Article pubs.acs.org/EF
Cluster of Asphaltene Nanoaggregates by DC Conductivity and Centrifugation Lamia Goual,† Mohammad Sedghi,† Farshid Mostowfi,‡ Richard McFarlane,§ Andrew E. Pomerantz,∥ Soheil Saraji,† and Oliver C. Mullins*,∥ †
Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States Schlumberger−DBR Technology Center, Edmonton, Alberta T6N 1M9, Canada § Alberta Innovates - Technology Futures, Edmonton, Alberta T6N 1E4, Canada ∥ Schlumberger-Doll Research, Cambridge, Massachusetts 02139, United States ‡
ABSTRACT: A model of the dominant molecular and stable colloidal structures of asphaltenes has been proposed, the Yen− Mullins model. The formation of clusters of asphaltene nanoaggregates in toluene was reported elsewhere to occur at a concentration of a few grams per liter with a cluster aggregation number of approximately 8 (Mullins, O. C. Energy Fuels 2010, 24, 2179−2207). Here, we measure the DC-conductivity signal of toluene as a function of asphaltene concentration obtaining support for the critical clustering concentration (CCC) of a roughly 1.7 g/L in toluene. In addition, the small change in the Stokes drag at the CCC indicates that the cluster aggregation number is small, less than 10. The temperature variation of the CCC is measured to be small and within error, suggesting that cluster formation is entropically driven. Centrifugation experiments were also performed on asphaltene−toluene solutions at different concentrations. These experiments confirmed that a significant change in asphaltene aggregation occurs in the concentration range of the CCC. The CCC values from centrifugation and DC-conductivity measurements are roughly the same. The centrifugation experiments confirm a cluster size of ∼5 nm corroborating the small aggregation number found in the DC-conductivity experiments. These results add to the growing body of literature validating the Yen−Mullins model.
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INTRODUCTION Asphaltenes are the solid, colloidal phase of crude oil.2 Their importance has long been understood within a flow assurance perspective.3 In addition, asphaltene gradients in oilfield reservoirs can be used to address a myriad of reservoir complexities such as reservoir connectivity.4−7 The utilization of asphaltene gradients to understand reservoir complexities is enormously enhanced by use of the industry’s first predictive equation of state of asphaltene gradients, the Flory−Huggins− Zuo EoS (FHZ EoS).8−10 This is the only formalism that has been shown to work for condensates through mobile heavy oils in field studies5−11 although there are other new formulations to treat asphaltene gradients as well.12 The FHZ EoS formalism depends explicitly on the size of the asphaltene molecular or colloidal species present in the corresponding reservoir crude oil. The molecular and stable nanocolloidal species of asphaltenes in crude oils and in laboratory solvents have been codified in the Yen−Mullins model (cf. Figure 1).1,13,14 Asphaltene Molecules. The mean asphaltene molecular weight is measured to be approximately 750 g/mol by many different mass spectral methods15−20 and molecular diffusion methods.21−25 The number of fused rings in the asphaltene polycyclic aromatic hydrocarbon (PAH) is approximately seven as obtained by NMR,26−28 Raman spectroscopy,29 direct molecular imaging,30,31 mass spectrometry18 and optical spectroscopy studies32,33 when combined with the importance of the aromatic sextets of the Clar representation from carbon X-ray Raman spectroscopy.34 Time-resolved fluorescence depolarization studies21,22 indicate that the predominant © XXXX American Chemical Society
Figure 1. Yen−Mullins model of asphaltenes showing the predominant molecular structure, nanoaggregate (with six molecules) and cluster (with eight nanoaggregates).1,13,14 In heavy oils, the clusters dominate.
molecular architecture consists of one or perhaps two PAHs per asphaltene molecule. Laser desorption, laser ionization mass spectrometry (L2MS) studies strongly support the predominance of one PAH per molecule, the so-called island model.35 Similar conclusions were corroborated by other mass spectral studies.36 Moreover, recent L2MS and surface assisted laser desorption ionization studies have shown that (1) laser mass spectrometry is sensitive to all asphaltenes including those that aggregate37 and (2) certain laser mass spectrometry studies have relatively uniform cross sections for asphaltene molecular types and additionally can lead to minimal fragmentation.38 Received: May 11, 2014 Revised: July 22, 2014
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Figure 2. Measured asphaltene concentration gradients in a large oilfield (green ellipse in figure) in Saudi Arabia.11 An enormous concentration gradient of asphaltenes is observed in the heavy oil rim of this four-way sealing anticline. Left: the fit of eq 1 to the data in local sections of this giant field is nearly perfect. The one adjustable parameter is the cluster size; fitting this data obtains 5.2 nm cluster size whereas Figure 1 shows a 5.0 nm nominal cluster diameter. Right: the gradient over the 100 km heavy oil rim matches eq 2; fitting to the data (red squares) again gives 5.2 nm cluster size.
asphaltene nanoaggregates do not go to the oil−water interface, only asphaltene molecules load onto the interface.39,40 This is quite logical given the structure of the asphaltene nanoaggregates with their hydrophobic alkyl substituents facing outward (cf. Figure 1). Laser mass spectral studies have recently confirmed that the aggregation number of asphaltene nanoaggregates is about six.50 NMR studies,51 AC-conductivity studies,52 DC-conductivity studies53−55 and centrifugation of asphaltene−toluene solutions55,56 and live crude oil centrifugation57 all gave results consistent with small (100 °C versus room temperature laboratory results supports only small or secondary temperature effects. For a fixed temperature, all factors in Stokes drag (such as viscosity) cancel except for the radius before and after cluster formation. The ratio of slopes is equal to the ratio of radii. The cube then gives the volume difference, thus the aggregation number of the cluster. If one truly only had monomers of nanoaggregates prior to CCC, then the cluster aggregation number would be predicted to be 2, a dimer. However, some nanoaggregate dimerization, trimerization, etc. is expected prior to CCC. We interpret the cube of the ratio to mean the cluster aggregation number is very small; we believe less than 10. To
Figure 9. DC-conductivity vs asphaltene concentration in toluene for LC asphaltene acquired at room temperature. The CCC is seen to be ∼1300 mg/L. The red diamonds were used for linear fitting below the CCC (but above the CNAC) and the black triangles were used for linear fitting above the CCC.
Figure 8 has a room temperature CCC of ∼1.8 g/L. Thus, we find that there is some variability of the CCC, a rough value for virgin crude oil asphaltenes in 10−3 mass fraction, thus 10 times larger than the CNAC. Centrifugation. A robust means of seeing variation of aggregation is through centrifugation. In this context, centrifugation is not a precise measurement for CCC for several reasons. For example, there is variable collection efficiency of asphaltene species in different volume elements in the centrifuge tube. Variations in collection efficiency exist both vertically and laterally in the centrifugation tube. Nevertheless, if aggregation changes substantially, centrifugation is an excellent means to confirm this; there is no equivocation of the results. Figure 10 shows the results of centrifugation of asphaltene solutions of varying concentration. Only a short spin of ∼3 days was used, thereby providing a “cutoff size” of 4 nm for the asphaltene particles. This represents a lower limit. If the particles are growing to 5 nm particle size, then the centrifugation experiments performed here (with longer spin times for smaller particles) would underestimate the CCC. H
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However, Figure 11 shows that there is an enormous increase in the fractional accumulation of HO asphaltenes over the range of concentration including the CCC. This is a robust confirmation that there is a large change in the nature of asphaltene aggregation at the CCC. The accumulated asphaltene fraction increases to nearly 100% meaning that these newly created asphaltene clusters are ≥4 nm per design of the experiment. These experiments, designed with a 4 nm cutoff diameter, are consistent with the finding of 5 nm asphaltene clusters from field studies.11 A different set of centrifugation experiments was performed in order to obtain size limits of asphaltene clusters. A solution of fixed, large concentration of both asphaltenes (3 g/L) was centrifuged for variable lengths of time. In these asphaltene sedimentation experiments, centrifugation was carried out at fixed rotational frequency and temperature, 18000 rpm and 25 °C, respectively. The (x,y) point for each curve (100%,100%) includes any collected asphaltene solid at the bottom of the tube. Figure 12 shows that for 0.47 h spin times, there was no
Figure 10. Distribution of HO asphaltenes (in solution and separated as a solid) in the centrifugation tube for different concentrations of the asphaltene solution (in mg/L). At high concentrations, there is substantial accumulation of asphaltenes at the bottom of the tube.
Figure 10 shows that at high concentrations there is a substantial accumulation of HO asphaltenes toward the base of the centrifuge tube consistent with the formation of clusters. Figure 11 shows the % asphaltene that accumulates toward or
Figure 11. Fractional accumulation of HO asphaltenes at the base of the centrifuge tube versus asphaltene concentration. There is a huge increase over the concentration range of the CCC going to nearly 100% of asphaltenes in clusters.
Figure 12. Cumulative collection of asphaltenes (y-axis) vs the cumulative solution (x-axis) as a function of centrifugation time for a fixed concentration of 3g/L. Short spin times yield no accumulation of asphaltenes toward the bottom (slope = 1), longer spin times yield almost total accumulation at the bottom of the centrifugation tube (slope ≪ 1), providing limits for the size of asphaltene clusters. Results for two different asphaltenes, LC and HO, are shown.
at the base of the centrifuge tube as a function of the solution concentration. The CCC, the concentration where growth in the cluster size ceases, is approximately 1.5 g/L. The lower limit of 4 nm designed for the centrifugation experiments might account for the difference in the comparison between the values of CCC from centrifugation versus DC-conductivity (which is ∼1.8 g/L). At low concentrations, there is some accumulation of HO asphaltenes at the base of the centrifuge tube. Some of this accumulation is due to different sensitivity of collection efficiency of asphaltene particles versus position in the tube. Smaller particles near the far wall of the tube and lower in the tube will have similar collection efficiency as larger particles further from the wall and higher up in the tube. Consequently, centrifugation is not precise in terms of particle sizing.
accumulation of asphaltenes toward the bottom of the centrifuge tube. Table 2 shows that 50 nm particles would travel the length of the centrifuge tube in this time; the asphaltene clusters are much small than 50 nm. This same conclusion was obtained from noting that such large particles would accumulate at the base of crude oil columns in reservoirs, thus would not be present in normal crude oils. Even for the 11.47 h spin, the accumulation is only partial, indicating that the clusters are less than 10 nm. Note that in this spin time, some accumulation of asphaltenes of 5 nm is expected. We have not tried to quantify spin time versus size except to zeroth order (Table 2) due to the variable g-forces in the centrifuge tube and the variable velocities of asphaltenes before and after they hit the sloped outer wall of the tilted centrifuge tube. At 1.96 days spin time, both asphaltenes show a high degree of accumulation at the base of the centrifuge tube. This I
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experiments provide robust determination of a large change in asphaltene aggregation at the CCC. In addition, the centrifugation and DC-conductivity experiments show that the cluster diameter is roughly 5 nm. Here, the temperature dependence of the critical cluster concentration (CCC) was found to be relatively small. This implies that, similar to nanoaggregate formation, cluster formation is predominantly mediated by entropy increase. The magnitude of the entropy change is ∼11 cal/K·mol, which is considerably smaller than that associated with nanoaggregate formation. Any high energy sites in asphaltene molecules would be (noncovalently) bonded in the nanoaggregate, thus only weak bonding remains for cluster formation. Analysis of the ratio of Stokes radii of the nanoaggregate and cluster (below and above the CCC) indicates that the cluster aggregation number is small and similar to many other findings such as in NMR and SANS and SAXS studies. The determinations found here for the number of nanoaggregates in a cluster and for the CCC are similar to those values in several other literature reports. The tight link between asphaltene nanoscience and oilfield reservoirs that are bigger by at least 13 orders of magnitude in linear dimension is a welcome development enabling a much clearer picture of reservoir fluid dynamics in geologic time. Prior to understanding asphaltene nanostructures, understanding these reservoir dynamics was simply precluded. In turn, reservoir studies are now constraining models of asphaltene nanostructures. This new feedback loop links fundamental science with major economic concerns, which is beneficial to all involved.
corresponds to a nominal asphaltene cluster size of 5 nm. A much longer spin did not cause accumulation of all asphaltene. A low concentration of asphaltene remained in the supernatant fluid after long spin times. This is consistent with the phase equilibrium model of asphaltene aggregation, which was shown to apply to high-Q ultrasonics data.44,45 That is, at low concentrations, asphaltene clusters are not expected, only asphaltene nanoaggregates or even molecules at lower concentrations. In other words, at high concentrations, most of the asphaltene population is in clusters, but there remains a small concentration of asphaltene nanoaggregates. The greater sedimentation of the HO asphaltene versus the LC asphaltene indicates that the HO clusters are somewhat larger than the LC clusters. One factor that is unknown is whether these asphaltenes slide down the test tube, tilted at 24°, at the same rate once the asphaltenes hit the far wall. Assuming this and other factors cancel out, the comparison of the data for both asphaltenes for the 11.74 h shows that the HO asphaltenes went roughly twice as far in the x-direction in Figure 12 from the x=y curve. The sedimentation velocity is proportional to the square of the particle size (cf. eq2). Thus, one obtains the estimate that the HO clusters are bigger by a factor of 1.4 than the LC clusters. If the LC clusters are 5 nm, then this gives HO clusters of 7 nm. However, comparison of the DC conductivity does not indicate that the HO clusters are much large than the LC clusters. It may be that more precise methods are needed to resolve subtle differences in cluster size. Current oilfield reservoir studies are incorporating the influence of asphaltene clusters in the dynamics of reservoir fluids.85 It appears that clusters play a critical role in establishing convective gravity currents that give rise to tar mat formation. These tar mats are often regional and can preclude aquifer support of production, leading to expensive alterations of field development planning.85 In other cases, where asphaltene cluster diffusion is overwhelmed by reservoir fluid processes that cause rapid asphaltene destabilization, upstructure mobile and permeable bitumen deposition has been encountered.85 The link between asphaltene nanoscience and large scale oil reservoir processes that occur in geologic time is enabling a first-principles approach to assessing oil production risks; this is beneficial for asset managers and oilfield scientists alike.
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AUTHOR INFORMATION
Corresponding Author
*O. C. Mullins. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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CONCLUSIONS Stable asphaltene clusters of nanoaggregates are an important component of the asphaltene nanostructure, as indicated in the Yen−Mullins model. Clusters play a crucial role in creating massive asphaltene and viscosity gradients in oilfield reservoirs. Prior to understanding the nature of asphaltene clusters, there was great uncertainty regarding the origin of heavy oil gradients in reservoirs. Now with the understanding of clusters and use of the Flory−Huggins−Zuo equation of state, large gradients of heavy oil are routinely interpreted to be either in thermodynamic equilibrium or in a state of disequilibrium due to oftenidentified reservoir fluid processes. In addition, asphaltene clusters are evidently important in reservoir processes that can result in formation of sealing tar mats that can have enormous impact on the economics of oilfields. It is essential to expand understanding of asphaltene clusters. The CCC for n-heptane asphaltenes in toluene determined by DC conductivity is at approximately 1.7 g/L, with comparable results found with centrifugation. The CCC in crude oils can be quite different and is often at a much higher concentration. The centrifugation J
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dx.doi.org/10.1021/ef5010682 | Energy Fuels XXXX, XXX, XXX−XXX
