tetramer geometry were simulated in both solvents.
All atom potentials (CHARMm 27 force field) were used throughout, and electrostatic
interactions were evaluated with an EWALD sum. All solvated simulations were performed
with DL_POLY.
Results & Feasibility
Two of the key issues to be addressed in this feasibility study were (i) whether the dynamics
that characterise asphaltene aggregation and dispersion are accessible on the MD timescale
(ca. 10-8 ps), and (ii) if so, whether the atomistic simulations can correctly reproduce the
observed solvent effects. The latter is particularly important, since if the simulations cannot
reproduce the difference between heptane (which induces aggregation) and toluene (which
promotes solubility), then they can say nothing reliable about strategies for inhibiting
asphaltene deposition.
The results of this study were very promising on both counts. In general, the dimeric
stacks showed clear evidence of decomposing in toluene on a nanosecond timescale, but no
such decomposition was evident in heptane. As an example, snapshots of the asphaltene stack
after 2 ns from one of the dimer simulations are given in Figure 2. The initial dimer geometry
was very similar to the left hand structure in Figure 2, with the aromatic sheets forming a
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slipped parallel arrangement with about 25% overlap of the planar sheet and an inter-sheet
distance of 3.8 Å; this geometry is characteristic of strong π-stacking interactions. For the
simulation in heptane (left hand picture) the dimer showed no indication of instability
throughout the simulation. Indeed, if anything the stack became more stable, with the intersheet distance decreasing to 3.5 Å; the overlap of the aromatic sheets and their relative
orientation showed no observable variation during the simulation. In contrast, there is clear
evidence of structural changes for the same dimer in toluene. After 2 ns the two aromatic
sheets were clearly not parallel, and there was no longer any significant overlap of the
aromatic regions. The rate of this decomposition was found to depend on the initial
configuration of the dimer, but was typically observable on a ns timescale.
Figure 2: asphaltene dimers after 2 ns simulations in heptane (left) or toluene (right). In both
cases the initial conformation looked very similar to that on the left. Grey indicates C atoms,
yellow S and blue N.
Simulations with the tetramers confirmed these trends. As the calculations were
intended to be indicative rather than definitive, less effort was made to anneal the starting
structure of the tetramer than was the case with the dimers. While the tetramer was built from
two well-annealed dimers, some strain did remain in the packing of the two dimers, which
lead to some disordering of the tetramer in heptane. None-the-less, strong evidence of
favourable π-stacking interactions between the asphaltene molecules was retained in heptane,
but completely absent in toluene.
Some factors that can affect the stability of the stacks emerged from this preliminary
study and warrant further investigation.
(1)
The role of the solvent: Close examination of the simulations indicated that
toluene can effectively substitute for the asphaltene on the aromatic stacks,
and thereby gradually displace the π-stacking interactions between the
asphaltenes. Indeed, average 3-D distributions of toluene about the asphaltene
showed that the toluene binds most strongly, and aligns best with the aromatic
sheets, precisely in the region where the aromatic sheets originally overlapped.
It is likely that resins, which act as natural dispersants, will have a similar
effect on the asphaltene stacks and would be amenable to molecular
simulation along the lines illustrated in this project.
(2)
The aliphatic side chains: The location and dimensions of the aliphatic side
chains was found to be a limiting factor for the size of asphaltenic stack that
could be built. At the same time, the side chains appeared to provide some
protection to the stack in heptane, wrapping around the stack and restraining
the relative motion of the aromatic sheets. The distribution of chain lengths
and locations in a crude oil is likely to have a considerable effect on the ease
of aggregation and the nature of any inhibitors that would be needed.
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Resource Implications
The 2 ns simulation of an asphaltene tetramer in heptane took about 1 month on an 8
processor PIII/866 MHz cluster. Simulations of the tetramer in toluene were about 30%
faster, but the later stages of the dissolution process were found to be hindered by the size of
the simulation box, and so larger system sizes would be desirable. While this is clearly a nontrivial exercise, and will not yet allow computational screening for new inhibitors, this project
has shown it to be a viable method for obtaining detailed mechanistic information at a
molecular level. Such information is not available from experimental sources. The
computational resource is, itself, quite modest now and so more complex simulations can be
envisaged. In particular, potential of mean force studies of the addition of asphaltene
molecules to form the primary stacks, and on the interaction between two stacks (i.e. the first
and second stage equilibria in Figure 1) have been shown by this project to be viable. Some
development of coarse-grained potentials for asphaltenes—along the lines that have already
been used for polymer and biomembrane simulations—would be useful in extending the
timescale on which the aggregation can be followed. However, in the final analysis this
project has shown that existing methods and potentials are sufficient to allow realistic
molecular simulations of the molecular processes that underlie asphaltene precipitation.
4
Research Impact and Benefits to Society
The ability to control asphaltene deposition is important for oil drilling and transport, and oil
companies currently have large capital and operating costs associated with trying to alleviate
the problems they cause. Also, many of the long-term environmental problems arising from
oil spills are related to the asphaltene deposition that occurs as the oil spillages. Given the
complexity and inhomogeneity of the compounds involved, this is a very difficult problem
and a complete solution will only result from the synthesis of a large number of different
research studies. This project has succeeded in highlighting molecular modelling as
potentially a very powerful tool to add to the methods that are already being used to solve this
problem.
The success of this project in achieving this aim, and the impact of the results on the
oil and gas industry, can best be seen from the continuing positive response from a number of
different sectors of the oil industry to the PI’s research. In particular, the PI has agreement in
principle from an Algerian oil company to fund a start-up molecular modelling study on their
asphaltenic crude oils. Further, an extended research contract with Cabot Specialty Fluids
(USA) and a new research contract with Lubrizol were obtained partly due to the greater
breadth of oil-related expertise the PI could offer as a result of this project.
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Dissemination Activities and Further Research
This project was a 15 month feasibility study, and represented a major new initiative. So far
the results have been presented at one international RSC conference on Chemistry in the Oil
Industry. The work will also be presented at the international conference on Heavy Organics
Deposition in Mexico, November 2002. Both of these conferences have a high profile in the
oil industry, and so are a very effective means of targeting the key group of beneficiaries
identified in the original proposal.
Dissemination is also being achieved through publication in the Journal of Physical
Chemistry, and Energy & Fuel; the former targets the relevant academic community while
the latter is probably the most appropriate journal for reaching the oil industry. This work has
also been publicised informally via the PI’s contacts in a number of relevant industrial
organisations, including BP Exploration, Cabot Specialty Fluids (UK), Conoco, TROSS,
Statoil and Exxonmobil.
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As a direct result of these activities, the PI was approached by Sonatrach/CRD
(Algeria), and discussions are now underway with a view to molecular modelling studies of
asphaltene inhibition in Algerian crude oil.
Research is still in progress on a number of the avenues identified in this feasibility
study. Initial work will focus on potential of mean force calculations for the aggregation of
two asphaltene stacks. Pending a successful outcome to the talks with Sonatrach/CRD, we
will also perform studies to characterise aggregation of asphaltenes with a different chemical
composition to that considered in this feasibility study. A grant application for a full-scale
study, focusing on aggregation in the presence of resins, will be submitted to the EPSRC later
this year.
References
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