CRUDE OIL POLARITY MEASURES QUALITY, PREDICTS BEHAVIOR

Inai Martins Ribeiro de Andrade Bruning
Consultant
Bad Godesberg, Germany

The classification of crude oil according to polarity provides a useful tool for the petroleum industry in predicting the behavior of crude in refining, production, and reservoir environments.

Polarity is a single value representing crude oil quality that can be used for comparison and evaluation purposes.

CRUDE OIL POLARITY

Petroleum consists of a complex mixture of substances, the predominant constituents being saturated and aromatic hydrocarbons. Minor components are heteroatomic compounds containing nitrogen, oxygen, and sulfur atoms.

The concentration of those constituents depends on the petroleum origin and geologic history. Because chemical composition and polarity are intimately associated, the polarity of a crude oil depends on its constituents-particularly, the heterocompounds.

Those heteroatomic compounds are concentrated in the heaviest fractions of crude oils, traditionally called resins and asphaltenes. They possess surface activity and stabilize water-in-oil emulsions.'

Heterocompounds include a wide variety of polar and polarizable molecules that interact with each other. The polarity resulting from their synergistic action among themselves, and with the predominantly paraffinic and aromatic medium, is the polarity shown by the crude.

Polarity rules oil-reservoir rock interactions. Consequently, migration and secondary and tertiary recoveries are also influenced.

Subsequent oil operations (demulsification, transportation, and refining) will also be affected, because polar oils cause water-oil emulsion stability, corrosion problems, catalyst poisoning, and product impurities.

Although its influence is widespread in the oil industry, oil polarity has not in the past been measured. It is usually inferred from the oil chemical characteristics (sulfur, nitrogen, aromatic hydrocarbon, and asphaltene contents, and acidity).

Recent work developed a method of determining and quantifying relative oil polarity using inverse gas chromatography.2 3 4. It uses the oil fraction that distills above 260 C. as the stationary phase, and a series of substances of known polarity as solutes.

The method was primarily developed to evaluate crude emulsion-forming ability in order to predict demulsification problems in recently discovered oil fields.

It soon became apparent that crude oil classification based on polarity is very useful to the petroleum industry because it provides a single number that can represent the behavior of the crude.

Polar compounds in crude oils are responsible for a series of undesirable characteristics that affect the oil industry from the reservoir to the refinery.

Consequently, the more polar crude will be the more interactive and less valuable one; its production and refining will be more difficult and costly.

This work reports the results obtained when this method was applied to 98 Brazilian and 5 foreign crudes.

EXPERIMENTAL

Experimental details for the preparation of the gas chromatographic column packings have been published previously.2 3 4 The same chromatographic conditions and reagents were employed in this study, and the measurement of the relative oil polarity used procedures that frequently characterize stationary phases in gas chromatography.5 6 Crude oil samples were typical of each field's production.

The oil fields were separated according to the five production areas of Brazil: Campos basin, with 21 oil fields; Espirito Santo, with 8; Bahia, with 21; and the Northeastern and North-northeastern areas, both with 24 oil fields.

For comparison purposes, five imported oils were also evaluated: Leona, Blend 1 (85% Qatar Marine and 15% Soviet Blend), Blend 11 (86% Soviet Blend and 14% Qatar Marine), Blend III (44% Heavy Maya, 36% Kirkuk, and 20% Cabiunas), and Kirkuk.

RESULTS

The Brazilian crudes were classified according to increasing polarity values.

These values ranged from 43.5 for Aracas crude oil, the least polar oil from Bahia, to 576.6 for Castanhal, a heavy and biodegraded crude from the Northeastern area.

Complete analyses of all oils were performed so that their other characteristics could be compared with their polarity values.

Although no direct correlation was found, polarity values increased with nitrogen, sulfur, and asphaltene contents; specific gravity; and acid numbers.

These results were anticipated because the polarity value is a measure of the resultant effect of all interactions of polar molecules in the crude. This is influenced by both quantitative and qualitative crude characteristics.4

The behavior of those crudes in demulsification operations and refining processes was already known. When these data were evaluated together with the determined polarity values, it was possible to establish the following classifications:

Nonpolar crudes, which possess relative polarity values up to 200, exhibit no affinity for water, are easily treated, and have excellent refining characteristics.
Low polar crudes, which have polarity values between 200 and 300, do not offer significant treatment and refining difficulties.
Polar crudes, which have polarities of 300-400, have emulsion-separation and refining problems that can be solved with the present technology of the oil industry. The great majority of processed petroleums are of this type.
Highly polar crudes, which have polarities between 400 and 500, have treatment and refining difficulties that require special attention in order to optimize operational procedures and diminish costs.
Extrapolar crudes, which have polarities above 500, are usually biodegraded oils with high heteroatomic compound contents. They have lower commercial values and form extremely stable water emulsions. Their separation and refining problems demand adequate solutions for economical processing.

Complete crude characterization and integrated interpretation with the polarity data are presented elsewhere.7

Highly polar and extrapolar oils were generally crudes that exhibited varied degrees of biodegradation.

In the Espirito Santo area, such a correlation was apparent because all crudes of this area are homogeneous and possess the same geological origin. Their polarity differences could be directly attributed to biodegradation, which caused the increased concentration in heteroatomic compounds.

Fig. 1 compares the gas chromatographic fingerprints of three of these oils, and Table 1 shows their characteristics.

Sao Mateus shows signs of biodegradation and Rio Itaunas is already highly biodegraded. The emulsion formed by the Rio Itaunas crude is so stable that it is demulsified after it is blended with the less polar oil of the region, Lagoa Suruaca, which has a polarity value of 237.1.

Of the crudes studied, Campos basin has the largest number of polar, highly polar, and extrapolar crudes, also derived from biodegradation.

On the other hand, the polarity of the Bahia crudes is low.

Eighteen oils are nonpolar, two of them being extremely nonpolar. These two are Aracas and Agua Grande crudes, which possess, respectively, polarities of 43.5 and 99.0.

The meaning of those values was fully understood when the polarity of mineral oil was measured under the same conditions and produced a value of 38.

Only three crudes are low polar: Estacao D. Joao, Estacao Nova Cassarongongo, and Buracica-Estacao Camboata, with polarity values of 211.6, 215.1, and 235.8, respectively.

Those results reflect the homogeneity of the crudes of Bahia, which are highly paraffinic and offer low sulfur, nitrogen, and asphaltene contents. Those oils are very easily treated, the region demulsifier consumption is low, and they provide excellent streams for the petrochemical industry.

The three crudes with higher polarity are produced from old wells. The oils have increased acid numbers, probably due to microbiological contamination during their production lifetime.

In the Northeastern region, most oils are polar or very polar. But their polarity derives from the high asphaltene and resin contents because they do not exhibit signs of biodegradation.

Only one oil in this region was found to be biodegraded. This is the Castanhal crude, with API gravity of 13.2 and extrapolarity of 576.6.

The majority of the North-northeastern crudes offers low polarity, although one oil is highly polar and two are extrapolar. Here again, high polarity can be attributed to biodegradation.

Fig. 2 compares the polarity of the Brazilian crudes studied.

Table 2 shows the properties of the imported crudes, including their polarities. Blend I and Kirkuk crudes are polar, Leona crude and Blend II are highly polar, and Blend III is extrapolar.

The characteristics of Blend I and Kirkuk are similar, and so are their polarities. Leona and Blend III have significantly higher asphaltene contents.

In Blend III, this is probably derived from the major contribution of Heavy Maya to the blend.

It should also be noted that the UOP K-factor is insufficient to explain the differences between crude properties. Blends II and III have the same UOP K, but their properties differ significantly.

Also, when one compares the oils from Fazenda Cedro and Sao Mateus in Table 1 with Leona crude, their diverse characteristics are evident in spite of having the same factor.

USES OF POLARITY

Classification according to polarity has proven to be advantageous for comparing and evaluating crude oils.

For refining purposes, the polarity measures the crude oil reactivity. Reactivity is evidently highly undesirable for fuel production and petrochemical streams.

For purposes of oil production, polarity can predict the emulsion-forming ability of a crude. Before this determination was made, possible oil treatment difficulties could only be evaluated through experimental bottle-tests, and the comparison of crudes was difficult and laborious.

For reservoir studies, knowing the polarity of a crude will help in understanding the oil-rock interactions, in establishing migration patterns, and in foreseeing oil behavior in secondary and tertiary recovery operations.

For commercial purposes, polarity reflects the quality of the crude more adequately than previous factors and indices.

ACKNOWLEDGMENT

The author thanks Petrobras Brasileiro S.A. (Petrobras) for permission to publish this work.

REFERENCES

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Bruning, I.M.R.A., Bol. tecn. Petro-bras, Vol. 32, 1989, p. 217.
Bruning, I.M.R.A., Patent BR PI 870.37.90, appl. July 21, 1987.
BrUning, I.M.R.A., Fuel, Vol. 69, 1990, p. 646-48.
McReynolds, W.O., J. of Chromatog. Sci., Vol. 8, 1970, p. 685.
Kovats, E., Helv. Chim. Acta, Vol. 41, 1958, p. 1915.
Bruning, I.M.R.A., Bol. tecn. Petrobras, Vol. 32, 1989, p. 207.