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The chemical and physical structure of petroleum: effects on recovery operations
Journal of Petroleum Science and Engineering 22 Ž1999. 3–15
The chemical and physical structure of petroleum: effects on recovery operations James G. Speight
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Western Research Institute, 365 North 9th Street, Laramie, WY 82070-3380, USA Accepted 21 January 1997
Abstract In a mixture as complex as petroleum, recovery chemistry can only be generalized because of the intricate and complex nature of the molecular species that make up the crude oil. It is this complexity that leads not only to difficulties in analyzing the recovered material but also in analyzing the original oil in place. Moreover, the incompatibility of crude oil constituents with each other is a continuing issue and the occurrence of suspended organic solids during recovery Žespecially thermal. reduces the efficiency of a variety of processes. More detailed knowledge of the composition and reactivity of petroleum will help in understanding the means by which models can be applied to understanding recovery processes. The models that are proposed as a means of being applicable to the prediction of sediment Ži.e., asphaltene, resin, wax. formation and deposition from petroleum due to changes in pressure, temperature and composition fall somewhat short in their structure. Further modeling needs involve an understanding of the chemistry of these materials and reflect the more modern approach to the physico-chemical structure of petroleum in order to more correctly predict the onset of precipitation as well as the location and amount of the sediment deposition in the producing wells and in oil-transport pipelines. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Complexity; Physico-chemical structure; Sediment
1. Introduction It is the general consensus that it is the polar, i.e., heteroatom, constituents of petroleum which are responsible for the formation of suspended organic solids during a variety of recovery processes ŽIslam, 1994; Park et al., 1994.. However, an area that remains largely undefined, insofar as the chemistry and physics are still speculative, is the phenomenon of the incompatibility of the crude oil constituents, as might occur during these operations. The formation )
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of a suspended solid phase during recovery Žas well as during refining operations. is related to the chemical and physical structure of petroleum; the latter is greatly influenced by the former. By way of a brief series of definition, the failure of petroleum fractions to mix and the separation of a separate phase is usually referred to as incompatibility. When incompatibility occurs, the constituents Žusually the asphaltenes and the resins. that form a separate phase are variously referred to as a precipitate, sediment, andror sludge formation depending upon the nature of the material and the causes of the separation.
0920-4105r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 4 1 0 5 Ž 9 8 . 0 0 0 5 1 - 5
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J.G. Speightr Journal of Petroleum Science and Engineering 22 (1999) 3–15
Fig. 1. The laboratory fractionation of petroleum.
Petroleum can be considered to be a delicately balanced system insofar as the different fractions which contain hydrocarbons Žsaturates and aromatics. as well as heteroatom constituents ŽFig. 1..
Although the heteroatom constituents tend to concentrate in the higher molecular weight fractions Žthe asphaltenes and resins. ŽFig. 2., nitrogen, oxygen, and sulfur species that are in near-neutral molecular
Fig. 2. The relative concentration of the heteroatoms after fractionation.
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locales will also occur in the saturates and aromatics, remembering that the nomenclature is not necessarily precise and that the composition of each fraction is a function of the separation process ŽSpeight, 1991.. Asphaltenes are recognized as being a complex mixture of species of varying molecular weight and polarity ŽFig. 3.. Thus, carbenes and carboids are lower molecular weight highly polar species that are predominantly products of thermal processes and might not occur in the typical recovery process. However, the application of thermal techniques, such as fire flooding, to petroleum recovery can produce such species and they will either deposit on the reservoir rock or appear as suspended solids in the oil. In fact, it is recognized that it is the polar species in the crude oil that govern the oil–rock interactions in the reservoir ŽBruning, 1991. from which many sediments can arise. With the exception of the carbenes and the carboids, the fractions are compatible proÕided there are no significant disturbances or changes made to the system. Such changes are: Ž1. the alteration of the natural abundance of the different fractions; Ž2. the chemical or physical alteration of the constituents as might occur during recovery, especially changes that might be brought on by thermal processes; and Ž3. alteration of the polar group distribution as might occur during oxidation by exposure to aerial oxygen during the recovery process. In the reservoir, asphaltene incompatibility can cause blockages of the pores
Fig. 3. Representation of asphaltenes and carbenesrcarboids on the basis of molecular weight and polarity.
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and channels through which the oil must move during recovery operations ŽPark et al., 1994; Islam, 1994; Leontaritis, 1996.. All of these incidents cause disturbances to the petroleum system. However, when such disturbances occur, it is the higher molecular weight constituents that are most seriously affected eventually leading to incompatibility Žprecipitation, sediment formation, sludge formation. depending upon the circumstances. Thus, the dispersibility of the higher molecular weight constituents becomes an issue that needs attention. And one of the ways by which this issue can be understood is to be aware of the chemical and physical character of the higher molecular weight constituents. By such means, the issue of dispersibility, and the attending issue of incompatibility can be understood and even predicted.
2. Discussion 2.1. Asphaltenes Asphaltenes are, by definition, a solubility class that is precipitated from petroleum, heavy oil, and bitumen by the addition of an excess of a liquid parrafinic hydrocarbon ŽGirdler, 1965; Andersen and Birdi, 1990; Speight, 1991, 1994; Speight and Long, 1996.. In addition, the composition of the asphaltenes fraction is dependent upon the nature of the hydrocarbon precipitant ŽFig. 4., the ratio of the volume of the precipitant to the volume of feedstock ŽFig. 5., to the contact time ŽFig. 6., and to the temperature at which the precipitation occurs ŽMitchell and Speight, 1973; Speight et al., 1984; Speight, 1991; Andersen and Stenby, 1996.. By virtue of the means of separation, the asphaltene fraction is chemically complex but it can be conveniently represented on the basis of molecular weight and polarity ŽFig. 3. ŽLong, 1979, 1981. and for different crude oils the slope of the line representing the distribution of molecular weight and the variation in polarity will vary ŽSpeight, 1994.. Asphaltenes can not be crystallized in the usual sense of the word. However, and by way of emphasis of the complex nature of this fraction, asphaltenes can
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Fig. 4. Asphaltene yield as a function of the carbon number of the paraffinic precipitant. Fig. 6. Asphaltene yield as a function of time.
be sub-fractionated by the use of a variety of techniques to produce fractions that vary considerably in molecular weight and which also vary considerably in terms of the functional group types and functional group content ŽFrancisco and Speight, 1984; Speight, 1991.. These data lend support to, and reinforce, the
Fig. 5. Asphaltene yield as a function of precipitant:oil ratio.
concept that asphaltenes are complex mixtures of molecular sizes and various functional types. In addition, any variation of the major parameters Žprecipitant, precipitant:oil ratio, time, and temperature. can cause substantial variations in the nature and amount of the separated asphaltenic material. It can vary from a dark brown amorphous solid to a black tacky deposit, either of which under the prevalent conditions could be termed an asphaltene. The elemental compositions of asphaltenes vary over only a narrow range corresponding to HrC ratios of 1.15 " 0.05%, although values outside of this range are often found ŽSpeight, 1991.. Notable variations do occur in the proportions of the heteroelements, in particular in the proportions of oxygen and sulfur. On the other hand, the nitrogen content of the asphaltenes has a somewhat lesser degree of variation. This is, perhaps, not surprising since exposing asphaltenes to atmospheric oxygen can substantially alter the oxygen content, and exposing a crude oil to elemental sulfur, or even to sulfur-containing minerals, can result in sulfur uptake. The data from the various studies intimate that asphaltenes, viewed structurally, contain condensed polynuclear aromatic ring systems bearing alkyl side
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chains. The formulae derived from the studies invoked the concept of large polynuclear aromatic systems and efforts were made to describe the total structures of asphaltenes in accordance with magnetic resonance data and results of other spectroscopic and analytical techniques. The heteroatoms Žnitrogen and sulfur. have also been identified as occurring in a variety of locations, including cyclic and non-cyclic systems ŽSpeight, 1991, 1994.. The oxygen functions are considerably different to the nitrogen and sulfur function there being a lesser occurrence of oxygen in being systems and a greater tendency for oxygen to occur as hydrogenated Že.g., phenolic. functions ŽSpeight, 1991, 1994.. Metals Ži.e., nickel and vanadium. occur predominantly in the asphaltene fraction with lesser amounts occurring in the resins fraction ŽReynolds, 1994.. However, from a structural viewpoint, metals are much more difficult to integrate into the asphaltene system. It is known that the nickel and vanadium occur as porphyrins but whether or not these are an integral part of the asphaltene structure is not known. Some of the porphyrins can be isolated as a separate stream from petroleum. Asphaltene molecular weights are variable ŽSpeight, 1991, 1994. there being the tendency to associate even in dilute solution in non-polar solvents. However, data produced using highly polar solvents indicate that the molecular weights, in solvents that prevent association, usually fall into the range 2000 " 500 ŽSpeight et al., 1985.. As a result of these studies, it has been possible to derive various models for petroleum asphaltenes, some of which can be used to explain the chemical and physical properties of the asphaltenes constituents ŽSpeight, 1991, 1994. and offers some degree of predictability as a function of the molecular model. These models, that might be more in keeping with behavioral characteristics, have smaller polynuclear aromatic systems and also span the range of functional types as well as molecular sizes ŽSpeight, 1991, 1994.. In addition, such models can have the large size dimensions that have been proposed for asphaltenes since mobility effects can increase the ‘effective diameter’ of the asphaltenes molecules thereby presenting larger dimensions that is the case for the stationary molecule. In fact, asphaltenes molecules can be considered to be ‘molecular
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chameleons’ insofar as they can vary in dimensions depending upon the degree of mobility and the angle of rotation about an axis andror the freedom of rotation about one, or more, of the bonds. 2.2. Structure of petroleum An early hypothesis of the physical structure of petroleum ŽPfeiffer and Saal, 1940. indicated that asphaltenes are the centers of micelles formed by adsorption, or even by absorption of part of the maltenes, that is, resin material, on to the surfaces or into the interiors of the asphaltene particles. Thus, most of those substances with greater molecular weight and with the most pronounced aromatic nature are situated closest to the nucleus and are surrounded by lighter constituents of less aromatic nature. The transition of the intermicellar Ždispersed or oil. phase is gradual and almost continuous. Since asphaltenes are incompatible with the oil Žsaturates and aromatics. fraction, asphaltene dispersion is mainly attributable to the resins Žpolar aromatics. indicating that the resins are, under ambient conditions, a necessary constituent and that by their presence they prevent incompatibility ŽSwanson, 1942; Witherspoon and Munir, 1960; Koots and Speight, 1975; Mushrush and Speight, 1995.. Furthermore, resins from one crude oil might disperse asphaltenes from a different crude oil with some difficulty, if at all ŽKoots and Speight, 1975.. The instability of many such ‘heterogeneous’ blends indicate that there may be significant structural incompatibility among the asphaltenes from one crude oil and the resins of another crude oil. The means by which asphaltenes and resins interact to exist in petroleum remains the subject of speculation but hydrogen bonding ŽMoschopedis and Speight, 1976a; Acevedo et al., 1985. and the formation of charge-transfer complexes ŽYen, 1974. have been cited as the causative mechanisms. There is evidence that asphaltenes participate in charge-transfer complexes ŽPenzes and Speight, 1974; Speight and Penzes, 1978. but the exact chemical or physical manner in which they would form in petroleum is still open to discussion. The original concept of the asphaltene–resin micelle invoked the concept of asphaltene–asphaltene association to form a graphite-like stack ŽFig. 7.
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celle, the structure is recognized as being complex ŽBardon et al., 1996..
2.3. Asphaltene deposition during recoÕery operations
Fig. 7. The stack-type concept of the asphaltene micelle.
which acted as the micelle core which, in turn, was stabilized by the resins. However, the concept of hydrogen-bonding interactions being one of the means of association between the asphaltenes and resins has, however, led to a reconsideration of the assumed cluster as part of the micelle. When resins and asphaltenes are present together, hydrogen-bonding studies show resin–asphaltene interactions are preferred over asphaltene–asphaltene interactions ŽMoschopedis and Speight, 1976a.. If the same intermolecular forces are projected to petroleum, asphaltenes in petroleum as single entities that are peptized, and effectively dispersed, by the resins ŽFig. 8.. However, whatever the means by which the individual molecular species are included in the mi-
Asphaltenes and asphaltene-related materials are known to deposit as sediments during recovery operations in the vicinity of production wells during miscible floods, after acid stimulation, or during pressure changes ŽBurke et al., 1990; Islam, 1994; Park et al., 1994.. Many reservoirs produce without any such problems until the oil stability is perturbed during later stages of oil production. The parameters that govern sediment formation and deposition of asphaltenic materials from petroleum are related to the composition of the crude oil ŽTable 1. as well as the parameters used for the recovery process ŽTable 2.. It must also be recognized that the material that deposits from the crude oil as a separate phase is more aromatic and richer in heteroatom compounds than the original crude oil. In fact, in some cases, especially when oxidation has occurred, the deposited material is more aromatic and richer in heteroatoms than in asphaltenes. In terms of the crude oil parameters that influence sediment formation, there has been considerable focus on the asphaltene content as well as the chemistry and physics of the asphaltenes relationship to the remainder of the oil. For example, while the asphaltene content of petroleum oils varies over a wide range ŽKoots and Speight, 1975; Speight, 1991., asphaltene content is not the single determining influence on sediment formation. It has been noted that
Fig. 8. The asphaltene–resin concept of the micelle.
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Table 1 Crude oil properties and their general relationship to sediment formation Parameter
Comments
Asphaltene content
affects light oil recovery and heavy oil recovery influences oil–rock interactions forms sediment when gases are dissolved in the oil thermal methods can change relationship to oil ) 1.0 Žoil is stable. - 1.0 Žoil is unstable. provides polarity, responsible for incompatibility preferential reaction with oxygen can cause incompatibility in paraffinic environments low gravity, high asphaltene content high viscosity, high asphaltene content can be changed by thermal recovery processes changes functional group composition
Asphaltene:resin ratio Heteroatom content Aromaticity API gravity Viscosity Oil medium Oxidation
asphaltene:resin ratios are usually below unity in oils that are stable and higher than unity in oils that exhibit ready precipitation of asphaltenic material ŽSachanen, 1945.. Even though it is generally understood that asphaltene content of a crude oil increases with decreasing API gravity, asphaltene precipitation has also been reported in light oils as well. Such an effect Žgas deasphalting. ŽSpeight, 1991. arises from the increased solubility of hydrocarbon gases in the petroleum as reservoir pressure increases during maturation. Incompatibility will also occur when asphaltenes interact with reservoir rock, especially acidic functions of rocks, through the functional groups Že.g., the basic nitrogen species. just as they interact with adsorbents. And, there is the possibility for interaction of the asphaltene with the reservoir rock through the agency of a single functional group in which the remainder of the asphaltene molecule remains in the liquid phase Žvertical association relative to the rock surface. ŽFig. 9.. On the other hand, the asphaltene constituents can react with the rock at
several points of contact Žhorizontal association relative to the rock surface. ŽFig. 10. thereby enhancing the bonding to the rock and, in some cases, effecting recovery operations to an even greater extent. Both modes of reaction can entrap other species Žsuch as resins and aromatics. within the space between the rock and the asphaltene. Another area where incompatibility might play a detrimental role during recovery operations occurs as a result of aerial oxidation. In general, it is the more polar species which oxidize first with or without the presence of catalysts leaving an oil that is relatively free of heteroatom species ŽMoschopedis and Speight, 1978.. Thus, oxidation is a means for the production of highly polar species in a hydrocarbon oil leading to the deposition of polar sediments and being analogous to the deasphalting procedure. Thus, after incorporation of oxygen to an oil-dependent limit, significant changes occur to asphaltenes and resins. These changes are not so much due to oxidative degradation but to the incorporation of oxygen functions that interfere with the natural order of intra-
Table 2 Recovery process parameters and their relationship to sediment formation Parameter
Comments
Carbon dioxide injection Miscible flooding Organic chemicals Acidizing Pressure decrease Temperature decrease
lowers pH by changing oil composition rich gas lowers solvent power of oil cause incompatibility through low solvent power interaction with constituents changes oil composition similar effect to pressure decrease
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Fig. 9. Vertically oriented asphaltene–mineral interactions.
molecular and intermolecular structuring leading to the separation of asphaltenic material ŽMoschopedis and Speight, 1976b, 1977.. Thus, with alterations in these parameters, the formation of a sediment can occur although the nature of the sediment will vary and be dependent upon the process of formation. There are also several process-related destabilizing forces that can cause precipitation of asphaltenic material and each involves disturbance to the equilibrium that exists within petroleum. For example, carbon dioxide causes the destabilization of the petroleum equilibrium by lowering pH, by changing oil composition, and by creating turbulence. Usually, asphaltene precipitation increases as the volume of carbon dioxide available to the crude oil increases during the later stages of carbon dioxide injection or stimulation. The most
noticeable primary locations of asphaltene deposition are the wellbore and the pump regions. In addition, flooding of a rich gas Žmiscible flooding. destabilizes the asphaltene–crude oil mixture by lowering the solvent power of the solution. The hydrocarbon gases used in such applications effectively cause deasphalting Žgas deasphalting, solvent deasphalting. of the crude oil. The negative effect of rich gas is at a maximum near the bubble point; this effect is alleviated after the bubble point is reached. Similarly, organic chemicals such as isopropyl alcohol, methyl alcohol, acetone, and even some glycol, alcohol, or surfactant based solvents, that do not have an aromatic component, may selectively precipitate asphaltenes and resins. Asphaltene precipitation may be caused by well stimulation, such as acidizing which involves a drastic shift in local chemical equilibria, pH and libera-
Fig. 10. Horizontally oriented asphaltene–mineral interactions.
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tion of carbon dioxide. It may also increase the concentration of some ions, such as iron, which will promote the formation of asphaltenic sediment. A decrease in the pressure is another important factor that influences the onset of solids deposition from petroleum. In fact, the effect of pressure particularly noticeable just above the bubble point for crude oils that are rich in light ends. Depending on the location of the pressure decrease, deposition may occur in different parts of the reservoir as well as in the wellbore and in the production stream. Furthermore, a decrease in pressure is usually accompanied by a decrease in temperature which can also cause physico-chemical instability leading to the separation of asphaltenic from the oil. Pressure change alone can also invoke similar asphaltene precipitation. The asphaltenic material in crude oil is electrically charged through the existence of zwitterions or polarization within the molecular species ŽPreckshot et al., 1943; Katz and Beu, 1945; Penzes and Speight, 1974; Speight and Penzes, 1978; Fotland and Alfindsen, 1996.. Therefore any process Žsuch as the flow through reservoir channels or through a pipe. which can induce a potential across, or within, the oil will also result in the electrodeposition of asphaltenic material through the disturbance of the stabilizing electrical forces. In addition, neutralization of the molecular charge will also result in the formation of a sediment. 2.4. Models of organic deposition The mechanism of asphaltene precipitation is very complex ŽSpeight et al., 1984; Andersen and Speight, 1993; Buckley, 1996. and controversy as to the nature of asphaltene solutions persists. However, petroleum consists of a mixture of oil, aromatics, resins, and asphaltenes ŽFig. 1. and it is necessary to consider each of the constituents of this system as a continuous or discrete mixture interacting with each other. The omission of any one fraction could lead to errors in the outcome ŽAndersen and Speight, 1993.. There are two general approaches to the consideration of the nature of asphaltenes in oil ŽHirschberg et al., 1984; Mansoori et al., 1987; Park et al., 1994; Islam, 1994.. The first approach considers asphaltenes to be dissolved in oil in a true liquid state. In this case, asphaltene precipitation is considered to
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depend on thermodynamic conditions of temperature, pressure and composition. This particular approach recognizes asphaltene precipitation as a thermodynamically reversible process. The second approach considers asphaltenes to be solid particles which are suspended colloidally in the crude oil and are stabilized by resin molecules; consequently, the deposition process is considered to be irreversible. If the material precipitated is a reacted derivative of the asphaltenes Ži.e., a sediment., this may be true but if the precipitated material is unreacted asphaltene then this assumption may need a correction. One question of major interest during recovery operations is the timing of sediment formationrdeposition and the amount of the organic material will separate from the oil under specific conditions. Two different models have been proposed to explain the behavior of petroleum and the potential for solids deposition during recovery operations ŽKawanaka et al., 1989; Islam, 1994; Park et al., 1994.. The continuous thermodynamic model utilizes the theory of heterogeneous polymer solutions is utilized for the predictions of the onset point and amount of organic deposits from petroleum crude. A steric colloidal model which is capable of predicting the onset of organic deposition has also been developed and a combination of these two models results in a fractal aggregation model. These efforts have generally been adequate to predict the asphaltene–oil interaction problems Žphase behavior andror flocculation. wherever it may occur during oil production and processing. In the continuous thermodynamic model the degree of dispersion of the high molecular weight organic constituents in petroleum depends upon the chemical composition of the petroleum. Precipitation of the high molecular weight material can be explained by a change in the molecular equilibria that exist in petroleum through a change in the balance of oil composition. Moreover, the precipitation process is considered to be reversible. Indeed, the reconstitution of petroleum after fractionation has been demonstrated ŽKoots and Speight, 1975. and lends support to this model. The ratio of polar to non-polar molecules and the ratio of high- to low-molecular-weight molecules in a complex mixture such as petroleum are the two
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factors primarily responsible for maintaining mutual solubility. The stability of the system is altered by the addition of miscible solvents causing the high molecular weight andror the polar molecules to separate from the mixture either in the form of another liquid phase, or as a solid precipitate. Hydrogen bonding and the sulfur- andror nitrogen-containing segments of the separated molecules could start to aggregate for polymerization and, as a result, produce a solid phase which separates from the oil. In the steric colloidal model, the high molecular weight materials in petroleum are considered to be solid particles of different sizes suspended colloidally in the oil and stabilized by other petroleum constituents Ži.e., resins. adsorbed on their surface. The original hypothesis of the physical structure of petroleum ŽPfeiffer and Saal, 1940. which resins played a role in the stabilization of the asphaltenes and later confirmation of the role of the resins in petroleum ŽSwanson, 1942; Witherspoon and Munir, 1960; Koots and Speight, 1975. is supportive of this model. In the fractal aggregation model, it is assumed that pi–pi interactions are the principal means by which asphaltenes associate. This assumption may not be completely valid because of the evidence that favors hydrogen bonding between the molecular species and the observation that asphaltene–resin interactions may predominate over asphaltene– asphaltene interactions in petroleum. The concept that asphaltene–asphaltene interactions may be the predominant interactions is true for solutions of asphaltenes, and this is reflected in the molecular weight data ŽSpeight et al., 1985. but there is no guarantee that these interactions are predominant in petroleum Žespecially with evidence that indicates the high potential for other interactions . ŽMoschopedis and Speight, 1976a; Speight, 1994.. In as much as the high molecular weight constituents of petroleum have a wide range of polarity and molecular weight distribution ŽFig. 3., such compounds have been considered to act as heterogeneous polydisperse polymers. However, knowing with a new understanding of the nature of asphaltenes ŽSpeight, 1994., there may be some difficulties with this assumption. Nevertheless, in order to predict the phase behavior of the these constituents, it has also been assumed that the properties
of these constituents depend on their molecular weights. The model also assumes that the asphaltenes are partly dissolved and partly in the colloidal state thereby accounting for both the solubility and colloidal effect of high molecular weight organic constituents in the lower molecular weight constituents. The proposed models can provide the tool for making satisfactory prediction of the phase behavior of the deposition of high molecular weight materials. Because the issue of the deposition of asphaltenic materials problem is complex, it is necessary to attempt an understanding of the deposition mechanism before an accurate and representative model can be formulated. Utilization of kinetic theory of fractal aggregation has enabled the development of the fractal aggregation model. This model allows to describe properly several situations, such as phase behavior of heavy organic deposition, the mechanism of the association of the high molecular weight constituents, the geometrical aspects of aggregates, the size distribution of the sediments, and the solubility of the high molecular weight constituents in the solution under the influence of a miscible solvent. Another potential model involves use of the solubility parameter of the asphaltenes and the surrounding medium. It is known that the solubility of asphaltenes varies with the solubility parameter of the surrounding liquid medium ŽMitchell and Speight, 1973. and calculation or estimation of the solubility parameters of various liquids is known. From these data, it is possible to estimate point at which asphaltenic material is precipitated when the composition of the oil is changed by the addition of a hydrocarbon liquid ŽMitchell and Speight, 1973.. In fact, the solubility parameter has been used successfully to determine the character of heavy oils and investigate the separation of asphaltenic material ŽWiehe, 1996.. The solubility parameter concept also recognized the gradation of polarity of the asphaltenes as selective precipitation occurs during the addition of a nonsolvent when the most polar constituents are precipitated first ŽAndersen and Speight, 1992.. The models which apply the solubility parameter concept calculate the interaction through an assumption of the total crude oil as formed by asphaltenes and deasphaltened oil; hence the system is regarded as a two component system. The changes in phase
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Fig. 11. Estimation of the solubility parameter from the HrC atomic ratio.
equilibrium is caused by the changes in the solubility parameter of either of the two pseudo-components, which may happen either by dissolution of gas or alkane in the deasphaltened oil phase or by changes in temperatures. The amount of precipitated asphaltenes is calculated as the differences in asphaltenes present in the oil and the solubility of asphaltenes at the saturation point. In the different models the change in the composition of the deasphaltened oil is taken as significant for the phase equilibrium, and various methods, i.e., cubic equations of state, are applied to determine the properties of this fraction. Further development of this concept ŽSpeight, 1994. has led to the graphical representation of the
solubility parameters of polynuclear aromatic systems and estimation of the solubility parameter of the asphaltenes based on hydrogen:carbon ratios ŽFig. 11.. Further development of this knowledge can allow the progress of asphaltene deposition to be followed ŽFig. 12. and the region of sediment formation Žor the region of instability and incompatibility. to be estimated using a simplified phase diagram ŽFig. 13.. Obviously more work is need to apply the
Fig. 12. Potential changes in the solubility parameter due to interactions that lead to sediment deposition.
Fig. 13. Representation of petroleum as a three-phase system showing a region of instability.
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mathematical procedures to this concept but it does invoke the principle of knowledge of the chemical and physical properties of the asphaltenes and the oil. In general, these proposed models are, to a degree, applicable to the prediction of heavy organic deposition Žasphaltene, paraffinrwax, resin. from petroleum due to changes in pressure, temperature and composition. However, the use of assumptions that do not reflect, or recognize, what might be the actual chemical and physical structure of petroleum can lead to errors in the data. Further modeling must involve recognition of the more modern concepts of the structure of petroleum as well as application of the models to the predictability of the location and amount of the deposition of the sediments inside the producing wells and oil-transport pipelines.
3. Conclusions Petroleum is a complex system that depends upon the relationship of the constituents fractions to each other and the relationships are dictated by molecular interactions. Thus, recovery chemistry can only be generalized because of the intricate and complex nature of the molecular species that make up the crude oil leading to difficulties in analyzing not only the recovered material but also the original oil in place. Moreover, the incompatibility of crude oils with each other is a continuing issue and the occurrence of suspended organic solids during recovery Žespecially thermal. reduces the efficiency of a variety of processes. Asphaltene precipitation or the mere presence of asphaltenes may invoke many implications in recovering asphaltic crude oils. Asphaltene precipitation may occur under various thermally or non-thermally enhanced oil recovery schemes or even primary production conditions. These models that are proposed models as being applicable to the prediction of sediment Ži.e., asphaltene, resin, wax. formation and deposition from petroleum due to changes in pressure, temperature and composition. Further modeling must involve an understanding of the chemistry of these materials and reflect the more modern approach to the physico-chemical structure of petroleum in order to
more correctly predict the onset of precipitation as well as the location and amount of the sediment deposition in the producing wells and in oil-transport pipelines.
References Acevedo, S., Mends, B., Rajas, A., Lairs, I., Rives, H., 1985. Fuel 64, 1741. Andersen, S.I., Birdi, K.S., 1990. Fuel Science and Technology International 8, 593. Andersen, S.I., Speight, J.G., 1992. Div. Fuel. Chem. Am. Chem. Soc. 37 Ž3., 1335, Preprints. Andersen, S.I., Speight, J.G., 1993. Fuel 72, 1343. Andersen, S.I., Stenby, E.H., 1996. Fuel Science and Technology International 14, 261. Bardon, C., Barre, L., Espinat, D., Guille, V., Li, M.H., Lambard, J., Ravey, J.C., Rosenberg, E., Zemb, T., 1996. Fuel Science and Technology International 14, 203. Bruning, I.M.R.deA., 1991. Oil and Gas Journal 89 Ž3., 38. Buckley, J., 1996. Fuel Science and Technology International 14, 55. Burke, N.E., Hobbs, R.E., Kashou, S.F., 1990. Journal of Petroleum Technology 42, 1440. Fotland, P., Alfindsen, H., 1996. Fuel Science and Technology International 14, 101. Francisco, M.A., Speight, J.G., 1984. Div. Fuel Chem. Am. Chem. Soc. 29 Ž1., 36, Preprints. Girdler, R.B., 1965. Proc. Assoc. Asphalt Paving Technol. 34, 45. Hirschberg, A., de Jong, L.N.J., Schipper, B.A., Meijer, J.G., 1984. Soc. Pet. Eng. J. 283. Islam, M.R., 1994. In: Yen, T.F., Chilingarian, G.V. ŽEds.., Asphaltenes and Asphalts. Developments in Petroleum Science. Vol. 40, Chap. 11, Elsevier, Amsterdam. Katz, D.L., Beu, K.E., 1945. Ind. Eng. Chem. 43, 1165. Kawanaka, S., Leontaritis, K.J., Park, S.J., Mansoori, G.A., 1989. In: Borchardt, J.K., Yen, T.F. ŽEds.., Oil Field Chemistry: Enhanced Recovery and Production Simulation. Symposium Series No. 396, Chap. 24, American Chemical Society, Washington, DC. Koots, J.A., Speight, J.G., 1975. Fuel 54, 179. Leontaritis, K.J., 1996. Fuel Science and Technology International 14, 13. Long, R.B., 1979. Div. Petrol. Chem. Am. Chem. Soc. 24 Ž4., 891, Preprints. Long, R.B., 1981. In: Bunger, J.W., Li, N. ŽEds.., The Chemistry of Asphaltenes. Advances in Chemistry Series No. 195, American Chemical Society, Washington, DC. Mansoori, G.A., Jiang, T.S., Kawanaka, S., 1987. Arabian J. Sci. Eng. 13 Ž1., 17. Mitchell, D.L., Speight, J.G., 1973. Fuel 52, 149. Moschopedis, S.E., Speight, J.G., 1976a. Proc. Fuel 55, 187. Moschopedis, S.E., Speight, J.G., 1976b. Proc. Assoc. Paving Technol. 45, 78. Moschopedis, S.E., Speight, J.G., 1977. J. Mater. Sci. 12, 990.
J.G. Speightr Journal of Petroleum Science and Engineering 22 (1999) 3–15 Moschopedis, S.E., Speight, J.G., 1978. Fuel 57, 25. Mushrush, G.W., Speight, J.G., 1995. Petroleum Products: Instability and Incompatibility. Taylor and Francis, Washington, DC. Park, S.J., Escobedo, J., Mansoori, G.A., 1994. In: Yen, T.F., Chilingarian, G.V. ŽEds.., Asphaltenes and Asphalts. Developments in Petroleum Science. Vol. 40, Chap. 8, Elsevier, Amsterdam. Penzes, S., Speight, J.G., 1974. Fuel 53, 192. Pfeiffer, J.P., Saal, R.N., 1940. Phys. Chem. 44, 139. Preckshot, G.W., Delisle, N.G., Cottrell, C.E., Katz, D.L., 1943. AIME Trans. 151, 188. Reynolds, J.G., 1994. In: Yen, T.F., Chilingarian, G.V. ŽEds.., Asphaltenes and Asphalts. Developments in Petroleum Science. Vol. 40, Chap. 10, Elsevier, Amsterdam. Sachanen, A.N., 1945. The Chemical Constituents of Petroleum. Reinhold, New York.
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Speight, J.G., 1991. The Chemistry and Technology of Petroleum, 2nd edn. Marcel Dekker, New York. Speight, J.G., 1994. In: Yen, T.F., Chilingarian, G.V. ŽEds.., Asphaltenes and Asphalts. Developments in Petroleum Science. Vol. 40, Chap. 2, Elsevier, Amsterdam. Speight, J.G., Long, R.B., 1996. Fuel Science and Technology International 14, 1. Speight, J.G., Penzes, S., 1978. Chemistry and Industry 729. Speight, J.G., Long, R.B., Trowbridge, T.D., 1984. Fuel 63, 616. Speight, J.G., Wernick, D.L., Gould, K.A., Overfield, R.E., Rao, B.M.L., Savage, D.W., 1985. Rev. Inst. Francais du Petrole 40, 51. Swanson, J., 1942. J. Phys. Chem. 146, 141. Wiehe, I.A., 1996. Fuel Science and Technology International 14, 289. Witherspoon, W., Munir, Z.A., 1960. Producers Monthly 24, 20. Yen, T.F., 1974. Energy Sources 1, 447.
The chemical and physical structure of petroleum: effects on recovery operations James G. Speight
)
Western Research Institute, 365 North 9th Street, Laramie, WY 82070-3380, USA Accepted 21 January 1997
Abstract In a mixture as complex as petroleum, recovery chemistry can only be generalized because of the intricate and complex nature of the molecular species that make up the crude oil. It is this complexity that leads not only to difficulties in analyzing the recovered material but also in analyzing the original oil in place. Moreover, the incompatibility of crude oil constituents with each other is a continuing issue and the occurrence of suspended organic solids during recovery Žespecially thermal. reduces the efficiency of a variety of processes. More detailed knowledge of the composition and reactivity of petroleum will help in understanding the means by which models can be applied to understanding recovery processes. The models that are proposed as a means of being applicable to the prediction of sediment Ži.e., asphaltene, resin, wax. formation and deposition from petroleum due to changes in pressure, temperature and composition fall somewhat short in their structure. Further modeling needs involve an understanding of the chemistry of these materials and reflect the more modern approach to the physico-chemical structure of petroleum in order to more correctly predict the onset of precipitation as well as the location and amount of the sediment deposition in the producing wells and in oil-transport pipelines. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Complexity; Physico-chemical structure; Sediment
1. Introduction It is the general consensus that it is the polar, i.e., heteroatom, constituents of petroleum which are responsible for the formation of suspended organic solids during a variety of recovery processes ŽIslam, 1994; Park et al., 1994.. However, an area that remains largely undefined, insofar as the chemistry and physics are still speculative, is the phenomenon of the incompatibility of the crude oil constituents, as might occur during these operations. The formation )
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of a suspended solid phase during recovery Žas well as during refining operations. is related to the chemical and physical structure of petroleum; the latter is greatly influenced by the former. By way of a brief series of definition, the failure of petroleum fractions to mix and the separation of a separate phase is usually referred to as incompatibility. When incompatibility occurs, the constituents Žusually the asphaltenes and the resins. that form a separate phase are variously referred to as a precipitate, sediment, andror sludge formation depending upon the nature of the material and the causes of the separation.
0920-4105r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 4 1 0 5 Ž 9 8 . 0 0 0 5 1 - 5
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J.G. Speightr Journal of Petroleum Science and Engineering 22 (1999) 3–15
Fig. 1. The laboratory fractionation of petroleum.
Petroleum can be considered to be a delicately balanced system insofar as the different fractions which contain hydrocarbons Žsaturates and aromatics. as well as heteroatom constituents ŽFig. 1..
Although the heteroatom constituents tend to concentrate in the higher molecular weight fractions Žthe asphaltenes and resins. ŽFig. 2., nitrogen, oxygen, and sulfur species that are in near-neutral molecular
Fig. 2. The relative concentration of the heteroatoms after fractionation.
J.G. Speightr Journal of Petroleum Science and Engineering 22 (1999) 3–15
locales will also occur in the saturates and aromatics, remembering that the nomenclature is not necessarily precise and that the composition of each fraction is a function of the separation process ŽSpeight, 1991.. Asphaltenes are recognized as being a complex mixture of species of varying molecular weight and polarity ŽFig. 3.. Thus, carbenes and carboids are lower molecular weight highly polar species that are predominantly products of thermal processes and might not occur in the typical recovery process. However, the application of thermal techniques, such as fire flooding, to petroleum recovery can produce such species and they will either deposit on the reservoir rock or appear as suspended solids in the oil. In fact, it is recognized that it is the polar species in the crude oil that govern the oil–rock interactions in the reservoir ŽBruning, 1991. from which many sediments can arise. With the exception of the carbenes and the carboids, the fractions are compatible proÕided there are no significant disturbances or changes made to the system. Such changes are: Ž1. the alteration of the natural abundance of the different fractions; Ž2. the chemical or physical alteration of the constituents as might occur during recovery, especially changes that might be brought on by thermal processes; and Ž3. alteration of the polar group distribution as might occur during oxidation by exposure to aerial oxygen during the recovery process. In the reservoir, asphaltene incompatibility can cause blockages of the pores
Fig. 3. Representation of asphaltenes and carbenesrcarboids on the basis of molecular weight and polarity.
5
and channels through which the oil must move during recovery operations ŽPark et al., 1994; Islam, 1994; Leontaritis, 1996.. All of these incidents cause disturbances to the petroleum system. However, when such disturbances occur, it is the higher molecular weight constituents that are most seriously affected eventually leading to incompatibility Žprecipitation, sediment formation, sludge formation. depending upon the circumstances. Thus, the dispersibility of the higher molecular weight constituents becomes an issue that needs attention. And one of the ways by which this issue can be understood is to be aware of the chemical and physical character of the higher molecular weight constituents. By such means, the issue of dispersibility, and the attending issue of incompatibility can be understood and even predicted.
2. Discussion 2.1. Asphaltenes Asphaltenes are, by definition, a solubility class that is precipitated from petroleum, heavy oil, and bitumen by the addition of an excess of a liquid parrafinic hydrocarbon ŽGirdler, 1965; Andersen and Birdi, 1990; Speight, 1991, 1994; Speight and Long, 1996.. In addition, the composition of the asphaltenes fraction is dependent upon the nature of the hydrocarbon precipitant ŽFig. 4., the ratio of the volume of the precipitant to the volume of feedstock ŽFig. 5., to the contact time ŽFig. 6., and to the temperature at which the precipitation occurs ŽMitchell and Speight, 1973; Speight et al., 1984; Speight, 1991; Andersen and Stenby, 1996.. By virtue of the means of separation, the asphaltene fraction is chemically complex but it can be conveniently represented on the basis of molecular weight and polarity ŽFig. 3. ŽLong, 1979, 1981. and for different crude oils the slope of the line representing the distribution of molecular weight and the variation in polarity will vary ŽSpeight, 1994.. Asphaltenes can not be crystallized in the usual sense of the word. However, and by way of emphasis of the complex nature of this fraction, asphaltenes can
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Fig. 4. Asphaltene yield as a function of the carbon number of the paraffinic precipitant. Fig. 6. Asphaltene yield as a function of time.
be sub-fractionated by the use of a variety of techniques to produce fractions that vary considerably in molecular weight and which also vary considerably in terms of the functional group types and functional group content ŽFrancisco and Speight, 1984; Speight, 1991.. These data lend support to, and reinforce, the
Fig. 5. Asphaltene yield as a function of precipitant:oil ratio.
concept that asphaltenes are complex mixtures of molecular sizes and various functional types. In addition, any variation of the major parameters Žprecipitant, precipitant:oil ratio, time, and temperature. can cause substantial variations in the nature and amount of the separated asphaltenic material. It can vary from a dark brown amorphous solid to a black tacky deposit, either of which under the prevalent conditions could be termed an asphaltene. The elemental compositions of asphaltenes vary over only a narrow range corresponding to HrC ratios of 1.15 " 0.05%, although values outside of this range are often found ŽSpeight, 1991.. Notable variations do occur in the proportions of the heteroelements, in particular in the proportions of oxygen and sulfur. On the other hand, the nitrogen content of the asphaltenes has a somewhat lesser degree of variation. This is, perhaps, not surprising since exposing asphaltenes to atmospheric oxygen can substantially alter the oxygen content, and exposing a crude oil to elemental sulfur, or even to sulfur-containing minerals, can result in sulfur uptake. The data from the various studies intimate that asphaltenes, viewed structurally, contain condensed polynuclear aromatic ring systems bearing alkyl side
J.G. Speightr Journal of Petroleum Science and Engineering 22 (1999) 3–15
chains. The formulae derived from the studies invoked the concept of large polynuclear aromatic systems and efforts were made to describe the total structures of asphaltenes in accordance with magnetic resonance data and results of other spectroscopic and analytical techniques. The heteroatoms Žnitrogen and sulfur. have also been identified as occurring in a variety of locations, including cyclic and non-cyclic systems ŽSpeight, 1991, 1994.. The oxygen functions are considerably different to the nitrogen and sulfur function there being a lesser occurrence of oxygen in being systems and a greater tendency for oxygen to occur as hydrogenated Že.g., phenolic. functions ŽSpeight, 1991, 1994.. Metals Ži.e., nickel and vanadium. occur predominantly in the asphaltene fraction with lesser amounts occurring in the resins fraction ŽReynolds, 1994.. However, from a structural viewpoint, metals are much more difficult to integrate into the asphaltene system. It is known that the nickel and vanadium occur as porphyrins but whether or not these are an integral part of the asphaltene structure is not known. Some of the porphyrins can be isolated as a separate stream from petroleum. Asphaltene molecular weights are variable ŽSpeight, 1991, 1994. there being the tendency to associate even in dilute solution in non-polar solvents. However, data produced using highly polar solvents indicate that the molecular weights, in solvents that prevent association, usually fall into the range 2000 " 500 ŽSpeight et al., 1985.. As a result of these studies, it has been possible to derive various models for petroleum asphaltenes, some of which can be used to explain the chemical and physical properties of the asphaltenes constituents ŽSpeight, 1991, 1994. and offers some degree of predictability as a function of the molecular model. These models, that might be more in keeping with behavioral characteristics, have smaller polynuclear aromatic systems and also span the range of functional types as well as molecular sizes ŽSpeight, 1991, 1994.. In addition, such models can have the large size dimensions that have been proposed for asphaltenes since mobility effects can increase the ‘effective diameter’ of the asphaltenes molecules thereby presenting larger dimensions that is the case for the stationary molecule. In fact, asphaltenes molecules can be considered to be ‘molecular
7
chameleons’ insofar as they can vary in dimensions depending upon the degree of mobility and the angle of rotation about an axis andror the freedom of rotation about one, or more, of the bonds. 2.2. Structure of petroleum An early hypothesis of the physical structure of petroleum ŽPfeiffer and Saal, 1940. indicated that asphaltenes are the centers of micelles formed by adsorption, or even by absorption of part of the maltenes, that is, resin material, on to the surfaces or into the interiors of the asphaltene particles. Thus, most of those substances with greater molecular weight and with the most pronounced aromatic nature are situated closest to the nucleus and are surrounded by lighter constituents of less aromatic nature. The transition of the intermicellar Ždispersed or oil. phase is gradual and almost continuous. Since asphaltenes are incompatible with the oil Žsaturates and aromatics. fraction, asphaltene dispersion is mainly attributable to the resins Žpolar aromatics. indicating that the resins are, under ambient conditions, a necessary constituent and that by their presence they prevent incompatibility ŽSwanson, 1942; Witherspoon and Munir, 1960; Koots and Speight, 1975; Mushrush and Speight, 1995.. Furthermore, resins from one crude oil might disperse asphaltenes from a different crude oil with some difficulty, if at all ŽKoots and Speight, 1975.. The instability of many such ‘heterogeneous’ blends indicate that there may be significant structural incompatibility among the asphaltenes from one crude oil and the resins of another crude oil. The means by which asphaltenes and resins interact to exist in petroleum remains the subject of speculation but hydrogen bonding ŽMoschopedis and Speight, 1976a; Acevedo et al., 1985. and the formation of charge-transfer complexes ŽYen, 1974. have been cited as the causative mechanisms. There is evidence that asphaltenes participate in charge-transfer complexes ŽPenzes and Speight, 1974; Speight and Penzes, 1978. but the exact chemical or physical manner in which they would form in petroleum is still open to discussion. The original concept of the asphaltene–resin micelle invoked the concept of asphaltene–asphaltene association to form a graphite-like stack ŽFig. 7.
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celle, the structure is recognized as being complex ŽBardon et al., 1996..
2.3. Asphaltene deposition during recoÕery operations
Fig. 7. The stack-type concept of the asphaltene micelle.
which acted as the micelle core which, in turn, was stabilized by the resins. However, the concept of hydrogen-bonding interactions being one of the means of association between the asphaltenes and resins has, however, led to a reconsideration of the assumed cluster as part of the micelle. When resins and asphaltenes are present together, hydrogen-bonding studies show resin–asphaltene interactions are preferred over asphaltene–asphaltene interactions ŽMoschopedis and Speight, 1976a.. If the same intermolecular forces are projected to petroleum, asphaltenes in petroleum as single entities that are peptized, and effectively dispersed, by the resins ŽFig. 8.. However, whatever the means by which the individual molecular species are included in the mi-
Asphaltenes and asphaltene-related materials are known to deposit as sediments during recovery operations in the vicinity of production wells during miscible floods, after acid stimulation, or during pressure changes ŽBurke et al., 1990; Islam, 1994; Park et al., 1994.. Many reservoirs produce without any such problems until the oil stability is perturbed during later stages of oil production. The parameters that govern sediment formation and deposition of asphaltenic materials from petroleum are related to the composition of the crude oil ŽTable 1. as well as the parameters used for the recovery process ŽTable 2.. It must also be recognized that the material that deposits from the crude oil as a separate phase is more aromatic and richer in heteroatom compounds than the original crude oil. In fact, in some cases, especially when oxidation has occurred, the deposited material is more aromatic and richer in heteroatoms than in asphaltenes. In terms of the crude oil parameters that influence sediment formation, there has been considerable focus on the asphaltene content as well as the chemistry and physics of the asphaltenes relationship to the remainder of the oil. For example, while the asphaltene content of petroleum oils varies over a wide range ŽKoots and Speight, 1975; Speight, 1991., asphaltene content is not the single determining influence on sediment formation. It has been noted that
Fig. 8. The asphaltene–resin concept of the micelle.
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Table 1 Crude oil properties and their general relationship to sediment formation Parameter
Comments
Asphaltene content
affects light oil recovery and heavy oil recovery influences oil–rock interactions forms sediment when gases are dissolved in the oil thermal methods can change relationship to oil ) 1.0 Žoil is stable. - 1.0 Žoil is unstable. provides polarity, responsible for incompatibility preferential reaction with oxygen can cause incompatibility in paraffinic environments low gravity, high asphaltene content high viscosity, high asphaltene content can be changed by thermal recovery processes changes functional group composition
Asphaltene:resin ratio Heteroatom content Aromaticity API gravity Viscosity Oil medium Oxidation
asphaltene:resin ratios are usually below unity in oils that are stable and higher than unity in oils that exhibit ready precipitation of asphaltenic material ŽSachanen, 1945.. Even though it is generally understood that asphaltene content of a crude oil increases with decreasing API gravity, asphaltene precipitation has also been reported in light oils as well. Such an effect Žgas deasphalting. ŽSpeight, 1991. arises from the increased solubility of hydrocarbon gases in the petroleum as reservoir pressure increases during maturation. Incompatibility will also occur when asphaltenes interact with reservoir rock, especially acidic functions of rocks, through the functional groups Že.g., the basic nitrogen species. just as they interact with adsorbents. And, there is the possibility for interaction of the asphaltene with the reservoir rock through the agency of a single functional group in which the remainder of the asphaltene molecule remains in the liquid phase Žvertical association relative to the rock surface. ŽFig. 9.. On the other hand, the asphaltene constituents can react with the rock at
several points of contact Žhorizontal association relative to the rock surface. ŽFig. 10. thereby enhancing the bonding to the rock and, in some cases, effecting recovery operations to an even greater extent. Both modes of reaction can entrap other species Žsuch as resins and aromatics. within the space between the rock and the asphaltene. Another area where incompatibility might play a detrimental role during recovery operations occurs as a result of aerial oxidation. In general, it is the more polar species which oxidize first with or without the presence of catalysts leaving an oil that is relatively free of heteroatom species ŽMoschopedis and Speight, 1978.. Thus, oxidation is a means for the production of highly polar species in a hydrocarbon oil leading to the deposition of polar sediments and being analogous to the deasphalting procedure. Thus, after incorporation of oxygen to an oil-dependent limit, significant changes occur to asphaltenes and resins. These changes are not so much due to oxidative degradation but to the incorporation of oxygen functions that interfere with the natural order of intra-
Table 2 Recovery process parameters and their relationship to sediment formation Parameter
Comments
Carbon dioxide injection Miscible flooding Organic chemicals Acidizing Pressure decrease Temperature decrease
lowers pH by changing oil composition rich gas lowers solvent power of oil cause incompatibility through low solvent power interaction with constituents changes oil composition similar effect to pressure decrease
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Fig. 9. Vertically oriented asphaltene–mineral interactions.
molecular and intermolecular structuring leading to the separation of asphaltenic material ŽMoschopedis and Speight, 1976b, 1977.. Thus, with alterations in these parameters, the formation of a sediment can occur although the nature of the sediment will vary and be dependent upon the process of formation. There are also several process-related destabilizing forces that can cause precipitation of asphaltenic material and each involves disturbance to the equilibrium that exists within petroleum. For example, carbon dioxide causes the destabilization of the petroleum equilibrium by lowering pH, by changing oil composition, and by creating turbulence. Usually, asphaltene precipitation increases as the volume of carbon dioxide available to the crude oil increases during the later stages of carbon dioxide injection or stimulation. The most
noticeable primary locations of asphaltene deposition are the wellbore and the pump regions. In addition, flooding of a rich gas Žmiscible flooding. destabilizes the asphaltene–crude oil mixture by lowering the solvent power of the solution. The hydrocarbon gases used in such applications effectively cause deasphalting Žgas deasphalting, solvent deasphalting. of the crude oil. The negative effect of rich gas is at a maximum near the bubble point; this effect is alleviated after the bubble point is reached. Similarly, organic chemicals such as isopropyl alcohol, methyl alcohol, acetone, and even some glycol, alcohol, or surfactant based solvents, that do not have an aromatic component, may selectively precipitate asphaltenes and resins. Asphaltene precipitation may be caused by well stimulation, such as acidizing which involves a drastic shift in local chemical equilibria, pH and libera-
Fig. 10. Horizontally oriented asphaltene–mineral interactions.
J.G. Speightr Journal of Petroleum Science and Engineering 22 (1999) 3–15
tion of carbon dioxide. It may also increase the concentration of some ions, such as iron, which will promote the formation of asphaltenic sediment. A decrease in the pressure is another important factor that influences the onset of solids deposition from petroleum. In fact, the effect of pressure particularly noticeable just above the bubble point for crude oils that are rich in light ends. Depending on the location of the pressure decrease, deposition may occur in different parts of the reservoir as well as in the wellbore and in the production stream. Furthermore, a decrease in pressure is usually accompanied by a decrease in temperature which can also cause physico-chemical instability leading to the separation of asphaltenic from the oil. Pressure change alone can also invoke similar asphaltene precipitation. The asphaltenic material in crude oil is electrically charged through the existence of zwitterions or polarization within the molecular species ŽPreckshot et al., 1943; Katz and Beu, 1945; Penzes and Speight, 1974; Speight and Penzes, 1978; Fotland and Alfindsen, 1996.. Therefore any process Žsuch as the flow through reservoir channels or through a pipe. which can induce a potential across, or within, the oil will also result in the electrodeposition of asphaltenic material through the disturbance of the stabilizing electrical forces. In addition, neutralization of the molecular charge will also result in the formation of a sediment. 2.4. Models of organic deposition The mechanism of asphaltene precipitation is very complex ŽSpeight et al., 1984; Andersen and Speight, 1993; Buckley, 1996. and controversy as to the nature of asphaltene solutions persists. However, petroleum consists of a mixture of oil, aromatics, resins, and asphaltenes ŽFig. 1. and it is necessary to consider each of the constituents of this system as a continuous or discrete mixture interacting with each other. The omission of any one fraction could lead to errors in the outcome ŽAndersen and Speight, 1993.. There are two general approaches to the consideration of the nature of asphaltenes in oil ŽHirschberg et al., 1984; Mansoori et al., 1987; Park et al., 1994; Islam, 1994.. The first approach considers asphaltenes to be dissolved in oil in a true liquid state. In this case, asphaltene precipitation is considered to
11
depend on thermodynamic conditions of temperature, pressure and composition. This particular approach recognizes asphaltene precipitation as a thermodynamically reversible process. The second approach considers asphaltenes to be solid particles which are suspended colloidally in the crude oil and are stabilized by resin molecules; consequently, the deposition process is considered to be irreversible. If the material precipitated is a reacted derivative of the asphaltenes Ži.e., a sediment., this may be true but if the precipitated material is unreacted asphaltene then this assumption may need a correction. One question of major interest during recovery operations is the timing of sediment formationrdeposition and the amount of the organic material will separate from the oil under specific conditions. Two different models have been proposed to explain the behavior of petroleum and the potential for solids deposition during recovery operations ŽKawanaka et al., 1989; Islam, 1994; Park et al., 1994.. The continuous thermodynamic model utilizes the theory of heterogeneous polymer solutions is utilized for the predictions of the onset point and amount of organic deposits from petroleum crude. A steric colloidal model which is capable of predicting the onset of organic deposition has also been developed and a combination of these two models results in a fractal aggregation model. These efforts have generally been adequate to predict the asphaltene–oil interaction problems Žphase behavior andror flocculation. wherever it may occur during oil production and processing. In the continuous thermodynamic model the degree of dispersion of the high molecular weight organic constituents in petroleum depends upon the chemical composition of the petroleum. Precipitation of the high molecular weight material can be explained by a change in the molecular equilibria that exist in petroleum through a change in the balance of oil composition. Moreover, the precipitation process is considered to be reversible. Indeed, the reconstitution of petroleum after fractionation has been demonstrated ŽKoots and Speight, 1975. and lends support to this model. The ratio of polar to non-polar molecules and the ratio of high- to low-molecular-weight molecules in a complex mixture such as petroleum are the two
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J.G. Speightr Journal of Petroleum Science and Engineering 22 (1999) 3–15
factors primarily responsible for maintaining mutual solubility. The stability of the system is altered by the addition of miscible solvents causing the high molecular weight andror the polar molecules to separate from the mixture either in the form of another liquid phase, or as a solid precipitate. Hydrogen bonding and the sulfur- andror nitrogen-containing segments of the separated molecules could start to aggregate for polymerization and, as a result, produce a solid phase which separates from the oil. In the steric colloidal model, the high molecular weight materials in petroleum are considered to be solid particles of different sizes suspended colloidally in the oil and stabilized by other petroleum constituents Ži.e., resins. adsorbed on their surface. The original hypothesis of the physical structure of petroleum ŽPfeiffer and Saal, 1940. which resins played a role in the stabilization of the asphaltenes and later confirmation of the role of the resins in petroleum ŽSwanson, 1942; Witherspoon and Munir, 1960; Koots and Speight, 1975. is supportive of this model. In the fractal aggregation model, it is assumed that pi–pi interactions are the principal means by which asphaltenes associate. This assumption may not be completely valid because of the evidence that favors hydrogen bonding between the molecular species and the observation that asphaltene–resin interactions may predominate over asphaltene– asphaltene interactions in petroleum. The concept that asphaltene–asphaltene interactions may be the predominant interactions is true for solutions of asphaltenes, and this is reflected in the molecular weight data ŽSpeight et al., 1985. but there is no guarantee that these interactions are predominant in petroleum Žespecially with evidence that indicates the high potential for other interactions . ŽMoschopedis and Speight, 1976a; Speight, 1994.. In as much as the high molecular weight constituents of petroleum have a wide range of polarity and molecular weight distribution ŽFig. 3., such compounds have been considered to act as heterogeneous polydisperse polymers. However, knowing with a new understanding of the nature of asphaltenes ŽSpeight, 1994., there may be some difficulties with this assumption. Nevertheless, in order to predict the phase behavior of the these constituents, it has also been assumed that the properties
of these constituents depend on their molecular weights. The model also assumes that the asphaltenes are partly dissolved and partly in the colloidal state thereby accounting for both the solubility and colloidal effect of high molecular weight organic constituents in the lower molecular weight constituents. The proposed models can provide the tool for making satisfactory prediction of the phase behavior of the deposition of high molecular weight materials. Because the issue of the deposition of asphaltenic materials problem is complex, it is necessary to attempt an understanding of the deposition mechanism before an accurate and representative model can be formulated. Utilization of kinetic theory of fractal aggregation has enabled the development of the fractal aggregation model. This model allows to describe properly several situations, such as phase behavior of heavy organic deposition, the mechanism of the association of the high molecular weight constituents, the geometrical aspects of aggregates, the size distribution of the sediments, and the solubility of the high molecular weight constituents in the solution under the influence of a miscible solvent. Another potential model involves use of the solubility parameter of the asphaltenes and the surrounding medium. It is known that the solubility of asphaltenes varies with the solubility parameter of the surrounding liquid medium ŽMitchell and Speight, 1973. and calculation or estimation of the solubility parameters of various liquids is known. From these data, it is possible to estimate point at which asphaltenic material is precipitated when the composition of the oil is changed by the addition of a hydrocarbon liquid ŽMitchell and Speight, 1973.. In fact, the solubility parameter has been used successfully to determine the character of heavy oils and investigate the separation of asphaltenic material ŽWiehe, 1996.. The solubility parameter concept also recognized the gradation of polarity of the asphaltenes as selective precipitation occurs during the addition of a nonsolvent when the most polar constituents are precipitated first ŽAndersen and Speight, 1992.. The models which apply the solubility parameter concept calculate the interaction through an assumption of the total crude oil as formed by asphaltenes and deasphaltened oil; hence the system is regarded as a two component system. The changes in phase
J.G. Speightr Journal of Petroleum Science and Engineering 22 (1999) 3–15
13
Fig. 11. Estimation of the solubility parameter from the HrC atomic ratio.
equilibrium is caused by the changes in the solubility parameter of either of the two pseudo-components, which may happen either by dissolution of gas or alkane in the deasphaltened oil phase or by changes in temperatures. The amount of precipitated asphaltenes is calculated as the differences in asphaltenes present in the oil and the solubility of asphaltenes at the saturation point. In the different models the change in the composition of the deasphaltened oil is taken as significant for the phase equilibrium, and various methods, i.e., cubic equations of state, are applied to determine the properties of this fraction. Further development of this concept ŽSpeight, 1994. has led to the graphical representation of the
solubility parameters of polynuclear aromatic systems and estimation of the solubility parameter of the asphaltenes based on hydrogen:carbon ratios ŽFig. 11.. Further development of this knowledge can allow the progress of asphaltene deposition to be followed ŽFig. 12. and the region of sediment formation Žor the region of instability and incompatibility. to be estimated using a simplified phase diagram ŽFig. 13.. Obviously more work is need to apply the
Fig. 12. Potential changes in the solubility parameter due to interactions that lead to sediment deposition.
Fig. 13. Representation of petroleum as a three-phase system showing a region of instability.
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J.G. Speightr Journal of Petroleum Science and Engineering 22 (1999) 3–15
mathematical procedures to this concept but it does invoke the principle of knowledge of the chemical and physical properties of the asphaltenes and the oil. In general, these proposed models are, to a degree, applicable to the prediction of heavy organic deposition Žasphaltene, paraffinrwax, resin. from petroleum due to changes in pressure, temperature and composition. However, the use of assumptions that do not reflect, or recognize, what might be the actual chemical and physical structure of petroleum can lead to errors in the data. Further modeling must involve recognition of the more modern concepts of the structure of petroleum as well as application of the models to the predictability of the location and amount of the deposition of the sediments inside the producing wells and oil-transport pipelines.
3. Conclusions Petroleum is a complex system that depends upon the relationship of the constituents fractions to each other and the relationships are dictated by molecular interactions. Thus, recovery chemistry can only be generalized because of the intricate and complex nature of the molecular species that make up the crude oil leading to difficulties in analyzing not only the recovered material but also the original oil in place. Moreover, the incompatibility of crude oils with each other is a continuing issue and the occurrence of suspended organic solids during recovery Žespecially thermal. reduces the efficiency of a variety of processes. Asphaltene precipitation or the mere presence of asphaltenes may invoke many implications in recovering asphaltic crude oils. Asphaltene precipitation may occur under various thermally or non-thermally enhanced oil recovery schemes or even primary production conditions. These models that are proposed models as being applicable to the prediction of sediment Ži.e., asphaltene, resin, wax. formation and deposition from petroleum due to changes in pressure, temperature and composition. Further modeling must involve an understanding of the chemistry of these materials and reflect the more modern approach to the physico-chemical structure of petroleum in order to
more correctly predict the onset of precipitation as well as the location and amount of the sediment deposition in the producing wells and in oil-transport pipelines.
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