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Study of Asphaltenes Aggregation Process in Crude Oils Using Confocal Microscopy J. Castillo,* J. Hung,† S. Goncalves, and A. Reyes Escuela de Quı´mica, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela 1020A, Apartado 47102, Venezuela Received February 25, 2003. Revised Manuscript Received September 17, 2003
Aggregation and growth of asphaltenes in crude oils of different stability were studied using the titration method and confocal microscopy. Confocal images show a new interpretation for the titration curve. Results show that the flocks growing process and flocks characteristics depend on the crude oil nature and can be related with the stability. High-resolution micrographic images demonstrate that titration trace behavior can be correlated with the crude oil stability.
Introduction Asphaltenes constitute a very important fraction of crude oils composed of molecules of different molecular weights, polarities, and solubility.1-3 They are precipitated from crude oils through the addition of low molecular weight paraffins.4 The importance of asphaltenes in the crude oil industry is due to the fact that they may flocculate during the reservoir exploitation because of changes in the thermodynamic (pressure, temperature, composition) or flow conditions. Asphaltene flocculation and deposition can block pore throats in the near well bore zone; the deposition can strongly modify stability properties and relative permeability, giving rise to a huge reduction in oil production. From a practical point of view, it is of interest to predict the conditions of incipient flocculation that delineate regions of colloidal stability from regions of precipitation. Several methods are employed to determine the colloidal stability. The p-shell point is a parameter widely used to characterize the crude oil stability; this method is very cumbersome and depends strongly on the operator experience. The aggregation and flocculation process in asphaltenes from crude oil has been investigated by different methods; one of the most used methods is based on the flocculation of asphaltene induced by the n-alkane addition (titration method).5,6 The titration method * Corresponding author. Phone: +58212-6051260. Fax: +582126052246. E-mail:
[email protected]. † Current address: Centro de Fı´sica, Instituto Venezolano de Investigaciones Cientı´ficas, Caracas 1020A, Apartado 21827, Venezuela. (1) Acevedo, S.; Escobar, G.; Ranaudo, M.; Pin˜ate, A. J. Amorin, A. Fuel 1999, 11, 774-778. (2) Acevedo, S.; Ranaudo, M. A.; Pereira, J. C.; Castillo, J.; Ferna´ndez, A.; Pe´rez, P.; Caetano, M. Fuel 1999, 78, 997-1003. (3) Rogel, E. Colloids Surf. A 1995, 104, 85-93. (4) Espinat, D.; Rosenberg, E.; Scarsella, M.; Barre, L.; Fenistein D.; Broseta, D. Colloidal Structural Evolution from Stable to Flocculated State of Asphaltene Solutions and Heavy Crudes. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; pp 183-201. (5) Laux, H.; Rahimian, I.; Butz, T. Fuel Process. Technol. 1997, 53, 69-79. (6) Andersen, S. I. Energy Fuels 1999, 13, 315-322.
consists of a plot of light intensity as a function of the added amount of n-alkane. For the plot interpretation, the trace is divided into three regions: in the first part of the curve, the asphaltenes start to form small aggregates in solution while the crude oil is being diluted, leading to an increase in the light transmission due to the dilution and lack of absorbents. When the peak maximum is reached, enough particles are present to scatter the light in such a way that the dilution is counteracted. This region is defined as the flocculation point. The behavior of the end region depends on crude oil characteristics and the effects of conditions of precipitation.5,6 In general, the intensity at the detector decreases when increasing the flocks, and when the flocks start to decant, the light intensity transmitted increases rapidly. The single-point titration of the crude oil mixture shows the influences of different factors on the stability of the colloid disperse crude oil system, but this point is useful only for observing macroscopic changes in the properties of the sample. This point reflects only average properties of the system under study. In this sense, submicroscopic studies at different points of the titration trace have been applied to follow the aggregation process when n-heptane is added. Techniques such as scanning electron microscopy (SEM) and Cryo-SEM are microscopic techniques widely used to visualize the aggregation and flocculation process.7-8 These techniques make it possible to observe the nature of the particle surface, the size distribution of the dispersed phase, and whether the dispersed phase is stabilized by solid particles. Mullins and Shue8 showed the applicability of Cryo-SEM to study the aggregation process when n-heptane is added. In these studies, they show two different images: the first image corresponds to special molecular interactions between asphaltene species, and the other corresponds to large (7) Sharma, A.; Groenzin, H.; Tomita, A.; Mullins, O. Energy Fuels 2002, 16, 490-496. (8) Ferworn, K. A.; Svrcek, W. Y. Characterization and Phase Behavior of Asphaltenic Crude Oils. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; pp 227-266.
10.1021/ef030047g CCC: $27.50 © 2004 American Chemical Society Published on Web 04/16/2004
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Figure 1. Experimental setup of the flocculation titration system.
Figure 3. Flocculation titration curve of crude oils of high stability.
Figure 2. Experimental setup of the confocal microscope: f1 and f2: single mode fibers; L1, L2, L3: lens; BS: beam-splitter.
aggregates coexisting with smaller asphaltene molecules. Many investigations around the influence of the n-alkane dilution have been carried out with Cryo-SEM and SEM.8 However, these observations are not necessarily connected to the real chemistry due to the fact that the sample requires a sample pretreatment and experimental artifacts might be introduced during the process. For this reason, in recent years, the search for a technique that involves the acquisition and interpretation of in-situ information has been of great importance. Confocal microscopy is known for its noncontact and nondestructive features, and has been used extensively in the biomedical area.9,10 Confocal microscopy excludes out-of-focus information by focusing through a small aperture (a confocal pinhole). This feature ensures that information in the image arrives only from a particular depth of the sample, hence providing a method to observe an optically sliced section of the object in the sample. The use of a confocal microscope is starting to increase in the areas of colloid physics.11-12 Thill et al., using a confocal microscope, estimate the dimension of complex heterogeneous aggregates, without disturbing samples during observation. The confocal method was used to analyze flocks of activated sludge material as (9) Sheppard, C. J. R.; Shotton, D. M. Confocal Laser Scanning Microscopy; Bios Scientific Publishers: USA, 1997. (10) Shotton, D. M. J. Cell Sci. 1989, 94, 175. (11) Mikula, R. J.; Munoz, V. A. Colloid Surf. 2000, 174, 26-36. (12) Disnmore, A. D.; Weeks, E. R.; Prasad, V.; Levitt, A. C.; Weitz, D. A. Appl. Opt. 2001, 4152-4159.
Figure 4. Flocculation titration curve of crude oils of low stability.
an example of the application of this method to more complex and chemically heterogeneous flocks.13 In this work, we present studies of the asphaltene flocculation process in crude oils of remarkably different stability using the titration method. The stability of crude oils is referred to as the p-shell parameter; crude oils with a p-shell bigger than 1.6 are considered stable, and for lower values the crude oil is considered of low stability. Titrations of crude oil solutions in toluene were measured with an automatic titration apparatus designed to work with a wide range of optical densities (0.1-6 OD). Significant differences were observed in the titration curve of stable and unstable crude oils, and a new interpretation of the titration curve based in microscopic analysis is proposed. Samples corresponding to different points of the titration curve were observed with a homemade confocal microscope. The results show two important features; the principal one is that the maximum in the titration curve is not the real flocculation point. We demonstrate that the flocculation process starts at the first volume of n-heptane added. Second, (13) Thill, A.; Veerapaneni, S.; Simon, B.; Wiesner, M.; Bottero, J. Y.; Snidaro, D. J. Colloid Interface Sci. 1998, 204, 357-362.
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Figure 5. Confocal images of different points of the titration curve for Boscan crude oil.
the form of the titration curve is related to the kind of aggregate formed. Analysis of titration trace joined with microscopy inspection shows a clear correlation between crude oil stability and the flocculation process. Experimental Section Sample Preparation. Samples of crude oils of low stability Furrial like the one of p-value < 1.6 and crude oils of high stability Boscan like the one of p-value > 2.2 were used directly from the source. The crude oils were diluted with a minimum of toluene to reduce viscosity and diminish the optical density of the sample. The onset of precipitation threshold was obtained through automatic titration of diluted toluene solutions of crude oil. All solvents used were HPLC grade. Flocculation Titration. Figure 1 shows a homemade titration apparatus setup. Radiation from a laser beam (632 nm, 5 mW) passes through a variable width optical cell. Afterwards, a small area of photodiode detects radiation transmitted. The crude oil solution is re-circulated through of the optical cell with a peristaltic pump (1). A second peristaltic pump (2) is used to add n-heptane at constant rate. With this system it is possible to study samples of different optical densities; it can be used to titrate crude oils with little dilution. For the titration: 10 mL of 1% crude oil in toluene is added to the sample reservoir, and pump 1 starts to re-circulate the solution through the optical cell (1 mm). This is the zero point of the titration. Once the intensity signal is stabilized at the detector (5 min), pump 2 is activated at 0.6 mL/min adding n-heptane and initiating the flocculation process in the sample reservoir. At different selected times during titration, some drops of the sample were taken to observe in the confocal microscope.
Confocal Microscopy. The schematic description of the confocal microscope is presented in Figure 2. The confocal microscope is homemade equipment, and the entrance and outlet of the microscope were equipped with an SMA fiber adaptor to handle them flexibly.14 The light emerging from the fiber (f1) is expanded using a beam-expander configuration with spatial filter (P1), to fill the entrance pupil of a 40×, 0.65 NA microscope objective lens. The objective lens focuses the light to a small spot (0.5 µm in diameter) on the sample, at the focal plane of the objective lens. The light reflected back from the illuminated spot on the sample is collected by the same objective lens and is reflected directly by a beam-splitter (BS) to the detection system. The microscope is equipped with a CCD camera (640 × 480 pixels) and a spectrometer system. Reflected light can be directed to a CCD camera in order to acquire total image; an aperture placed before the CCD produces the confocal effect and permits obtaining an image with enhanced spatial resolution of less than 300 nm. For microscopic observation, approximately 3 drops of asphaltenes solutions from titrated solution taken at different percentages of n-heptane were deposited onto glass slides without previous sample treatment.
Results and Discussion Figure 3 shows a typical titration curve of corrected light intensity versus added n-heptane volume for three crude oils of high stability. For Boscan crude oil (Figure 3 insert), we divide the intensity curve into four zones: the initial part of the titration curve (zone a) shows a (14) Hung, J.; Castillo, J.; Jime´nez, G.; Hasegawa, M.; Rodriguez, M. Spectrochim. Acta Part A 2003, 59/13, 3177-3183.
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Figure 6. Confocal images of point (D) of the titration curve for Boscan crude oil and its respective profile plot.
constant intensity as a function of the volume of n-heptane added. This behavior is indicative that flock is not formed in this zone. The increment of the light intensity represented in zone b is due to the dilution and decrease of the absorption by small aggregates formed. When the maximum is reached (zone c), the dimensions of the particles are bigger than the light wavelength; as a consequence, these particles begin to scatter the light, diminishing the intensity at the detector. Finally, a decrease of the light intensity (zone d) is due to dispersion induced by the aggregation and coagulation process of big flocks. Figure 4 shows the titration curve for crude oils of low stability; these samples present significant differences compared with the crude oils of high stability described previously. In this case, only three zones can be defined. For the case of Furrial crude oil (Figure 4 insert), the corrected light intensity versus added nheptane volume shows a curve that begins with a constant increment of intensity; therefore, the constant intensity zone (a) does not appear, indicating that for little volume of n-heptane added a decrease of absorbance is observed. The increase of the light intensity with a high value slope is indicative that the dilution and lack of absorber effect are dominant. This effect is indicative of aggregate formation at the beginning of the titration. When the intensity maximum is reached and then remains constant (zones c and d), this is an indicator that the formation of aggregates of dimensions comparable with the light wavelength have begun. The
low variation in the slope of the titration curve shows that the dimensions of this flock do not change at the same rate as those of the sample studied previously. These differences in the titration curve can be definitively related to the stability of th crude oils. The addition of n-heptane in the crude oils solutions causes changes in the solubility parameters of the solvent. If the beginning of the titration curve is represented for a flat zone, the crude oil is stable to variations in the solubility properties of the medium. For crude oils of low stability, little variations in the solubility of the medium induce the aggregation process and the curve begins with a positive slope. The behavior of the titration curve after the maximum intensity suggests differences in the kind of flocks formed. The scattered light increases rapidly and the intensity at the detector decreases when the forming flocks increase in size. These big flocks decant and the solution begins to transmit more light. On the other hand, when the flocks increase in number faster than in size, the small flocks are less efficient for scattering the light and the intensity remains constant. Figure 5 shows confocal images of four different points of the titration curve for Boscan crude oil. Images 5a and 5b correspond at crude oil solution with 5 and 15 mL of n-heptane added. These images show little particles present in the crude oil solution. These bright sphere-like particles correspond to small flocks with diameters between 100 and 200 nm. Due to the small size of the particles, they show up as brilliant light
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Figure 7. Confocal images of different points of the titration curve for Furrial crude oil.
Figure 8. Confocal images of point (D) of the titration curve for Furrial crude oil and its respective profile plot.
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scattering points. Image 5c corresponds to a sample with 22 mL of titulant added; this image shows clusters formed by aggregation of small flocks. Clusters E and G that showed on image 5c are small aggregates whose size is approximately 6 µm. This sample exhibits a large polydispersity, the particle size varies from hundred of nanometers to several micrometers. Figure 6 shows a line profile curve for clusters E and F. This plot describes the topographies of the surface of this cluster. High values in the y-axis correspond to higher points in the surface. This cluster described by the line profile is a big particle of 1-3 µm in diameter formed by small asphaltenes particles unified like a mass with some irregularities at the surface. Figure 7 displays four confocal images of different points of titration curve for the Furrial crude oil. Image 7a corresponds to 3 mL of titulant added; this image shows flocks formed in crude oil solution whose size is approximately 500 nm. The image shows dark points bordered by brilliant zones. This image is typical of spherical particles. Images 7b and 7c correspond to samples with 15 and 25 mL of titulant added. They show the aggregation between the flocks producing clusters of different sizes. Image 7c is just like a macrostructure. An excess of precipitant results in a collapse of the aggregates into a single huge aggregate, arranged in a compact structure of large size. Figure 8 shows a line profile of image 7d. This plot presents a successive pick valley structure demonstrating that the big flock is composed of small flocks joined to form a structure with some local order and long-range disorder; this behavior is in agreement with extensive
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work previously published by others authors.9-10 The last point of the titration curve for Boscan and Furrial crude oils can be related to the differences in the coagulation process of the sample. For Boscan crude oil, the coagulations of aggregates increase themselves in size faster than in number. In the case of Furrial crude oils, the growing process of the aggregates is faster in number than in size and the coagulation and decantation is slower; therefore, the intensity trace does not show a well-defined maximum. Conclusions From our experiments, it is possible to conclude that the behavior of the titration trace can be correlated with the crude oils stability. With changes in the medium solubility, crude oils with low tendency to form flocks show a titration curve that begins with a slope approximately zero and presents a well-defined maximum. Crude oil of low stability shows a titration curve that begins with a pronounced slope. Image confocal microscopy shows that the flock grown and its characteristics depend on the nature of the crude oils and illustrate the existence of different aggregates. Acknowledgment. We thank Lab. Fisicoquı´mica de Hidrocarburos from the University Central of Venezuela for the crude oils samples. We also thank Socrates Acevedo for clarifying discussions. CONICIT Grant 97000722. EF030047G
