Paraffin Problems in Crude Oil Production And Transportation: A Review
- Misra Sanjay (Oil & Natural Gas Corp. Ltd.) | Baruah Simanta (Oil & Natural Gas Corp. Ltd.) | Singh Kulwant (Oil & Natural Gas Corp. Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- February 1995
- Document Type
- Journal Paper
- 50 - 54
- 1995. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 4.3.4 Scale, 4.3.3 Aspaltenes, 4.1.5 Processing Equipment, 4.2 Pipelines, Flowlines and Risers, 2.4.3 Sand/Solids Control, 4.2.3 Materials and Corrosion
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Problems related to crystallization and deposition of paraffin waxes during production and transportation of crude oil cause losses of billions of dollars yearly to petroleum industry. The goal of this paper is to present the knowledge on such problems in a systematic and comprehensive form. The fundamental aspects of these problems are defined, and characterizatin of paraffins and their solubility tendencies have been discussed. It has been established conclusively that n-paraffins are predominantly responsible for this problem. Comprehensive discussion on the mechanism of crystallization of paraffins has been included. Compounds other than n-paraffins, especially asphaltenes and resins, have profound effects on solubility of n-paraffins. In evaluations of the wax potential of a crude, the climate of the area concerned should be considered. Under the most favorable conditions, n-paraffins form clearly defined orthorhombic crystals, but unfavorable conditions and the presence of impurities lead to hexagonal and/or amorphous crystallization. The gelation characteristics are also affected the same way. An attempt was made to classify the paraffin problems into those resulting from high pipeline pressure, high restarting pressure, and deposition on pipe surfaces. Fundamental aspects and mechanism of these dimensions are described. Wax deposition depends on flow rate, the temperature differential between crude and pipe surface, the cooling rate, and surface properties. Finally, methods available in the literature for predicting these problems and evaluating their mitigatory techniques are reviewed. The available methods present a very diversified picture; hence, using them to evaluate these problems becomes taxing. A top priority is standardizing these methods for the benefit of the industry.
Problems related to crystallization and deposition of paraffin waxes during production and transportation of crude oil are well known. Extensive research by many workers has enriched our knowledge on the subject. Paraffin problems are causing losses of billions of dollars per year to petroleum industry worldwide through the cost of chemicals, reduced production, well shut-in, less utilization of capacity, chocking of the flowlines, equipment failure, extra horsepower requirement, and increased manpower attention. Indepth understanding of such problems is of paramount importance to oilfield operators in their search for technical/economic solutions. This paper reviews the fundamentals of these problems; the mechanisms of wax crystallization, gelation, and deposition; and laboratory methods for predicting and quantifying these problems.
Characterization of Paraffin Wax
By historical definition of the problem, the organic compounds of the crude, called paraffin, must be insoluble in the crude at the producing conditions.1 They must be higher-molecular-weight compounds of various homologous series. The classes of compounds recognized as possibly being in the deposits are (1) aliphatic hydrocarbons (both straight and branched chain), (2) aromatic hydrocarbons, (3) naphthenes, and (4) resins and asphaltenes. Gruse and Stevens2 described some representatives of each of these classes of chemicals, their structures, and their boiling and melting points. In reality, however, these compounds can be present in crude oil in pure generic forms or mixture of these forms. For example, in a given compound, alicyclic and aromatic rings can coexist in a streight, chain moiety. The length and number of side chains and the presence of alicyclic, aromatic, and condensed rings have a profound effect on melting point, boiling point, and solubility of these compounds in crude oil.
Recognized as the principal constituents of macrocrystalline waxes, n-paraffins give rise to clearly defined, needle-shaped crystals. The branched-chain paraffins make up the major portion of microcrystalline waxes. The long, straight-chain naphthenic and aromatic paraffins also contribute to microcrystalline waxes and have a marked effect on the type of crystal growth of macrocrystalline waxes.1 Macrocrystalline waxes lead to paraffin problems in production and transportation; microcrystalline waxes contribute most to the tank bottom sludges.
It has been established conclusively that wax deposited during production and transportation of crude oil predominantly consists of n-paraffins with smaller amounts of branched-chain and cyclic paraffins and aromatics.3 In a typical example, analysis of oil well equipment deposits showed that paraffins are the dominant species (52% )besides asphaltenes and resins (<5%)4; the balance is made up of crude oil, water, and mechanical impurities.
Paraffin wax molecules are straight-chain alkanes that contain more than 15 carbon atoms and have very little branching. Swetgoff5 listed the melting points of some paraffin molecules. Waxes containing up to C80 paraffin compounds have been reported.6
Paraffin compounds containing more than 20 carbon atoms are considered potential troublemakers for oilfield operators. Studies by Holder and Winkler7 showed that wax that came out of heavy fuel oil on cooling from 9 to -10°F had a composition centering on C20 and a spread from C16 to C27. As the temperature increases, the average number of carbon atoms of the crystallized wax in creases. For example, wax composition centered on C22 when the same oil was cooled between 33 and 9°F with a spread of C17 to C29. Leonidov et al.8 demonstrated that the average carbon numbers of waxes precipitated at 14, 32, 68, and 86°F were 22, 23, 26, and 29 respectively.
While evaluating wax problem potentials of a given crude. we must consider the following points: (1) the concentration of n-paraffins; (2) their carbon number distribution; (3) the concentration of branched paraffins, naphthenes, and aromatics; (4) the concentration of resins and asphaltenes; and (5) the climate of the area or the temperature regimes.
While the first, second, and fifth factors would help us predict the paraffin (macrocrystalline) wax deposition potential, the others would indicate moderation in the extent of problem.
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