An Improved Paraffin Inhibitor
- S.M. Bucaram (Sinclair Oil And Gas Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- February 1967
- Document Type
- Journal Paper
- 150 - 156
- 1967. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 2.4.3 Sand/Solids Control, 4.2 Pipelines, Flowlines and Risers, 4.3.3 Aspaltenes, 4.1.9 Tanks and storage systems, 4.1.5 Processing Equipment, 3.4.1 Inhibition and Remediation of Hydrates, Scale, Paraffin / Wax and Asphaltene, 4.1.2 Separation and Treating
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Laboratory investigations of the paraffin deposition process, and the possibility of inducing favorable changes in the crystal structure of the precipitated waxes, resulted in the development of several paraffin crystal modifiers. Of these, polyethylene was found the most effective because of its ability to modify and/or reduce the size and structure of paraffin precipitated from crude oil. Changes induced by polyethylenes of various molecular structures and weights were investigated and results were recorded by means of photomicrographs. It was found that high molecular weight polyethylene possessing an amorphous structure was the most effective paraffin crystal modifier. Field testing of polyethylene was conducted with excellent results. Inhibitor combinations incorporating polyethylene are currently in use to combat paraffin deposition.
The nature and occurrence of paraffin deposits have been extensively studied, and literature is available dating as far back as 1865. The patent literature describes many products developed to remove or prevent paraffin deposits, but in spite of these contributions the problem of paraffin deposition was not solved. Shock et al. reported that paraffin problems occur in almost every oil-producing area. Their survey covering 69 producing areas in 19 states showed that paraffin problems are present in 59 of these areas and in 18 states. The problems range from year-around deposition in the formation, down-hole equipment, surface lines, separators and storage tanks to seasonal problems which occur only in winter months. Paraffin deposits form by two mechanisms: (1) a growth process directly on the pipe surface and (2) accumulations of wax crystals in the oil which in time attach to each other and to the metal. Paraffin inhibition can be obtained either by affecting the wettability of metal surfaces through film-forming action, or by the use of materials which modify and disperse wax crystals. The literature reports that inhibition by crystal modification is the most promising and potentially the most economical method for paraffin control. Our work in the area of paraffin control substantiates this conclusion.
Composition of Paraffin Deposits
Paraffin deposits range from almost pure white paraffin wax to those which are almost totally asphaltic in nature. Most deposits fall between these extremes and contain paraffin waxes, microcrystalline waxes, asphaltic material, resins, oil, water, sand and silt.
These waxes constitute about 40 to 60 percent of an average crude paraffin deposit. They can be described as long-chain compounds containing from about 26 to 50 carbon atoms. Solid paraffin waxes consist of large, well-formed, needle-shaped crystals which agglomerate and form large masses.
These waxes account for less than 10 percent of the crude paraffin deposit. They are long-chain hydrocarbon compounds with branched-chain and cyclic-ring molecules located at random along the carbon chain. Microscopic examination of microcrystalline waxes shows the crystalline structure to be very small and irregular. When suspended in a solvent, no well-formed crystals are observed. The small, irregular crystals tend to remain dispersed in fluid and show little tendency to agglomerate.
Resins and Asphaltenes
Asphaltic materials are sticky, dark, semi-solid amorphous substances. The amount present in crude oil deposits has been reported to range from 10 to about 56 percent. The asphaltenes are made up of condensed aromatic rings with few side chains and have carbon-hydrogen ratios of approximately 10. Molecular weights have been found to range from less than 1,000 to about 140,000. Resins are almost identical to asphaltenes but have slightly lower molecular weights.
Theory of Inhibition
The controlling factor in the formation of paraffin crystals is nucleation. For a crystal to form, a stable nucleus must first exist to act as a growth center for the attachment of paraffin molecules. According to Walton, prior to nucleation there is a continuous forming and dissociation of molecular clusters. As more paraffin molecules precipitate from solution, the concentration becomes sufficiently large to disturb the equilibrium resulting in more clusters forming than dissociating.
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