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Asphaltenes and waxes
Deposition of the high-molecular-weight components of petroleum fluids as solid precipitates in surface facilities, pipelines, downhole tubulars, and within the reservoir are well-recognized production problems. Depending on the reservoir fluid and the type of recovery process, the deposited solid may consist of:
- A mixture of these materials
The deposits also can contain resins, crude oil, fines, scales, and water.
Asphaltenes and waxes are a general category of solids and, thus, cover a wide range of materials. Understanding the fundamental characteristics that define the nature of asphaltenes and waxes is valuable in reducing or avoiding the production impacts of their deposition. This page examines the general chemical classifications and types of asphaltenes and waxes, in addition to their solidification behaviors.
Chemical classification of petroleum fluids
Petroleum-reservoir fluids are complex multicomponent mixtures. The chemical constituents of petroleum may be classified broadly as belonging either to the C6- or the C6+ fraction. The light end, or C6- fraction, of petroleum fluids is composed of well-defined pure hydrocarbon components with carbon numbers up to 5 and the light gases nitrogen (N2), carbon dioxide (CO2), and hydrogen sulfide (H2S). The hydrocarbons in the light end primarily are straight-chain normal alkanes (n-alkanes) and their branched isomers (i-alkanes). The heavy end, or C6+ fraction, consists of all the components with carbon numbers of 6 or greater.
Classification of petroleum constituents
A classification system and nomenclature commonly used in the petroleum industry describes components as belonging to the paraffinic (P), naphthenic (N), or aromatic (A) fractions. These are often referred to jointly as PNA.
This class includes n-alkanes and i-alkanes that consist of chains of hydrocarbon segments (-CH2-, -CH3) connected by single bonds. Methane (CH4) is the simplest paraffin and the most common compound in petroleum reservoir fluids. The majority of components present in solid wax deposits are high-molecular-weight paraffins.
This class includes the cycloalkanes, which are hydrocarbons similar to paraffins but contain one or more cyclic structures. The elements of the cyclic structures are joined by single bonds. Naphthenes make up a large part of microcrystalline waxes.
This class includes all compounds that contain one or more ring structures similar to benzene (C6H6). The carbon atoms in the ring structure are connected by six identical bonds that are intermediate between single and double bonds, which are referred to as:
- Hybrid bonds
- Aromatic double bonds
- Benzene bonds
Resins and asphaltenes
Resins and asphaltenes primarily are a subclass of the aromatics, although some resins may contain only naphthenic rings. They are large molecules consisting primarily of hydrogen and carbon, with one to three sulfur, oxygen, or nitrogen atoms per molecule. The basic structure is composed of rings, mainly aromatics, with three to ten or more rings per molecule.
SARA classification of petroleum constituents
The components of the heavy fraction of a petroleum fluid can be separated into four groups: saturates, aromatics, resins, and asphaltenes (SARA).
- Saturates include all hydrocarbon components with saturated (single-bonded) carbon atoms. These are the n-alkanes, i-alkanes, and cycloalkanes (naphthenes).
- Aromatics include benzene and all the derivatives composed of one or more benzene rings.
- Resins are components with a highly polar end group and long alkane tails. The polar end group is composed of aromatic and naphthenic rings and often contains heteroatoms such as oxygen, sulfur, and nitrogen. Pure resins are heavy liquids or sticky solids.
- Asphaltenes are large highly polar components made up of condensed aromatic and naphthenic rings, which also contain heteroatoms. Pure asphaltenes are black, nonvolatile powders.
The experimental method used to determine the weight fractions of these groups is called SARA analysis.
Nature of asphaltenes
Asphaltenes are a solubility class that is soluble in light aromatics such as benzene and toluene but is insoluble in lighter paraffins. They normally are classified by the particular paraffin used to precipitate them from crude (e.g., n-pentane or n-heptane). Fig. 1 from Mitchell and Speight shows that different alkane solvents yield different amounts of precipitates. Fig. 2 from Speight et al. shows dependence of the aromacity (hydrogen/carbon atomic ratio) and molecular weight of asphaltene on the precipitating solvent. These figures also indicate that the amounts and natures of asphaltenes precipitated with n-heptane or heavier alkanes are very similar. Speight, Long and Trowbridge provides a summary of standard analytical methods for asphaltene separation with either n-pentane or n-heptane.
Fig. 2 – Relationship of asphaltene aromaticity to carbon number of the paraffin. (Reprinted from Fuel, J.G. Speight, R.B. Long, and T.D. Trowbridge, “Factors Influencing the Separation of Asphaltenes From Heavy Petroleum Feedstocks,” pages 616-620, Copyright 1984, with permission from Elsevier Science.)
Although the exact nature of the original state of equilibrium of asphaltenes in petroleum fluids is still under investigation, one characteristic is the tendency of asphaltenes to form aggregates in hydrocarbon solutions. These aggregates are called micelles. The micelles and the hydrocarbon medium form a colloidal system. One commonly held view is that the colloids are stabilized by resins adsorbed on their surface, and the dispersion of colloids in the fluid form a two-phase system. Fig. 3 from Leontaritis schematically shows asphaltene-resin micelles that are suspended in the oil. Colloids also may be solvated by the surrounding medium, forming a true single-phase solution. Thermodynamic models (e.g., the solubility-parameter model of Hirschberg et al.) inherently assume the single-phase view. The role of resins in the single-phase or two-phase solution models may be quite different. Changes in pressure, temperature, and composition may alter the solubility parameter of the oil and/or the asphaltene-resin association and cause asphaltene precipitation. See also wax precipitation.
Fig. 3 – Asphaltene-resin micelles.
The definition of asphaltenes as compounds that are soluble in aromatics such as toluene and insoluble in light alkanes are referred to as laboratory asphaltenes by Joshi et al. Asphaltenes that precipitate in the field from a depressurization process are called field asphaltenes and contain different constituents. Laboratory and field precipitates contain combinations of asphaltenes and resins. Speight referred to them as asphalts, but that distinction is not made here.
Stability of asphaltenic crudes
SARA ratios play an important role in the solubility of asphaltenes. Avila et al. performed SARA analyses on 30 Venezuelan oil samples and attempted to associate the SARA contents with asphaltene precipitation observed in the field. Fig. 4 shows the SARA contents of crude oils that experience asphaltene precipitation in the field and those that do not. Crude oils with a high content of saturates and low contents of aromatics and resins clearly are more prone to asphaltene precipitation.
Fig. 4 – SARA effect on the stability of several crude oils regarding asphaltene precipitation.
Correlation for asphaltene precipitation with alkanes
Asphaltene precipitation at laboratory and field conditions can be predicted with thermodynamic models. For precipitation with alkanes at atmospheric conditions, a simple correlation from Rassamdana et al. and Sahimi et al. can be used.
Fig. 5 from Sahimi et al. shows the experimental weight percents of precipitated asphaltene, W (g of precipitated asphaltene/g of crude oil × 100%), as a function of the solvent to crude oil ratio, R (cm3 of solvent/g of crude oil), from precipitation experiments of an Iranian crude oil with n-C5, n-C6, n-C7, n-C8, and n-C10 at 26°C and atmospheric pressure. As expected, the amount of precipitates decreases with increasing solvent carbon number. Rassamdana et al. and Sahimi et al. found that the experimental points in Fig. 5 could be collapsed onto a scaling curve of Y vs. X with
where Ma is the molecular weight of the alkane solvent. Fig. 6 shows the resulting scaling curve. This curve can be represented accurately by a cubic order polynomial:
The critical solvent ratio, Rc, where precipitation starts to occur obeys the correlation
The factor 0.275 corresponds to a temperature of 26°C. For other temperatures, the following correlation is proposed.
where Tc is a temperature-dependent parameter.
Fig. 5– Experimental data for the weight percent, W, of precipitated asphaltene as a function of the solvent to crude oil ratio, R, in cm3/g. The results are, from top to bottom, for n-C5, n-C7, n-C8, and n-C10 as the precipitating agent.
Fig. 6 – Scaling curve for data in Fig. 5.
Characteristics of petroleum waxes
This section discusses the phase behavior and properties of wax-forming components, primarily normal alkanes, relevant to understanding and modeling wax phase behavior.
Types of petroleum waxes
Petroleum waxes are complex mixtures of n-alkanes, i-alkanes, and cycloalkanes with carbon numbers ranging approximately from 18 to 65. The minimum energy-chain structure of alkanes is a flat zig-zag of carbon atoms with the hydrogen atoms located in planes passing through the carbon atoms perpendicular to the chain axes. Fig. 7 shows this structure schematically for typical petroleum wax components.
There are two general classes of petroleum waxes. Waxes composed primarily of normal alkanes crystallize in large flat plates (macrocrystalline structures) and are referred to as paraffin waxes. Waxes composed primarily of cycloalkanes and i-alkanes crystallize as small needle structures and are referred to as microcrystalline waxes. Table 1 shows a comparison of the properties of paraffin and microcrystalline waxes as given by Gilby. Musser and Kilpatrick isolated waxes from sixteen different crude oils and found that paraffinic waxes had molecular weight ranges of 350 to 600, while microcrystalline waxes had large molecular weight ranges of 300 to 2,500. Of the 16 oils analyzed, five exhibited microcrystalline wax deposition, six precipitated paraffinic waxes, and the remaining five showed a mixture of paraffinic and microcrystalline waxes.
Table 1 - Typical composition and properties of commercially available paraffin and microcrystalline waxes
In addition to the possibility of precipitating mixtures of the two different types of waxes, the crystal structures in solid-wax deposits will be malformed to some degree because of the complex precipitation environment encountered in petroleum production. Crystal imperfections may occur when the temperature of the solution is decreased rapidly or when heavy aromatic components of the oil are incorporated into the lattice structure. The presence of molecules that hinder the lattice formation result in a wax phase composed of many small, independent crystal lattices.
Precipitation of petroleum waxes
Solid-wax formation consists of two distinct stages: nucleation and crystal growth. As the temperature of a liquid solution is lowered to the wax appearance temperature (WAT), the wax molecules form clusters. Wax molecules continue to attach and detach from these clusters until they reach a critical size and become stable. These clusters are called nuclei and the process of cluster formation is called nucleation. Once the nuclei are formed and the temperature remains below the WAT, the crystal-growth process occurs as further molecules are laid down in a lamellar or plate-like structure.
Nucleation is described as either homogeneous or heterogeneous. Homogeneous nucleation occurs in liquids that are not contaminated with other nucleating materials. In this case, the development of nucleation sites is time dependent. Heterogeneous nucleation occurs when there is a distribution of nucleating material throughout the liquid. If there is sufficient nucleating material, heterogeneous nucleation can be nearly instantaneous. Pure hydrocarbon mixtures in laboratories rarely undergo heterogeneous nucleation, whereas crude oil in the reservoir and production tubing will most likely nucleate this way because of the presence of:
- Formation fines
- Corrosion products
Solidification behavior of normal alkanes
Turner reviewed the properties of normal alkanes found in petroleum waxes, including:
- Solid-phase transitions
- Crystal structures
- Phase behavior of binary mixtures
Fig. 8 shows experimental data and correlation predictions for normal alkane melting temperatures at atmospheric pressure as a function of carbon number. In addition to the solid/liquid-phase transition indicated in this figure, many normal alkanes undergo solid/solid-phase transitions within a few degrees below the melting point.
Normal alkanes can assume four different crystal structures:
For normal alkanes with odd carbon numbers from 11 to 43 and even carbon numbers from 22 to 42, the crystal structure formed on cooling from a melt is hexagonal. This structure has a high degree of molecular-rotational freedom and is characteristically plastic and translucent. All the other crystal structures are restricted rotationally, resulting in a hard deposit and opaque appearance. The even-carbon-number alkanes from 12 to 20 form a triclinic structure on cooling from the melt, whereas all alkanes with carbon numbers 43 or greater form an orthorhombic structure on cooling from the melt. This is also the stable low-temperature form of the alkanes with odd carbon numbers less than 43, which is achieved by further cooling from the hexagonal structure. The monoclinic structure is never attained directly from the melt but is assumed by the even-carbon-number alkanes on cooling from the hexagonal or orthorhombic structures.
Solidification behavior of alkane mixtures
Binary mixtures of wax forming n-alkanes are completely miscible in the liquid state. In general, these binary mixtures form continuous-solid solutions if both molecules are similar in form and dimension and exhibit the same crystal structure in their pure state. Practically, this means that single-phase-solid solutions form when the molecular length difference is less than 6%. For n-alkanes with carbon numbers 18 to 35, the critical length difference is 2 to 6 carbon atoms. The behavior of binary mixtures depends on whether the constituents are both odd-numbered alkanes, both even-numbered alkanes, or a mixture of odd- and even-numbered alkanes because of the different pure component crystal structures.
The solid-phase behavior of binary mixtures also has been observed to be time and temperature dependent. Dorset shows that some mixtures, such as C30 with C36, form metastable continuous-solid solutions that separate into eutectics with complete fractionation of the constituents over a period of days. Other mixtures, such as C30 with C40, show complete immiscibility immediately on cooling.
For binary mixtures that form continuous-solid solutions, the stable low-temperature configuration is an orthorhombic structure, which is slightly different from the pure component orthorhombic crystal. This occurs for systems in which one alkane is contaminated with even 1 or 2% neighboring alkanes. This same structure has been observed for synthetic ternary and higher mixtures, as well as for diesel fuels. The diesel fuels exhibited an amorphous (microcrystalline) solid phase in addition to the orthorhombic macrocrystalline phase. Pedersen et al. and Hansen et al. also noted the probable existence of solid/solid-phase transitions with variations in temperature in their studies on a number of North Sea crude oils.
In contrast with the phase separations observed in binary mixtures of alkanes with significant length differences, Dirand et al. and Chevallier et al. found that commercial paraffin waxes with continuous distributions of 20 to 33 consecutive n-alkanes formed single-phase orthorhombic-solid solutions at room temperature. The wax deposit from one crude oil also showed the same single-phase macrocrystalline structure; however, an amorphous solid was also present. Increasing the temperature of the commercial waxes to their melting points of 55 to 65°C showed the existence of several different two-phase solid domains for these mixtures.
Significance of experimental solidification behavior for model development
As indicated in the previous discussion, solidification behavior of petroleum-mixture components can range from the relatively simple crystallization of pure n-alkanes into well-defined solid structures to the very complex precipitation of solids from live reservoir fluids into multiphase microcrystalline and imperfect macrocrystalline domains. Development of thermodynamic models for predicting the equilibrium-phase behavior of solid waxes depends on which phenomena are to be modeled and on the availability of experimental data for estimating parameters and testing models. The determination of the properties and phase behavior of solid waxes is an area of active research.
The simplest models are written for a single-component single-phase solid. Models of this type may be applied to pure component solidification cases or as an approximation in which a multicomponent wax is treated as one lumped component. More common is the solid-solution model in which a single-phase multicomponent solid deposit is assumed. Some researchers have extended the experimental evidence of immiscible pure solid phases for binary mixtures to the multicomponent case. Lira-Galeana et al. proposed a multisolid wax model in which the solid deposit is assumed to consist of a number of immiscible solid phases, each of which is composed of a single pure component. Generally, the solid deposit is considered to be made up of a number of multicomponent phases, as in the work of Coutinho.
The experimental work discussed generally supports the assumption of multiple solid phases, although Dirand et al. and Chevallier et al. have shown that commercial waxes with a large number of consecutive n-alkanes can form a single multicomponent solid solution at room temperature. As discussed in Thermodynamic models for wax precipitation, the models currently available are able to operate in predictive mode for some well-defined systems, but reservoir-fluid modeling still relies heavily on the availability of experimental data.
|Ma||=||molecular weight of alkane solvent, m|
|r||=||radial distance, L|
|R||=||solvent to crude oil ratio|
|Rc||=||critical solvent ratio|
|W||=||weight percent of precipitated asphaltene, m/m|
|X||=||defined in Eq. 1|
|Y||=||defined in Eq. 2|
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Noteworthy papers in OnePetro
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Jamaluddin, Abul. 2013. Flow Assurance – Managing Flow Dynamics and Production Chemistry. https://webevents.spe.org/products/flow-assurance-managing-flow-dynamics-and-production-chemistry-2
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