Energy & Fuels 2009, 23, 1155–1161
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Characterization of the Asphaltene Onset Region by Focused-Beam Laser Reflectance: A Tool for Additives Screening† Javier Maruga´n,*,‡ Jose´ A. Calles,‡ Javier Dufour,‡ Rau´l Gime´nez-Aguirre,‡ Jose´ Luis Pen˜a,‡,§ and Daniel Merino-Garcı´a§ Department of Chemical and EnVironmental Technology, ESCET, UniVersidad Rey Juan Carlos, C/Tulipa´n s/n, 28933 Mo´stoles, Madrid, Spain, and Alfonso Cortina Technology Center Repsol-YPF S.A., Carretera A-5 Km 18, 28931 Mo´stoles, Madrid, Spain ReceiVed August 1, 2008. ReVised Manuscript ReceiVed NoVember 10, 2008
Deposition problems have more commonly been reported in crude oils with low asphaltene concentrations rather than in crude oils with high contents of asphaltenes, which have been shown to have a very stable behavior. Therefore, it is more important to determine the onset conditions at which asphaltenes start to flocculate rather than the amount of solids determined by standard procedures (i.e., IP-143 standard), because it typically does not provide any data about their stability. This work deals with the study of asphaltenes aggregation kinetics near the onset by using focused-beam laser reflectance measurements (FBRMs). Four crude oils and a 190+ residue have been investigated to determine the onset ratio and the kinetic evolution of the particle size distributions. The results show that there is no relation between the total amount of asphaltenes and their stability. The kinetic study of the asphaltene aggregation near the onset has shown that the time needed to reach the equilibrium is much longer than for greater n-heptane/crude oil ratios. Moreover, information on the existence of a delay and the growth rate is also provided. Concerning the use of asphaltene inhibitors, experimental data show that they act by shifting the onset n-heptane/oil ratio to higher values, whereas the kinetics and particle size distributions are unaffected. This unexpected conclusion should not be generalized to other n-heptane/oil ratios far from the onset region or other additives that may have a different inhibition mechanism. Considering that the results of the onset n-heptane/oil ratios, aggregation kinetics and particle size distributions show no apparent relation to the total asphaltene content. FBRM data constitute a valuable tool for the assessment of asphaltene deposition problems and the screening of efficient additives to control asphaltene stability.
Introduction During oil production, the changing temperature and pressure conditions of the fluids may induce the formation of solids that reduce the diameter of flowlines or even completely block them. These solids come either from the crude itself (paraffins and asphaltenes) or from the water that is in oilfield together with the crude (hydrates and scales). Asphaltenes are defined as a solubility class, being the fraction that is insoluble in n-heptane and soluble in toluene.1 As a first approach, asphaltenes can be described as the fraction of crude oil that comprises the most polar components of crude oil, because they concentrate the majority of the heteroatoms (nitrogen, sulfur, and oxygen)2 and the metals (mainly iron, nickel, and vanadium).3 Asphaltenes combine this polarity with a high molecular weight and also a high aromaticity (carbon/hydrogen ratio around 1.0 or 1.2). They are a nuisance not only by their possible segregation as solids, † Presented at the 9th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed: Dpto. de Tecnologı´a Quı´mica y Ambiental, ESCET, Universidad Rey Juan Carlos, 28933 Mo´stoles, Madrid, Spain. Telephone: +34-916447466. Fax: +34-914887068. E-mail:
[email protected]. ‡ Universidad Rey Juan Carlos. § Alfonso Cortina Technology Center Repsol-YPF S.A. (1) IP 143/90. Standards for Petroleum and Its Products; Institute of Petroleum: London, U.K., 1985. (2) Creek, J. L. Energy Fuels 2005, 19 (4), 1212–1224. (3) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1999.
which, as mention before, may cause formation damage and problems, such as the reduction in flow rates and pipe blocking, but also indirectly by the stabilization of emulsions.4 Even if the economics associated with paraffins and hydrates are greater than those for asphaltenes, this fraction poses a significant challenge from the scientific point of view, because most of the available analytical techniques suffer when they have to characterize a sample of the complexity of asphaltenes. In addition to this, attempts to standardize experimental procedures to obtain and analyze asphaltenes have failed.5 Research on asphaltenes has evolved from the pioneering studies of Sachanen6 and Pfeiffer and Saal,7 and the picture of asphaltene structure and interaction with the surrounding medium has changed accordingly; nowadays, asphaltenes are believed to form stable aggregates in the nanometer range that are destabilized because of the modifications in the bulk that are caused by the pressure and temperature changes during oil production. The contribution of the resin fraction to the stability of these nanoaggregates is still a matter of discussion.8,9 Research on this issue is enormously impeded by the fact that (4) Yen, T. F. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; p 6. (5) http://www.ualberta.ca/dept/chemeng/asphaltenes. (6) Sachanen, A. N. Petroleum Zeit 1925, 21, 1441. (7) Pfeiffer, J. P.; Saal, R. N. Phys. Chem. 1940, 44, 139–149. (8) Merino-Garcia, D.; Andersen, S. I. Langmuir 2004, 20 (11), 4559– 4565. (9) Zhao, B.; Shaw, J. M. Energy Fuels 2007, 21, 2795–2804.
10.1021/ef800626a CCC: $40.75 2009 American Chemical Society Published on Web 12/23/2008
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Table 1. Characteristics of Four Crude Oils and the 190+ Residue Used crude oil
°API
source
ASPa (wt %)
A B C D D-190+
33 21 31 27
Middle East South America South America South America South America
1.3 7.2 1.7 1.2 1.7
a
Measured by IP-143 standard.
no unified definition is currently used to separate resins and that resins and asphaltenes may also be considered as a continuum. In any case, the final goal has to be kept in mind: asphaltenes are relevant in upstream operations because of their tendency to deposit during production when the fluids are depressurized, and investigation has to accommodate this fact. From this point of view, research efforts should be focused on (i) the transition from nanoaggregates into particles, including a generalized description of the kinetics involved and the development of new anti-asphaltene chemical additives testing methods, (ii) the mechanism of deposition of unstable asphaltenes onto the walls in dynamic experiments in flowloops, (iii) the relationship between pressure and n-heptane asphaltenes, (iv) the correlation between asphaltene properties and their stability in the crude, and (v) the interaction between additives and asphaltene molecules, to explain how parts per million (ppm) amounts of additives are enough to inhibit deposition. In this work, the first point is taken into consideration: flow assurance problems caused by asphaltenes deposition are related to the asphaltene content in crude oils. However, the amount of solids determined by standard procedures (i.e., IP-143 standard) is not the only information required to predict the formation of deposits, because it does not provide any data about their stability. It is more important to determine the onset conditions at which asphaltenes start to flocculate. Moreover, the kinetics of particle growth in the first moments of instability may shed some light on the destabilization process. This work deals with the study of asphaltenes aggregation kinetics near the onset by using focused-beam laser reflectance measurements (FBRMs). Other techniques have been used in the past to assess particle sizes in model mixtures, but the first results on crude oil samples with the FBRM technique were reported by this group in a previous work.10 Herein, the approach of that work is extended to assess growth in the region near the onset. Moreover, it is demonstrated that this technique provides a tool to determine the onset n-heptane/oil mass ratio. The experimentation has also allowed for the observation of differences in behavior in the onset region, depending upon the origin and stability of the crudes. Finally, experiments with a 190+ residue are shown, to illustrate the effect of asphaltene inhibitors on both the onset and the particle size distribution (PSD). Experimental Section Chemicals and Materials. Four crude oils and the 190+ residue of one of them were studied to determine the onset ratio and kinetic evolution of the PSD. Table 1 shows the density, source, and asphaltene content of the different oils tested. Asphaltene contents were determined by means of the IP-143 standard.1 The asphaltene content of the D-190+ residue is in agreement with the asphaltene wt % of crude oil D, considering that the residue represents 74.8 wt % of the crude oil. Crude oils A, B, and D have shown to be (10) Calles, J. A.; Dufour, J.; Maruga´n, J.; Pen˜a, J. L.; Gime´nez-Aguirre, R.; Merino-Garcı´a, D. Energy Fuels 2008, 22, 763–769.
Figure 1. Example of the conversion of CLD to PSD for crude C.
stable from the point of view of asphaltene deposition on upstream applications, whereas C showed deposition problems. n-Heptane reagent grade (density of 0.68 g/cm3; 99% pure) was obtained from Scharlab S.A. Three commercial additives have been used to assess their efficiency on the stability of one selected crude oil. They were used as received, at concentrations ranging from 50 to 200 ppmV. p Value Measurement. Crude oil is stirred in a thermostatted beaker at 25 °C. Heptane is used to titrate the crude, by means of discontinuous additions every 10 min. After each titration, a sample is observed at the microscope, to detect asphaltene particles. Onset Detection with Near-Infrared (NIR) Spectroscopy. Following the same procedure as in the p value measurement, changes in absorbance at 1600 nm are used to detect the presence of asphaltene particles. PSD. A FBRMs equipment FBRM S400A LASENTEC manufactured by Mettler Toledo (Columbus, OH) was employed. The FBRM instrument operates by scanning a highly focused laser beam at a fixed speed across particles in suspension. As the solution is stirred, particles move along with the fluid. When a particle crosses the area where the beam is focused, it reflects part of the light. The duration of the signal of the backscattered light from these particles renders a characteristic measurement of the particle geometry, i.e., a chord.10 The number of recorder chords is reported as counts. More details of the application of this technique in flocculation processes can be found elsewhere.11 Crude oil samples were tested at room temperature by placing 40 g of crude oil in a stirred glass beaker. The stirring rate was fixed at 400 rpm, after tests at different rates. To reach the desired onset n-heptane/oil ratio (R), a micropipette was used to titrate the sample with n-heptane using volumetric steps of 0.1 mL/g of oil. Following the evidence that state that the aggregation of asphaltenes is slow,12 10 min of stabilizing time was allowed between injections. After each injection, chord length distributions (CLDs) were recorded every 10 s in 90 channels, of sizes from 1 to 1000 µm, following a geometric progression of ratio 101/30, ∼1.0798. Record time was optimized to obtain representative results with a low signal-to-noise ratio. The raw outcome of the FBRM instrument is a CLD, which is influenced not only by the particle size but also by their shape. To estimate the actual PSD, the inversion of a model that gives the probability of every chord length as a function of the particle size is required. This model allows for the estimation of the CLD corresponding to a given PSD, after including a shape factor. The (11) Blanco, A.; Fuente, E.; Negro, C.; Tijero, J. Can. J. Chem. Eng. 2002, 80, 734–740. (12) Correra, S.; Donaggio, F.; Capuano, F.; Onorati, N. In Proceedings of the 4th Petroleum Phase Behavior and Fouling Conference, Trondheim, Norway, 2003.
Characterization Onset Asphaltenes FBRM AdditiVes
Energy & Fuels, Vol. 23, 2009 1157 Table 2. Asphaltene Onset Values Determined by FBRM crude oil A B C D D-190+
Figure 2. Example of the asphaltene onset determination for crude C. The inset shows the experiment and the onset for a longer time.
model for spherical particles by Hukkanen and Braatz13 has been applied herein. Figure 1 illustrates the transformation of the distribution once the CLD counts are converted into the numberbased PSD (also reported as counts). As clearly seen, the maximum in size is shifted to longer values while the number of counts at the maximum is increased. More details about this procedure can be found elsewhere.10
Results and Discussion Asphaltene Onset Determinations. A preliminary study was performed to tune up the experimental procedure, mainly the time scale and the amount of sample. Both parameters determine the instrument signal noise at the onset, and thus, optimal values were selected to minimize it. On the other side, for crude oils with high values of the wax appearance temperature (WAT), the measurements were affected by the presence of paraffin solids. However, whereas the recorded signals for paraffin solids decreased along time because of the dilution and solubility effects of the added n-heptane, a clear increase was observed when the asphaltene solids appeared, allowing for a clear distinction between the two types of solids. After the preliminary experiments, a set of tests were performed to determine the onset of the different crude oils using the procedure described in the Experimental Section. Figure 2 represents an example of the onset determination, showing the evolution along time of the number of counts in the range from 0 to 30 µm. As seen, the baseline of the plot was above zero, indicating the presence of solids in the sample. However, these solids do not correspond to asphaltenes but paraffins, as proven by the decrease in the signal observed after the initial additions of n-heptane. In contrast, subsequent additions of n-heptane (every 10 min in Figure 2) produced a clear increase in the recorded counts. Before reaching the asphaltene onset, the signal decreased after a short time, indicating that asphaltene solids appeared because of local concentration effects. However, once the asphaltene onset was reached, the increase in the signal was not recovered (showing a null slope in Figure 2), indicating the presence of stable asphaltene particles in suspension. This decreasing behavior below the onset and stable signal above it has been confirmed in experiments where the samples have been allowed to evolve for longer times at the onset ratio (see the inset in Figure 2). Table 2 summarizes the values of the asphaltene onset n-heptane/oil ratios for the crude oils studied in this work. The (13) Hukkanen, E. J.; Braatz, R. D. Sens. Actuators 2003, 96, 451–459.
Ronset (mLn-C7/gcrude oil) 0.70 ( 0.05 0.80 ( 0.05 0.50 ( 0.05 2.60 ( 0.05 2.00 ( 0.05
error interval corresponds to the volume of the subsequent n-heptane aliquots. The reproducibility of the measurements has been assessed by repeating 3 times the experiments with crude B. The three tests led to the same value, indicating that the standard deviation is in the order of magnitude of the error in the n-heptane incremental additions. As seen, the onset ratios (Ronset) were influenced by neither crude oil sources nor the total asphaltene content determined by IP-143 standard. The value corresponding to the D-190+ residue is in agreement with that of crude D, considering that the residue represents 74.8 wt % of the crude oil. This seems to indicate that the removal of lightend compounds does not modify the stability of the asphaltenes. This effect is reasonable considering that the asphaltene onset ratios were around 2, which means that the concentration of heptane significantly exceeds that of the crude. The conclusion is that the effect of the dilution with n-heptane seems to be more significant than the light-end composition of the oil. For comparison purposes, asphaltene onset of crude D has been determined by two conventional techniques: (i) determination of the p value microscope and (ii) a NIR spectroscopic measurement, leading to values of Ronset of 2.27 ( 0.05 and 2.12 ( 0.05 mLn-C7/gcrude oil, respectively. Aggregation Kinetics of Asphaltenes at Onset. A study of the kinetics of the asphaltene aggregation was carried out to observe differences between crude oils related to their source and sample characteristics. Figure 3 shows the evolution with time of the number of counts in the 0-30 µm range recorded in samples with the onset values of n-heptane/oil ratios reported in Table 2. In all cases, zero time corresponds to the mixing time of the n-alkane with the sample. Important differences can be observed in the asphaltene aggregation kinetics shown by every sample when analyzing the main features of the recorded plots: (i) Initial delay. Although the onset value corresponding to the appearance of the first asphaltene solids can be observed by small increases in the recorder signals, in some cases, the macroscopic precipitation leading to significant amounts of solids took place from the beginning, whereas in other cases, an initial delay was observed. This fact is obviously related to the stability of the asphaltenes nanoaggregates against growing into bigger particles. (ii) Slope. Once the macroscopic precipitation of asphaltene starts, the slope of the plot is a measurement of the asphaltene precipitation rate. (iii) Absolute signals. The values of the number of counts registered in the experiments are related to the amount of solids in the sample. (iv) Time to reach equilibrium. A constant value of the recorded signal is achieved after short times, indicating that the equilibrium of the asphaltene solids formation is achieved. Figure 3 shows that different crude oils could present very different behaviors attending to these criteria. For instance, crude A showed no initial delay for the asphaltenes precipitation, but the slope of the plot is sufficiently low not to reach equilibrium after more than 13 h (see the inset in Figure 3a), even considering that the absolute values of the signal indicated the formation of a high number of particles. Consequently, the aggregation kinetics near the onset is much slower than for a
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Figure 3. Kinetics of asphaltene aggregation at the onset n-C7/oil ratio: (a) crude A, (b) crude B, (c) crude C, (d) crude D, and (e) D-190+ residue.
high n-heptane/crude oil ratio. In a previous work,10 we showed that, for high values of the n-heptane/crude oil ratios, equilibrium is reached after less than 100 s. On the other hand, the amount of solids (and consequently, the number of recorded counts) is very dependent upon the n-heptane/crude oil ratio. For that reason, the comparison of the different crudes in Figure 3 has been focused on the kinetics during the first 4 h. Crude B presented an initial delay of more than 1.5 h, but once the macroscopic precipitation starts, it took place at a very high rate, leading to very high values of the signals without reaching equilibrium, which could be explained by the high amount of asphaltenes of this sample (Table 1). Crudes C and D showed similar patterns, with relatively short initial delays (a bit longer
for crude D) but fast aggregation kinetics that led to precipitation equilibriums after a few hours. However, although these two samples present similar total asphaltene contents (Table 1), the absolute values of the signal for crude D was several times higher, indicating that the fraction of the total asphaltenes that precipitates at the onset is greater than the one of crude C. Finally, the D-190+ residue presented a behavior very similar to crude D but with a slightly higher initial delay and somewhat slower kinetics. However, the most important difference is that the absolute values of the signals are 20 times lower. This fact indicates that, although the onset ratios of the residue and the crude are very similar, the amount of solids that precipitate at the onset are much lower for the residue.
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Figure 4. PSDs at the onset n-heptane/oil ratio: (a) crude A, (b) crude B, (c) crude C, (d) crude D, and (e) D-190+ residue.
Onset PSD. PSDs of the precipitated asphaltenes can be estimated from the original CLDs provided by the FBRM instrument using the inversion of a PSD-CLD model, as shown by Calles et al.10 Figure 4 represents the evolution with time of the PSDs for all crude oils tested in this work during the kinetic tests at the asphaltene onset ratio shown in Figure 3. Similar trends were observed in all cases, with increasing values of the amount of precipitated asphaltenes, whereas the shape of the PSD remains unaffected. Concerning the shapes of the PSDs, in contrast with the almost Gaussian distributions exhibited by crude C, D, and D-190+ residue, crude A and B show wider curves with long
Table 3. Values of the Maximum of the PSD and the Mean Particle Size of the Tested Crude Oils after 15 000 s at the Asphaltene Onset Ratio crude oil
maximum of the PSD after 15 000 s (µm)
mean particle size after 15 000 s (µm)
A B C D D-190+
5.0 5.0 3.0 4.0 3.5
7.5 7.0 3.0 3.7 3.1
tails toward higher values of particle size. Table 3 summarizes the values of the maximum of the PSDs and the mean size after
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Figure 5. Effect of anti-asphaltene additives on the onset n-heptane/ oil ratio of the D-190+ residue.
15 000 s at the asphaltene onset ratio kinetic curves. Although all of the values of the maxima are quite similar, significant differences are observed in the mean particle size estimated for crude A and B experiments. When the results shown in Table 3 are compared to those reported in Tables 1 and 2, no apparent correlation can be found. Whereas crude oils A and B show similar PSDs and asphaltene onset values, their aggregation kinetics and total asphaltene contents are quite different. In contrast, crudes C and D show very similar PSDs and kinetics but very different onset values. These results indicate that the formation of large aggregates of asphaltenes cannot be easily predicted by either the total asphaltene content of the crude oil or the onset n-heptane/oil ratio. Testing of Anti-asphaltene Additives. Once the onset n-heptane/oil ratios and the kinetics and PSDs were determined, the next study was focused on the evaluation of the effect of asphaltene inhibitors on the asphaltenes aggregation. The study has been carried out with the D-190+ residue, because it does not present manipulation problems because of the loss of light ends and is therefore easier to handle. Three different commercial additives have been used (named as A, B, and C). Additives were employed in three increasing concentrations in crude oil from 50 to 200 ppmV. Figure 5 illustrates the increase in the onset n-heptane/oil ratio induced by the presence of the additives. Even at very low concentrations, the additives showed a great efficiency in shifting this ratio to greater values. In this example, with D-190+ residue, additive A is shown to be the most appropriate chemical (from those studied) to increase the stability of asphaltenes. Concerning the influence of the additives on the asphaltene aggregation kinetics near the onset, Figure 6 shows the comparison between the time evolution profiles of the recorded signals in the absence of additives and after addition of 100 ppmV of each one. Tests have been carried out at the Ronset ratios stated in Table 2. No significant differences can be observed in the trends. Similar results are obtained when analyzing the PSDs (Figure 7). In this case, neither the shape nor the mean particle sizes of the asphaltene particles were affected by the presence of the additives. Only small differences in the absolute values of the signal are observed, in both Figures 6 and 7, which cannot be considered as indicators of significant differences in the amount of solids precipitated. Summarizing, it seems that the inhibition effect of antiasphaltenes chemical additives is based on the shifting of the
Maruga´n et al.
Figure 6. Effect of 100 ppmV of the additives on the kinetics of asphaltene aggregation for D-190+ oil.
Figure 7. Effect of 100 ppmV of the additives on the PSDs recorded after 2000 s for D-190+ oil.
onset n-heptane/oil ratio to higher values, somehow stabilizing the asphaltene nanoparticles against the aggregation. However, once the Ronset is achieved, the growing kinetics and PSDs of the asphaltene particles are not modified. Further experiments are in progress to generalize this conclusion to other n-heptane/ oil ratios far from the onset region or other additives (maybe with different inhibition mechanisms). Conclusions The potential of a crude oil for generating asphaltene deposition problems in industrial applications is obviously related to the stability of asphaltenes (represented by the onset) but also the kinetics of the aggregation phenomena. FBRM allows not only the determination of the n-alkane/oil ratio of the asphaltene onset but also the kinetics of the aggregation, including the possible existence of initial delays and the time required to reach equilibrium, and information about the amount of solids that really precipitates near the onset. In this sense, this work proves that there is no relation between the total amount of asphaltenes (determined through a usual protocol, i.e., IP-143 standard) and their stability, presenting FBRM as a very sensitive technique to test the kinetic behavior of the asphaltene aggregation in crude oil near the onset to predict
Characterization Onset Asphaltenes FBRM AdditiVes
the possible generation of deposition problems in exploration and production facilities. According to the results reported in this work, the tested antiasphaltenes additives act by shifting the onset n-heptane/oil ratio to higher values, whereas the kinetics and PSDs are unaffected. This unexpected conclusion should not be generalized to other n-heptane/oil ratios far from the onset region without further experimentation or other additives (maybe with different inhibition mechanisms). In any case, this work has shown that FBRM constitutes a very powerful technique for the characterization of the asphalt-
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ene aggregation phenomena near the onset. Results of the onset n-heptane/oil ratios, aggregation kinetics, and PSDs show no apparent relation to the total asphaltene content but represent very valuable information for the prediction of asphaltene deposition problems and the screening of efficient anti-asphaltene additives. Acknowledgment. Thanks are due to L. Gamo for his help with some of the experiments. The authors also acknowledge the financial support of Repsol-YPF. EF800626A
