© 2001,

AAPG/DEG

, 1075-9565/01/$15.00/0Environmental Geosciences, Volume 8, Number 2, 2001 123–129

B A R K E R A N D B U F A R S A N : E V A P O R A T I V E L O S S E S

123

Evaporative Losses from the Surface “Oil Lakes” of Southern Kuwait

COLIN BARKER* and AHMED BUFARSAN

‡

*

Geosciences Department, University of Tulsa, Tulsa, OK 74104

‡

P.O. Box 14572, Fiha, Kuwait

ABSTRACT

Much of the oil spilled into the Kuwait desert during the conflict of

1991 still survives as “oil lakes.” Oil spills are usually rapidly

degraded by a combination of biodegradation, water washing,

evaporation, and oxidation, but because of the low rainfall and

lack of near-surface water in the Kuwaiti desert, the first two of

these processes cannot operate. Evaporation appears to be the

dominant degradation mechanism. We have simulated evapora-

tion of a Kuwait crude oil (from Burgan field) and a Venezuelan

crude oil at temperatures of 25, 30, 40, and 50

�

C and at various

air flow rates from 25 to 100 mL/min. Weight loss is initially

rapid but subsequently decreases through time. After 30%

weight loss the API gravity of the Burgan crude decreased to 22

�

from an initial value of 30

�

, and viscosity increased from 65.3 to

300.5 centipoise. Evaporative reduction of oil lake volume and

reduced surface area lead to shoreline movement. Applying a

cone-shaped model that relates volume loss to distance moved

by the shoreline showed that the retreat is much smaller than

predicted, implying that the lakes have lost considerably less

volume by evaporation than expected. It seems likely that a

devolatilized, high-viscosity, high-density, near-surface layer

develops, protecting the underlying oil and decreasing the over-

all rate of evaporation.

Key Words:

Kuwait, evaporative loss, oil lakes, Gulf War.

INTRODUCTION

When invading Iraqi forces withdrew from Kuwait inMarch 1991, they left behind a large number of damaged oilwells. While the many burning wells received most of themedia attention, damaged wells also spilled large quantitiesof oil that spread across the desert and accumulated in de-pressions as so-called “oil lakes.” Nearly a decade after theyfirst formed, numerous oil lakes up to a kilometer in diame-ter still remain. Photographs of selected present-day oillakes are given by El-Baz and Al-Sarawi (2000).

Several different processes operate to degrade oil spilledat the surface, the most important being bacterial degrada-tion, water washing, evaporation, and photooxidation. Inaddition, some of the surface oil lakes in Kuwait burned

•

•

(Williams et al., 1991), and presumably this lead to a prefer-ential loss of the more volatile light ends. Aerobic bacteriadegrade oils when oxygen and nutrients are present andremove compound types in a well-documented sequencestarting with the n-alkanes (Connan, 1984; Volkman et al.,1983). Bacteria cannot live in oil and are only effective indegrading oils when an aqueous phase is present. Liquidwater is also needed for water washing to be significant, andthis degradation mechanism removes the most soluble com-ponents, which are the gasoline range hydrocarbons, espe-cially aromatics (Lafargue and Barker, 1988; Palmer, 1993).

Average annual rainfall in Kuwait from 1960–1990 is re-ported by Al-Ruwaih (1995) to be 105 mm. For the period1955–1986 Al-Sulaimi et al. (1997) report 106 mm/yr, butwith considerable variations from year to year (the maxi-mum being 242 mm in 1976). Most rain falls from Octoberto May with January being the wettest month. Annual meanevapotranspiration for the period 1955–1986 was 2266 mm,ranging from 67 mm in January to 324 mm in July. Averagedesert temperatures ranged from 7

�

C in January to 39.3

�

C inJuly with high values approaching 50

�

C (Al-Sulaimi et al.,1997). The highest temperature ever recorded is 50.8

�

C (El-Baz and Al-Sarawi, 2000). Because of the low rainfall andthe corresponding lack of water there are no standing bodiesof water in southern Kuwait. In this high-temperature, aridenvironment with high evapotranspiration rates, evapora-tion is likely to play a very important role in modifying thecomposition of surface oil spills in the desert.

The source of the oil in the three lakes studied was proba-bly the nearby giant Burgan oil field (Figure 1). Determina-tion of pristane/nC

17

and phytane/nC

18

ratios, together withsterane and hopane distributions for the Burgan crude andthe oil in the lakes, shows that they are essentially identical(BuFarsan, 1999; BuFarsan et al., 2001, personal communi-cation). This confirms that the Burgan oil field was indeedthe source of the oil now in the oil lakes. Gas chromato-graphic analysis of crude oil from the giant Burgan field insoutheast Kuwait (Figure 2) shows the presence of light endsand a smooth distribution of n-alkanes up to approximatelynC

32

. In contrast, oils from the southern oil lakes in thesame general area (locations 1, 2, and 3 of Figure 1) lackcompounds with less than about 14 carbon atoms, but do con-tain abundant long-chain n-alkanes (Figure 3). The presence

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E N V I R O N M E N T A L G E O S C I E N C E S

of these abundant n-alkanes confirms that biodegradationhas not been of major significance, as expected from the lowrainfall and lack of surface water. This suggests that evapo-ration has probably been the dominant process controllingoil composition in the oil lakes of southern Kuwait. Evapo-ration of crude oil from the Burgan field has been simulatedin laboratory experiments to provide information on changesin physical characteristics and chemical composition.

EXPERIMENTAL PROCEDURES

Three southern oil lakes were sampled, but oils couldonly be collected from the margins because of the danger ofunexploded ordnance that has not yet been cleared from un-der the main part of the lakes. A Burgan oil sample was ob-

tained from a producing well located immediately east oflake 3 (Figure 1).

Evaporative losses from crude oils were investigated inlaboratory experiments using the simple apparatus shown inFigure 4. In this system air from a Gast Model 1398 com-pressor was blown over the oil sample and weight and com-position monitored as a function of time. Temperature wascontrolled at 25, 30, 40, or 50

�

C by an external water bath,and air flow rate controlled with a valve and monitored witha rotameter to give flow rates of 20, 50, or 100 mL/min. Ini-tial oil sample weights were approximately 12 g and exam-ples of weight-loss profiles are given below.

Burgan oil and evaporated oil samples were diluted 2:1with methylene chloride and analyzed gas chomatographi-cally. Analyses were carried out using a Hewlett PackardHP5890A gas chromatograph equipped with a HP G1875AA

FIGURE 1. Outline map of Kuwait showing the locations of the three oil lakes stud-ied (1, 2, 3). The position of the Burgan oil field (solid line) and surface exposures of“tarcrete” (dashed line) are also shown. (Distribution of tarcrete was taken from El-Baz and Al-Sarawi, 2000.)

FIGURE 2. Gas chromatogram of oil from the Burgan Oilfield, southeast Kuwait.Carbon numbers are shown for selected normal alkanes.

FIGURE 3. Gas chromatogram of an oil sample taken from the edge of one of thesouthern “oil lakes” in Kuwait. Carbon numbers are shown for selected normal alkanes.

FIGURE 4. Schematic flow diagram of the system used for the evaporation experiments.

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Chemstation and data handling system for data acquisitionand data analysis. A 30 m, Rtx-1 Restek glass capillary col-umn, 0.25 mm in diameter with a 25

�

m film thickness op-erating with a 100:1 split ratio, was used for all the analyses.

RESULTS

Preliminary experiments carried out with a Venezuelancrude oil evaluated experimental procedures and establishedoptimum operating conditions. This oil, VLC-99 from theBolivar Coastal Fields, Lake Maracaibo, was readily avail-able in large quantities and had very similar properties tothe Burgan crude oil (Bockmeulen et al., 1983). Table 1shows cumulative weight losses after 8 hr and 200 hr for airflow rates ranging from 25 to 100 mL/min and temperaturesfrom 25

�

C to 50

�

C. The data for weight loss at 200 hr havebeen summarized in the three-dimensional diagram given asFigure 5. As expected, the percentage evaporative loss in-creases with both increasing temperature and with increas-ing air flow. Figure 6 compares the cumulative weight-losscurves for the VLC-99 oil after 200 hr at a constant temper-ature of 25

�

C and air flows of 25, 50, and 100 mL/min. Atthe highest flow rate the final weight loss was 26.15%. Thisis very close to the weight loss of 25.67% observed for theBurgan oil under similar conditions (Table 1).

Based on the preliminary experiments with VLC-99 crudeoil, a constant air flow of 100 mL/min was selected for theevaporation experiments with the Burgan oil. Four tempera-tures (25, 30, 40, and 50

�

C) were used (Table 1). A typicalweight-loss profile through time is shown in Table 2 and Fig-ure 7. In this example Burgan oil was evaporated at 40

�

Cwith an air flow rate of 100 mL/min. This weight-loss profile

has the same general character as those obtained for differenttemperature/flow rate conditions and shows that weight lossis initially quite rapid but then decreases to much slowerrates. However, for the time intervals used in the experi-ments, the rate of weight loss did not drop to zero. Fingas(1995, 1996) reported similar behavior for a wide range ofcrude oils, and for each oil he derived a relationship betweenpercentage weight loss, temperature (T,

�

C), and time (t, min-utes). The equations had the form %loss

�

A

�

B

�

T

�

ln(t), where

A

and

B

are constants. Although no data werereported for the Burgan crude, he did study an “ArabianMedium” crude oil where

A

�

1.89 and

B

�

0.045 and an“Arabian Light” oil with

A

�

2.25 and

B

�

0.37. The cor-responding cumulative weight-loss curves calculated from

TABLE 1.

Percentage evaporative weight losses for Burgan (B) and Venezuelan (V) crude oils at various temperatures and air flow rates.

Temperature (°C) Air Flow(mL/min) Crude Oil

Evaporation Time

8 Hours 200 Hours

25 25 V 6.62 23.2425 50 V 7.93 24.2525 100 V 9.63 26.1525 100 B 9.08 25.67

30 25 V 7.12 23.5330 50 V 8.94 26.2630 100 V 13.73 27.6730 100 B 12.42 25.75

40 25 V 9.60 24.5240 50 V 11.66 28.1240 100 V 16.35 30.2640 100 B 17.78 31.09

50 25 V 15.64 29.1450 50 V 12.65 30.0050 100 V 17.01 36.9950 100 B 17.47 36.26

FIGURE 5. 3-D block diagram showing the relationship among temperature, airflow rate, and percent evaporative loss for the VLC99 Venezuelan crude oil that wasused to investigate experimental conditions.

FIGURE 6. Cumulative percent weight loss from VLC-99 Venezuelan crude oil at40�C and 25 mL/min (diamonds), 50 mL/min (squares), and 100 mL/min (triangles).

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E N V I R O N M E N T A L G E O S C I E N C E S

his equations for a temperature of 40

�

C are plotted in Figure 7and compared with the evaporation trend for Burgan oil ob-tained at the same temperature. Surprisingly, the Arabian Me-dium crude oil appears to lose weight faster, and ultimatelyloses a larger weight percentage, than the Arabian Light. Thecurves for the Arabian oils are not directly comparable withthe Burgan crude because Fingas (1996) reported air flowover oil samples in terms of velocity (i.e., m/sec), while inthis study air flow was measured as mL/min. However, theshapes of the curves are very similar and both show a rapidinitial weight loss that subsequently decreases with time. Thelower weight loss from the Arabian oils is probably due to alower air flow rate during the evaporation experiments. Sinceall experiments with the Burgan oil were carried out using anair flow of 100 mL/min, it was not possible to test this. How-ever, experiments with the Venezuelan crude oil at 25 and 50mL/min showed experimental trends that overlapped the cal-culated Arabian trends (although the Venezuelan crude oildid lose weight faster in the early stages).

Longer-term experiments were carried out by leaving oilsamples exposed in beakers in an outdoor, but protected, en-

vironment. After 50 days of exposure during an Oklahomaspring the Burgan oil lost 17% of its weight. Again the ini-tial loss was quite rapid but decreased in rate with time (Fig-ure 8). For comparison an Oklahoma crude oil was treatedin the same way. This lighter crude oil lost 30% of itsweight in the 50-day exposure.

PHYSICAL CHANGES

Evaporative loss of a crude oil’s more volatile compo-nents leads to changes in both chemical composition andphysical characteristics. Variations in the composition ofBurgan crude oil with progressive evaporation have beendescribed by BuFarsan (1999) and BuFarsan et al. (2001, per-sonal communication), and these changes in composition pro-duce changes in viscosity and density that are particularlyimportant in controlling the behavior of an oil spill. Stan-dard analytical techniques for determining API gravity andviscosity need somewhat larger samples than those used inthe experiments for establishing weight loss. Three 17 mLsamples of the Burgan crude oil were placed in separate35 mL beakers and stirred with rotating magnetic stir barsas air was blown over them at 100 mL/min. Weight loss wasmonitored by periodic weighings and samples were pre-pared with cumulative weight losses of 12.9, 20, and 30%.API gravities for these evaporated fractions and the originalBurgan oil were determined with hydrometers. Values de-creased steadily with progressing evaporative loss, fallingfrom an initial value of 30

�

API to 22

�

API after 30% ofthe oil had been lost. Viscosity was measured with a cone-in-plate apparatus at 25

�

C. Values increased steadily from65.3 to 300.5 centipoise with progressive evaporative loss(Table 3) and are shown in Figure 9.

TABLE 2.

Cumulative evaporative weight loss from the Burgan crude oil at 40°C with an air flow of 100 mL/min.

EvaporationTime (hours)

Cumulative EvaporativeLoss (%)

2 9.104 12.396 14.888 17.78

32 22.3156 24.4080 26.00

104 26.85128 28.15152 29.92176 30.66

FIGURE 7. Cumulative weight loss through time. (Top) Experimental data for theBurgan crude oil evaporated at 40�C and 100 mL/min air flow. (Middle, bottom) Cal-culated weight-loss curves for “Arabian Medium” and “Arabian Light” oils, respec-tively, using the equations developed by Fingas (1996).

FIGURE 8. Cumulative percent weight loss over 1200 hr (50 days) for samples ofthe Burgan crude oil (bottom) and Oklahoma crude oil (top) evaporated in exposedbeakers.

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MODELING LAKEAREA REDUCTIONS

It is difficult to determine what fraction of the oil in thelakes has been lost over the last nine years. A rough esti-mate can be made using a simple model to calculate the dis-tance moved by the oil shoreline when one-third of the oil islost by evaporation, since this is the approximate percentagelost during laboratory experiments (see for example, Table1). In the absence of any topographic data to constrain theshape of the bottom of the oil lake, a wedge-shaped volumewas considered by BuFarsan (1999). With this model thedistance moved by the lake shoreline is independent of thedepth at the center of the lake and is also independent oflake width. The modeling showed that for a volume loss ofone-third the shoreline would move approximately 55 m, adistance considerably greater than the observed changes,which are of the order of 15 m. This suggests that the lakeshave lost significantly less than a third of their volume byevaporation.

Here we consider a slightly more elaborate model thatuses a cone-shaped volume for the lake (Figure 10). Again,the relationship between shoreline movement (D) and thepercentage of oil lost is independent of initial lake depth.Calculations were carried out for lakes with diameters of500, 1000, and 2000 m and for various percentages of evap-orative loss. These curves are plotted in Figure 11 where

they are compared with a total evaporative loss of one-third,as measured in the laboratory simulation experiments. For a1000 m diameter lake the shoreline would move 62.5 m,and for a 500 m diameter it would move 31.3 m. In largerlakes the estimated distance is even greater. Because the ex-act shape of the lakes is unknown there is considerable un-certainty in the modeling, but the fact that the shorelineshave retreated only about 15 m suggests that the oil lakeshave lost much less than a third of their volume in the lastnine years.

During the process of evaporation the oil lakes lose lightends from the exposed surface and, if evaporation is to con-tinue, these volatile compounds must be replaced from thebody of the oil, either by convection or by diffusion to thefree surface. Table 3 shows that as the oils lose their lightends by evaporation, API gravity decreases and there is apossibility that these denser oils could sink, initiating con-vection. However, the increase in density when API gravitydrops from 30 to 22

�

API is only 0.046 g/mL, and it seems

FIGURE 9. Viscosity of the Burgan crude oil after varying percentages of evapora-tive loss.

FIGURE 10. Diagram of the cone-shaped lake volume used to model the distance moved by the shore, D, when the lake level drops due to various percentages of evaporative loss.The schematic diagram is drawn with considerable vertical exaggeration.

TABLE 3.

Changes in density, API gravity, and viscosity of the Burgan crude oil with progressing evaporation.

Percent Lost byEvaporation

Density(g/ml)

Gravity (

o

API) Viscosity

(Centipoise)

Original oil 0.876 30 65.312.9 0.893 27 120.420.0 0.904 25 132.530.0 0.922 22 300.5

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E N V I R O N M E N T A L G E O S C I E N C E S

unlikely that this small difference is enough to overcome theconsiderable increase in viscosity that also occurs (Figure 9).

Diffusion will also be restricted by the changing characterof the near surface layer with the more degraded part of theoil remaining as a protective layer. This viscous layer of re-sidual heavy ends acts as a cap, preventing evaporation fromthe deeper part of the oil column. This could explain why, af-ter nine years, the volume of oil in the lakes has not shownmajor decreases and why hydrocarbons heavier than aboutC

15

are still abundant. In the desert environment it is likelythan windblown sand and finer material become incorporatedinto the near-surface, high-viscosity layer and further in-crease its effectiveness as a barrier to continued evaporation.However, no experiments were carried out to document this.Compositional stratification in oil columns of Burgan crudeoil has been observed in laboratory experiments (BuFarsan,1999; BuFarsan et al., 2001, personal communication).

CONCLUSIONS

In the arid environment of the Kuwait desert the majorprocess degrading oil spills is evaporation. Laboratory sim-ulation experiments using Burgan and other crudes haveshown that evaporation is initially rapid, but the rate ofweight loss decreases until only approximately two-thirdsof the oil remains. Evaporative losses produce a residualcrude oil that has decreased API gravity (increased density)and increased viscosity. Although details of the lake shapesare unknown, it is possible to make an estimate of lossesfrom the lakes in the natural environment by considering thedistance the shoreline has moved. Using a conical model(which is independent of depth) suggests that considerablyless than one-third of the oil has been lost by evaporation inthe nine years since the lakes accumulated. It seems likelythat evaporation may be a self-limiting process because ofthe formation of a high-viscosity, high-density, devolatil-

•

ized surface “skin” that acts as a protective layer inhibitingfurther evaporation.

ACKNOWLEDGMENTS

We are pleased to acknowledge support from the KuwaitEnvironmental Public Authority and the University ofTulsa. Dr. Al-Sarawi is thanked for his cooperation and forobtaining the Burgan oil sample. Phillips Petroleum pro-vided facilities for determining viscosity, and William A.Redman helped with laboratory analyses.

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•

•FIGURE 11. Movement of lake margins for lakes contracting due to evaporation.Distance moved by the shoreline was calculated using the cone-shaped model shownin Figure 9. Curves are given for lakes with initial diameters of 0.5, 1.0, and 2.0 km.

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ABOUT THE AUTHOR•

Colin Barker

Colin Barker received a B.A. in chemis-

try and a D.Phil. in geology from Oxford

University and is the McMan Professor and

Chairman of the Geosciences Department

at the University of Tulsa. He worked for

Exxon Production Research before joining

the faculty at the University of Tulsa in

1969. Dr. Barker has been an AAPG Dis-

tinguished Lecturer (1980–1981), a Mat-

son Award winner (1978 and 1982), and Chairman of the Visiting

Geologists Program (1996–2000). His research interests include or-

ganic geochemistry, ultra-deep gas, and anomalous subsurface pres-

sures.