Thermodynamic and Colloidal Models of Asphaltene Flocculation

Chapter 24

Thermodynamic and Colloidal Models of Asphaltene Flocculation S. Kawanaka, Κ. J . Leontaritis, S. J . Park, and G. A. Mansoori

Downloaded by UNIV OF SYDNEY on May 17, 2013 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch024

Department of Chemical Engineering (M/C 110), University of Illinois, Chicago, IL 60680

This paper reviews the experiences of the oil industry in regard to asphaltene flocculation and presents justifications and a descriptive account for the development of two different models for this phenomenon. In one of the models we consider the asphaltenes to be dissolved in the oil in a true liquid state and dwell upon statistical thermodynamic techniques of multicomponent mixtures to predict their phase behavior. In the other model we consider asphaltenes to exist in oil in a colloidal state, as minute suspended particles, and utilize colloidal science techniques to predict their phase behavior. Experimental work over the last 40 years suggests that asphaltenes possess a wide molecular weight distribution and they may exist in both colloidal and dissolved states in the crude oil. The key to solving many of the technical problems that face the fossil fuel industries in our modern technological society today lies heavily in understanding the thermodynamic and transport aspects of these problems. Most of the irreplenishable energy resources available are mainly mixtures of gases, liquids and solids of varying physical and chemical properties contained in the crust of the earth in a variety of geological formations. Knowledge of the fluid phase equilibrium thermodynamic and transport characteristics of these mixtures is a primary requirement for the design and operation of the systems which recover, produce and process such mixtures. There has been extensive progress made in the past several years in the formulation of statistical thermodynamics of mixtures and transport phenomena modeling of multiphase flow in composite media. This knowledge may now be applied to the understanding and prediction of the phase and transport behavior of reservoir fluids and other 0097-6156/89/0396-O443$06.00/0 o 1989 American Chemical Society

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hydrocarbon mixtures. The present report is designed with the purpose of describing the role of modern theoretical and experimental techniques of statistical thermodynamics and transport and electrokinetic phenomena 10 oevelop methods that will predict asphaltene and asphalt flocculation during the production, transportation, and processing of petroleum. The mechanisms of gas injection and oil recovery involved with miscible gas flooding are basically of three kinds [1,2]: (i) The firstcontact miscible gas drive, (ii) The condensing gas drive (or the enriched gas drive), and (iii) The vaporizing gas drive (or the high pressure gas drive) processes. The first and second processes are based on the injection of hydrocarbons that are soluble in the residual oil, while the third process involves injection of a high density gas, such as highpressure nitrogen or carbon dioxide. In the case of the first-contact miscible process, a typical injection fluid is propane, which is soluble in oil. For the condensing gas drive process the injection fluid could be natural gas containing relatively high concentration of intermediate hydrocarbons, such as ethane, propane, and butane. Miscible flooding of petroleum reservoirs by carbon dioxide, natural gas, and other injection fluids has become an economically viable technique for petroleum production (1,2). The most common problem in petroleum recovery is poor reservoir volumetric sweep efficiency, which is due to channeling and viscous fingering because of the large difference between mobilities of the displacing and displaced fluids. Introduction of a miscible fluid in the petroleum reservoirs in general will produce a number of alterations in the flow behavior,phase equilibrium properties, and the reservoir rock characteristics. One such alteration is asphaltene and wax precipitation, which is expected to affect productivity of a reservoir in the course of oil recovery from the reservoir. (3,4,5). In most of the instances observed asphaltene and wax precipitation may result in plugging of or wettability reversal in the reservoir. Effect of asphaltene deposition could be positive (like prevention of viscous fingering) or negative (complete plugging of the porous media) depending on whether it could be controlled and predicted before it occurs. The parameters that govern precipitation of asphaltene and wax appear to be composition of crude and injection fluid, pressure, and temperature of the reservoir. With alterations in these parameters the nature of asphaltene and wax substances which precipitate will vary. Also, precipitation of asphaltene is generally followed with polymerization or flocculation of the resulting precipitate, which produces an insoluble material in the original reservoir fluid (6,7,8). Because of the complexity of the nature of asphaltic and wax substances the phenomena of precipitation and flocculation of these substances are not well understood. Also in view of the complexity of the petroleum reservoirs, study and understanding of the in situ precipitation of asphaltene and wax seems to be a challenging and timely task. Such an understanding will help to design a more profitable route for miscible gas flooding projects.



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In part II of the present report the nature and molecular characteristics of asphaltene and wax deposits from petroleum crudes are discussed. The field experiences with asphaltene and wax deposition and their related problems are discussed in part III. In order to predict the phenomena of asphaltene deposition one has to consider the use of the molecular thermodynamics of fluid phase equilibria and the theory of colloidal suspensions. In part IV of this report predictive approaches of the behavior of reservoir fluids and asphaltene depositions are reviewed from a fundamental point of view. This includes correlation and prediction of the effects of temperature, pressure, composition and flow characteristics of the miscible gas and crude on: (i) Onset of asphaltene deposition; (ii) Mechanism of asphaltene flocculation. The in situ precipitation and flocculation of asphaltene is expected to be quite different from the controlled laboratory experiments. This is primarily due to the multiphase flow through the reservoir porous media, streaming potential effects in pipes and conduits, and the interactions of the precipitates and the other in situ material presnet. In part V of the present report the conclusions are stated and the requirements for the development of successful predictive models for the asphaltene deposition and flocculation are discussed.

Nature and Properties of Asphaltenes The classic definition of asphaltenes is based on the solution properties of petroleum residua in various solvents. The word asphaltene was coined in France by J.B. Boussingault in 1837. Boussingault described the constituents of some bitumens (asphalts) found at that time in eastern France and in Peru. He named the alcohol insoluble, essence of turpentine soluble solid obtained from the distillation residue "asphaltene", since it resembled the original asphalt. In modern terms, asphaltene is conceptually defined as the normalpentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. The generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale (8,9). With this extension there has been much effort to define asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous source. It was demonstrated that the elemental compositions of asphaltene fractions precipitated by different solvents from various sources of petroleum vary considerably (see Table I). Figure 1 presents hypothetical structures for asphaltenes derived from oils produced in different regions of the world. Other investigators (10,11) based on a number of analytical methods, such as NMR, GPC, etc., have suggested the hypothetical structure shown in Figure 2. It has been shown (9) that asphaltenes contain a broad distribution of polarities and molecular weights. According to these studies, the concept of asphaltenes is based on the solubility behavior of high-boiling hydrocarbonaceous materials in benzene and low-molecular weight nparaffin hydrocarbons. This solubility behavior is a result of physical effects that are caused by a spectrum of chemical properties. Long also


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H


I

3

Figure 1. An example of a hypotetical structure of asphaltene, among the many suggested, showing their aromatic character.

Figure 2. Asphaltene structure deduced from microscopic and macroscopic analysis, showing their micro- and macro-molecular bonding. T. F. Yen, 1972, first suggested this type of structure.



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explained that by considering molecular weight and molecular polarity as separate properties of molecules, the solvent-precipitation behavior of materials derived from various carbonaceous sources can be understood. Future quantification of Long's approach probably can be achieved by developing a polarity scale based on solubility parameter. According to Marcusson (11) there is a close relation between asphaltenes, resins, and high molecular weight polycyclic aromatic hydrocarbons which may exist in petroleum. The heavy polycyclic aromatics of petroleum, on oxidation gradually form neutral resins (and probably asphaltogenic acids, ); asphaltenes are made as a result of further oxidation of neutral resins. On the contrary, the hydrogenation of asphaltic products containing neutral resins and asphaltenes produces heavy hydrocarbon oils, i.e., neutral resins and asphaltenes are hydrogenated into high molecular weight polycyclic aromatic structures. Asphaltenes are very similar to neutral resins in the ultimate analysis. The process of transformation of neutral resins into asphaltenes is very simple even at low temperatures. Specially neutral resins can easily be converted to asphaltenes in the presence of air or oxygen at elevated temperatures. Thus neutral resins and asphaltenes are similar in their chemical structure. The physical and physico-chemical properties of asphaltenes are different with those of neutral resins. The molecular weight of asphaltenes is very high. Published data (9,12,13,14,15,16) for the molecular weight of petroleum asphaltenes range from approximately 1,000 to 2,000,000. The reported molecular weight of asphaltene varies considerably depending upon the method of measurement. A major concern in reporting molecular weights is the association of asphaltenes which can exist at the conditions of the method of measurement. Vapor pressure osmometry (VPO) has become the most prevalent method for determining asphaltene molecular weights. However, the value of the molecular weight from VPO must be weighed carefully since, in general, the measured value of the molecular weight is a function of the solvent and its dielectric constant. Reported molecular weights from ultracentrifuge and electron microscope studies are high. To the contrary, those from solution viscosity and cryoscopic methods are low. Asphaltenes are lyophilic with respect to aromatics, in which they form highly scattered colloidal solutions. Specifically, asphaltenes of low molecular weight are lyophobic with respect to paraffins like pentanes and petroleum oils. Thus the degree of dispersion of asphaltenes in petroleum oils depends upon the chemical composition of the oil. In heavy highly aromatic oils the asphaltenes are colloidally dispersed; but in the presence of an excess of petroleum ether and similar paraffinic hydrocarbons they are coagulated and precipitated. The coagulated and precipitated asphaltenes can be re-peptized (colloidally dispersed) by the addition of aromatics. Neutral resins are particularly effective as peptizers. Such components as resins or high molecular weight aromatics are readily adsorbed by asphaltenes and act as protective layers, isolating the colloidal particles from the coagulative action of

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lyophobic constituents of petroleum oils. If the proportion of the peptizing constituents in a petroleum oil is sufficient, the asphaltenes form stable suspensions. Sachanen (11) explained that the solution of asphaltenes in an aromatic solvent is preceded by swelling of the powdered asphaltenes accompanied by evolution of heat. Nellensteyn discovered that the peptizing or precipitating properties of different liquids with respect to asphaltenes are closely related to the surface tension. Flocculation occurs when the solvent has a surface tension below 24 dynes/ cm at 25°C. Total peptization takes place when the surface tension exceeds 26 dynes/ cm. In the intermediate zone between 24 and 26 dynes/ cm, either flocculation or peptization may occur, depending on the properties of the asphaltenes. Asphaltenes in solution are generally recognized as colloidal systems. Several investigators have observed that colloidal solutions can be precipitated by flow through capillaries and porous media (4,7). The precipitation is believed to result from the electrical interactions with the walls which disturb the stabilizing electrical forces around the colloidal particles and the particles agglomerate to a precipitate. The asphaltene particles are electrically charged and thus can be precipitated by application of an electrical potential or by flow of the asphaltene-containing product through the sand, due apparently to electrical effects resulting from the flow. On the contrary, application of a counter-potential may prevent precipitation of asphaltenes from the crude oil flowing through a porous material. Asphaltenes are not crystallized and cannot be separated into individual components or narrow fractions. Thus, the ultimate analysis is not very significant, particularly taking into consideration that the neutral resins are strongly adsorbed by asphaltenes and probably cannot be effectively separated from them. Not enough is known of the chemical properties of asphaltenes. On heating, they are not melted, but decompose, forming carbon and volatile products above 300-400 ° C . They react with sulfuric acid forming sulfonic acids, as might be expected on the basis of the polyaromatic structure of these components. The color of dissolved asphaltenes is deep red at very low concentration in benzene as 0.0003 per cent makes the solution distinctly yellowish. The color of crude oils and residues is due to the combined effect of neutral resins and asphaltenes. The black color of some crude oils and residues is related to the presence of asphaltenes which are not properly peptized.

Field Experiences Asphaltene deposition during oil production and processing is a very serious problem in many areas throughout the world. In certain oil fields (12,13,14) there have been wells that, especially at the start of production, would completely cease flowing in a matter of a few days after an initial production rate of up to 3,000 BPD. The economic implications of this problem are tremendous considering the fact that a problem well workover cost could get as high as a quarter of a million


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dollars. In another field the formation of asphaltic sludges after shutting in a well temporarily and/or after stimulation treatment by acid has resulted in partial or complete plugging of the well (7). Still in another field, deposit of asphaltenes in the tubing was a very serious production problem and necessitated frequent tubing washings or scrapings to maintain production (17). Asphaltenes have played a significant role in the production history and economics of the deep horizons of some oil reservoirs (18). For example in one case asphaltene problems have ranged from asphaltene deposition during early oil production to asphaltene flocculation and deposition resulting from well acidizing and carbon dioxide injection for enhanced oil recoverv.

Table I. Elemental compositions of asphaltenes precipitated by different flocculants from various sources (16) Elemental Compo§ition(wt%) Source FiQccutent £. hi M Q Canada n-pentane 79.5 8.0 1.2 3.8 n-heptane 78.4 7.6 1.4 4.6 Iran n-pentane 83.8 7.5 1.4 2.3 n-heptane 84.2 7.0 1.6 1.4 Iraq n-pentane 81.7 7.9 0.8 1.1 n-heptane 80.7 7.1 0.9 1.5 Kuwait n-pentane 82.4 7.9 0.9 1.4 n-heptane 82.0 7.3 1.0 1.9

3. 7.5 8.0 5.0 5.8 8.5 9.8 7.4 7.8

H/C 1.21 1.16 1.07 1.00 1.16 1.06 1.14 1.07

AtomicRatios N/C O/C SZC .013 .036 .035 .015 .044 .038 .014 .021 .022 .016 .012 .026 .008 .010 .039 .010 .014 .046 .009 .014 .034 .010 .017 .036

Even for reservoirs in which asphaltene deposition was not reported previously during the primary and secondary recovery, it was reported that asphaltene deposits were found in the production tubing during carbon dioxide injection enhanced oil recovery projects (18). Asphaltene precipitation, in many instances, carries from the well tubing to the flow lines, production separators, and other downstream equipment. It has also been reported (19) that asphaltic bitumen granules occured in the oil and gas separator with oil being produced from certain oil fields. The downtime, cleaning, and maintenance costs are a sizable factor in the economics of producing a field prone to asphaltene deposition. Considering the trend of the oil industry towards deeper reservoirs, heavier and as a result asphaltic crudes, and the increased utilization of miscible gas injection techniques for recovering oil, the role of asphaltene deposition in the economic development of asphaltene containing oil discoveries will be important and crucial.


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Modeling of Asphaltene Flocculation, and its Interaction with Oil Solution of the asphaltene problem calls for detailed analyses of asphaltene containing systems from the statistical mechanical standpoint and development of molecular models which could describe the behavior of asphaltenes in hydrocarbon mixtures. From the available laboratory and field data it is proven that the asphaltene which exists in oil consists of very many particles having molecular weights ranging from one thousand to several hundred thousands. As a result distributionfunction curves are used to report their molecular weights. The wide range of asphaltene size distribution suggests that asphaltenes may be partly disolved and partly in colloidal state (in suspension) peptized (or stabilized) primarily by resin molecules that are adsorbed on asphaltene surface (12,13,14,19,20). As a result, a realistic model for the interaction of asphaltene and oil should take into account both the solubility in oil of one segment and suspension characteristic (due to resins) of another segment of the molecular weight distribution curve of asphaltene. We have proposed two different models (12,13,21,22) which are based on statistical mechanics of particles (monomers and polymers) dissolved or suspended in oil. A combination of the two models is general enough to predict the asphaltene-oil interaction problems (phase behavior or flocculation) wherever it may occur during oil production and processing. Such a model may be constructed by joining the concepts of continuous thermodynamic theory of liquid-solid phase transition, fractal aggregation theory of colloidal growth, and steric colloidal collapse and deposition models. Solubility Model of Interaction of Asphaltene and Oil : It is generally assumed that two factors are responsible for maintaining the mutual solubility of the compounds in a complex mixture such as the petroleum crude: These are the ratio of polar to nonpolar molecules and the ratio of the high molecular weight to low molecular weight molecules in the mixture. Of course, polar and nonpolar compounds are basically immiscible, and light and heavy molecules of the same kind are partially miscible depending on the differences between their molecular weights. However, in the complex mixture of petroleum crude or coal liquids and the like all these compounds are probably mutually soluble so long as a certain ratio of each kind of molecule is maintained in the mixture. By introduction of a solvent into the mixture this ratio is altered. Then the heavy and/or polar molecules separate from the mixture either in the form of another liquid phase or to a solid precipitate. Hydrogen bonding and the sulfur and/or the nitrogen containing segments of the separated molecules could start to aggregate (or polymerize) and as a result produce the irreversible asphaltene deposits which are insoluble in solvents. In order to formulate the necessary model for prediction of the "onset of deposition" of asphaltene we have taken advantage of the theories of polymer solutions (22,23,24). Both asphaltenes and asphaltene-free crude consist of mixtures of molecules with a virtually


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continuous molecular weight distributions in order to formulate the theory of interaction of oil and asphaltene systems we can utilize the concept of continuous mixture (16,21) joined with the thermodynamic theory of heterogeneous polymer solutions (22,24). We have already formulated the necessary continuous mixture model for the prediction of the "onset of deposition" of asphaltene due to injection of a miscible solvent. In the course of our ongoing research we intend to extend our model to varieties of crude oils and miscible solvents of practical interest (Figure 3). Suspension Model of Interaction of Asphaltene and Oil : This model is based upon the concept that asphaltenes exist as particles suspended in oil. Their suspension is assisted by resins (heavy and mostly aromatic molecules) adsorbed to the surface of asphaltenes and keeping them afloat because of the repulsive forces between resin molecules in the solution and the adsorbed resins on the asphaltene surface (see Figure 4). Stability of such a suspension is considered to be a function of the concentration of resins in solution, the fraction of asphaltene surface sites occupied by resin molecules, and the equilibrium conditions between the resins in solution and on the asphaltene surface. Utilization of this model requires the following (12): 1. Resin chemical potential calculation based on the statistical mechanical theory of polymer solutions. 2. Studies regarding resin adsorption on asphaltene particle surface and measurement of the related Langmuir constants. 3. Calculation of streaming potentials generated during flow of charged asphaltene particles. 4. Development and use of asphaltene colloidal and aggregation models for estimating the amount of asphalt which may be irreversibly aggregated and flocculated out. The amount of resins adsorbed is primarily a function of their concentration in the liquid state (the oil). So, for a given system (i.e., fixing the type and amount of oil and asphaltenes) changing the concentration of resins in the oil will cause the amount of resins adsorbed on the surface to change accordingly. This means that we may drop the concentration of resins in the oil to a point at which the amount of resins adsorbed is not high enough to cover the entire surface of asphaltenes. This may then permit the asphaltene particles to come together (irreversible aggregation), grow in size, and flocculate. One major question of interest is how much asphaltene will flocculate out under certain conditions. Since the system under study consist generally of a mixture of oil, aromatics, resins, and asphaltenes it may be possible to consider each of the constituents of this system as a continuous or discrete mixture (depending on the number of its components) interacting with each other as pseudo-pure-components. The theory of continuous mixtures (24), and the statistical mechanical theory of monomer/polymer solutions, and the theory of colloidal aggregations and solutions are utilized in our laboratories to analyze and predict the phase behavior and other properties of this system.


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Figure 3. Molecular weight distributions of asphaltenes before and after flocculation predicted by our continuous mixture model.



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NOTES

1.0 O 2.0

represents resin molecules

• represents aromatic molecules

3.0 — represents oil molecules of different size and paraffinic nature

represents asphaltene particles of different sizes and shapes

Figure 4. Asphaltene particle peptization effected by adsorbed resin molecules. This physical model is the basis of our asphaltene Thermodynamic-Colloidal Model.


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Discussions It has been shown experimentally (4,7,12,13,14) beyond any reasonable doubt that the electrical charge of colloidal asphaltenes is a very important property and, regardless of the charge sign, it seems possible to device colloidal asphaltene deposition preventive measures by controlling the electrical effects attributed to the charge of asphaltenes. The primary electrokinetic phenomenon in effect is the "streaming potential" generated by the movement of the electrically charged colloidal asphaltene particles due to the flow of oil. This streaming potential seems to neutralize the similar charge of the colloidal asphaltene particles and cause them to flocculate. The electric charge of colloidal asphaltenes has not been explained yet, primarily because of the complexity of the composition of asphaltic materials. The difference in charge (+or-) displayed by asphaltene particles derived from different crudes has not been explained either. One suggestion has been that the large quantities of nickel and vanadium found in asphaltene deposits may hold the key to these charges (7). This idea may be investigated by analyzing metal contents of asphaltene deposits that contain colloidal asphaltene particles with different electric charge. One thing that appears to be universally accepted is that resins in the petroleum act as peptizing agents of the colloidal asphaltene particles. A number of experiments have been performed that point out the peptizing role of resins (4,7,12,19,25). However, because of the significance of the resins as peptizing agents of the colloidal asphaltene particles and the fact that based on current experimental information they appear to be the main hope for combating the colloidal asphaltene problem in the field, more experiments must be performed to establish clearly and beyond any doubt the resin role in the asphaltene deposition problem and generate enough thermodynamic properties of the resins to be utilized in modeling efforts to the problem. Experimental evidence (4,25) suggests that for an oil mixture there is a critical concentration of resins below which the colloidal asphaltene particles may flocculate and above which they cannot flocculate regardless of how much the oil mixture is agitated, heated, or pressurized sort of changing its composition. The authors believe that there should be certain unique geological conditions which favor the formation of an oil whose actual resin concentration is less than its critical resin concentration. Such conditions are responsible for the transformations the hydrocarbon deposits undergo. These geological conditions could conceivably be identified and established after sufficient experimental data on actual resin concentration and critical resin concentration are generated for different crude oils around the world. As a result, geological conditions alone could provide clues for predicting potential asphaltene problems for oils that have not yet been produced. The postulated and sufficiently proven notion that asphaltenes are oxidation products of resins and that resins are oxidation products of oil (11) sort of makes the probability of finding oils whose actual resin concentration is less than their critical resin concentration small. In other



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words, an oil deposit must have a substantial amount of resins before starting to form asphaltenes during the natural geological transformation process. Table II seems to corroborate this statement. Thus, it would be interesting to find out what kind of geological conditions are suitable for generating oils whose actual resin concentration is less than their critical resin concentration. A model which is based on the notion of the resin critical concentration described above and predicts the phase behavior of asphaltenes in oil mixtures was published recently by the authors (12). Since asphaltene deposition takes place during primary, secondary and tertiary oil recovery, and in the refinery, injection of peptizing agents (i.e.: resins) in proper amounts and places could prevent or at least control the colloidal asphaltene deposition problem. Furthermore, experiments could be performed (i.e.: of the coreflood type) where peptizing agents are injected to study their effect on inhibition of asphaltene deposition or permeability reductions.

Table II. Resin and Asphaltene Content of Crude Oils (11)

Crude Oil

Sp.gr. (60°/60°F)

Pennsylvania Oklahoma, Tonkawa Oklahoma, Okla. City Oklahoma, Davenport Texas, Hould Texas, Mexia Louisiana, Rodessa Calif., Huntington Beach Mexico, Panuco Russia, Surachany Russia, Balachany Russia, Bibi-Eibat Russia, Dossor Russia, Kaluga Asia, Iraq (Kirkuk) Mississippi, Baxterville

18

0.805 0.821 0.835 0.796 0.936 0.845 0.807 0.897 0.988 0.850 0.867 0.865 0.862 0.955 0.844 0.959

Resins (% by wt) 1.5 2.5 5.0 1.3 12.0 5.0 3.5 19.0 26.0 4.0 6.0 9.0 2.5 20.0 15.5 8.9

Asphaltenes (% by wt) 0.0 0.2 0.1 0.0 0.5 1.3 0.0 4.0 12.5 0.0 0.5 0.3 0.0 0.5 1.3 17.2

One interesting question posed by previous researchers (14,19) is why there was asphaltic bitumen deposited at the bottom of the well considering that no phase change or any substantial temperature or pressure changes had taken place. The conclusion was that the question


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could only be answered after considerable light was thrown upon the nature of the asphaltic bitumen prior to its separation from the crude oil in the well. There were a few efforts to try to determine the size and nature of asphaltene particles while they still are in the original oil (15,19). Katz and Beu (19) did not see any asphaltene particles in the original oil of size 65 A or larger, but they did see these particles after mixing the crude with solvents. They concluded that the particles, if they do exist, must be smaller than 65 A. Witherspoon et al. (15), using ultracentrifuge techniques, found that the particles that eluded Katz and Beu do exist and are of the 35-40 A range. By careful work of electron micrography with rapid lyophilization, the size of asphaltene is found to be 20-30 A (27). In native oil or solutions, the asphaltene particle size can be doubled (28,29).

Conclusions Because the asphaltene problem is so elusive, it seems that, before one can formulate a comprehensive model describing the problem, the true asphaltene deposition mechanism(s) must be clearly understood and backed by field and experimental data; then an accurate and representative model can be formulated. In this report we have tried to address this basic question. The mere fact that there are several basic schools of thought with regards to the asphaltene deposition problem points out that we are still far from formulating a universally accepted model for describing the behavior of asphaltenes in crude oil.Establishing the state of the asphaltene particles in the original crude oil seems to be a basic building block in the scientific quest to find a solution to the asphaltene deposition problem. More experiments must be done to duplicate Witherspoon et al.s' ultracentrifuge work for different oils and possibly utilize other contemporary experimental techniques to establish the state of asphaltenes in crude oils. Meanwhile, a model that describes the phase behavior of asphaltenes in oil must take into account the lack of positive information on the structure of asphaltenes in the original oil and rely as little as possible on the concept of an asphaltene molecule or of specific properties of asphaltenes. This was the philosophy that we followed in our models, mentioned earlier, predicting the behavior of asphaltenes in petroleum.

Acknowledgments This research is supported in part by the National Science Foundation Grant CBT-8706655 and in part by the Shell Oil Company.

Literature Cited 1. 2.

Mansoori, G.A.; Jiang, T.S. Proc. of the 3rd European Conference on Enhanced Oil Recovery, Rome, Italy, April,1985. Stalkup, F.I. Miscible Displacement: Society of Petroleum Engineers Monograph, June 1983; Chapters 1,2,3


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David, A., Asphaltenes Flocculation During Solvent Simulation of Heavy Oils. American Institute of Chemical Engineers, Symposium Series 1973, 69 (no. 127), 56-8. 4. Preckshot, C.W.; Dehisle, N.G.; Cottrell, C.E.; Katz, D.L., Asphaltic Substances in Crude Oil Trans. AIME 1943, 151, 188. 5. Shelton, D.A.; Yarborough, L. J. of Petroleum Technology 1977,1171. 6. Cole, R.J.; Jessen, F.W. Oil and Gas Journal 1960, 58, 87. 7. Lichaa, P.M. and Herrera, L. Society of Petroleum Engineers Journal, paper no. 5304, 1975, 107. 8. Speight, J.G.; Long, R.G.; Trawbridge, T.D. Fuel 1984, 63, no. 5, 616. 9. Long, R.B. In Cemistry of Asphaltenes; Bunger, J.W.; Li, N.C., Eds.; Advances in Chemistry Series No. 195; Washington, DC, 1981; p 17. 10. Yen, T.F., Present Status of the Structure of Petroleum Heavy Ends and its Significance to Various Technical Applications. Preprints ACS. Div. Pet. Chem. 1972, 17, No. 4, 102. 11.Sachanen, A.N. The chemical constituents of petroleum: Reinhold Publishing Corp., 1945. 12. Leontaritis, K.J.; Mansoori, G.A. SPE Paper#16258; Proceedings of the 1987 SPE Symposium on Oil Field Chemistry, Society of Petroleum Engineers, Richardson, TX, 1987. 13. Leontaritis, K.J.; Mansoori, G.A.; Jiang, T.S., Asphaltene Deposition in Oil Recovery: a Survey of Field Experiences and Research Approaches J. of Petr. Sci. and Eng.,1988 (to appear). 14. Adalialis, S. M.Sc. thesis, Petroleum Engineering Department, Imperial College of the University of London, London, 1982. 15. Ray, B.R.; Witherspoon, P.A.; Grim, R.E. J. Phys. Chem. 1957, 61, 1296,. 16. Speight, J.G.; Moschopedis, B.C. In Cemistry of Asphaltenes; Bunger, J.W.; Li, N.C., Eds.; Advances in Chemistry Series No. 195; Washington, DC, 1981; p 115. 17. Haskett, C.E.; Tartera, M. Journal of Petroleum Technology 1965, 387-91. 18. Tuttle, R.N. Journal of Petroleum Technology 1983, 1192. 19. Katz, D.L.; Beu, K.E. Ind. and Eng. Chem. 1945, 37, 195. 20. Koots. J.A.; Speight, J.C., Fuel, 54,1975, p179. 21. Mansoori, G.A.; Jiang. T.S.; Kawanaka, S. Arabian J. of Sci. & 1988, 13, No. 1, 17. 22. Du, P.C.; Mansoori, G.A. SPE Paper #15082. Proc. of the 1986 California Regional Meeting of SPE, Society of Petroleum Engineers, Richardson, TX, 1986. 23. Hirschberg, A.; de Jong, L.N.J.; Schipper, B.A.; Meijers, J.G., Society of Petroleum Engineers Journal 1984, 24, No 3, 283-293. 24. Scott, R.L.; Magat, M. J. Chem. Phys., 1945, 13, 172; Scott, R.L. J. Chem. Phys. 1945, 13, 178. 25. Swanson, J. Phys. Chem. 1942, 46, 141.


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26. Kawanaka, S.; Park, S.J.; and Mansoori, G.A. SPE/DOE Paper #17376; Proc. 1988 SPE/DOE Symposium on Enhanced Oil Recovery, p 617, Society of Petroleum Engineer, Tulsa, OK, 1988. 27. Dickie, J.P.; Haller, M.N.; Yen, T.F. J. Coll. Interface Sci. 1969, 29, 475. 28. Dwiggens, C.W. J. Phys. Chem. 1965, 69, 3500. 29. Pollack, S.S.; Yen, T.F. Anal. Chem. 1970, 42, 623.


R E C E I V E D March 8, 1989


Thermodynamic and Colloidal Models of Asphaltene Flocculation

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