Prediction of wax Deposition Risk of Malaysian Crude

International Journal of Science and Advanced Technology (ISSN 2221-8386) Volume 1 No 6 August 2011

http://www.ijsat.com

89

Prediction of Wax Deposition Risk of Malaysian Crude from Viscosity-Temperature

Correlation for Dead Crude

Ekeh Modesty Kelechukwu

Dept. of petroleum engineering, UCSI University

56000 Cheras, Kuala Lumpur

E-mail: [email protected]

Abstract- Malaysian oil production occurs offshore

and extreme environment, its cold temperature

could make wax deposition problems almost

inevitable as most of the oilfields approaching their

matured stage of production. Wax deposition had

demonstrated a critical concern in the quest to

increase oil recovery from producing reservoirs.

The unwanted effect of wax deposition could cause

reduced productivity, complete plugging of

pipeline, minimum profitability and other hazardous

risks; while its mitigation operations and production

losses are expensive and economically unjustified.

This study employed Standing correlation model for

the viscosity-temperature relationship of dead crude

to predict wax crystallization point. The predicted

wax appearance points were in good agreement

with the experimental values of the Malaysian dead

crude oils, with average absolute deviation (AAD

%) of 11.71%. However, the Standing model varied

significantly with the measured viscosity data. The

main goal of this work was to predict wax

deposition threat or related problems in Malaysian

fields.

Keywords- Prediction of wax; potential wax

problem; Malaysian crude.

I. I. Introduction

The presence of paraffin waxes in crude oils

presents a multitude of problems to the producers.

The problems associated with their presence range

from minor to severe, and depend on their quantity

and composition. Petroleum production can be

significantly affected by deposition of paraffin wax

during crude production, with devastating economic

consequences. Hence, predicting wax problems

within the production tubing and flowlines that

could decrease or halt production is essential in

optimizing production and operating efficiency. The

liquid hydrocarbons initially are in equilibrium in

the reservoir under super-saturated temperature-

pressure conditions. Paraffin generally consist of

straight and branched chain hydrocarbons and

precipitates out of waxy crude when there is a slight

change in equilibrium conditions, causing a loss of

solubility of the wax in the crude. A decrease in

temperature is the most common cause of paraffin

mailto:[email protected]



90

wax precipitation, though many other factors could affect the process

However, the solubility of paraffin waxes is not

only sensitive to temperature variation, but also an

integration of physiochemical properties of the

crude and other operation factors in production

system. Thus, accurate knowledge of the nature of

any crude oil, as characterized by its

physiochemical properties or classification is vital

in the quest for solution to production risk or

intervention strategy for handling waxy crude. The

main goal of this work was to predict wax

deposition threat or related problems in Malaysian

fields, using experimental methodology to simulate

wax deposition in the laboratory.

II. Wax Deposition Mechanism

A problem of paraffin wax may be described as a

situation in which a predominantly organic deposit

hampers the production of crude oil; the loss crude

production from well depends on the severity and

location of the deposition. In a pioneering work,

Burger et al, (1981) investigated four wax

deposition mechanisms: the mechanism of paraffin

wax deposition are governed by molecular diffusion

of wax molecules; shear dispersion of wax

crystallites and Brownian diffusion of wax

crystallites. Gravity settling of paraffin crystals in

flow line conditions is negligible, because it’s

dominated by shear dispersion (Burger et al., 1981).

Molecular diffusion is the deposition mechanism

prevalent for tubing deposition in flowing well that

maintains oil temperature, well above the cloud

point until the oil is coming up the tubing (Bern et

al., 1980; Leiroz and Azevedo, 2005). Deposition is

enhanced as result of lateral transportation by the.

In wax deposition mechanism process, a

concentration gradient is produced in the oil as a

result of temperature gradient profile, due to

increasing solubility of waxes with increasing

temperature. The concentration caused waxes in

solution to diffuse from the warmer oil, which has a

greater concentration of dissolved waxes, to the

colder oil, which has a lower concentration,

resulting to molecular diffusion of the paraffin

crystals towards the surface wall.

III. Reference Crude Oils Characterization

Physiochemical properties (API gravity and

pour point), density, viscosity, Wax Appearance

Temperature as well as Wax content of the five

reference crude oils were measured. Table 1

presents results analysis of crude samples from

various oil fields in Malaysia, generated at the

unipem laboratory of the Universiti Teknologi

Malaysia. The crudes varied greatly in their

characteristics and composition. They varied in

color from black to light brown, and density

measured at 15 °C, varies from 0.80 to 0.98 kg/L,

viscosity at 40 °C varied between 3.83 to 37.50 cSt

and at 70 °C varies from 2.25 to 32.50 cSt. Water

content in all the sampled crude were very

minimum, (less than 1% vol.). The API Gravity

ranges from light oil to heavy crude oil, (12.6 to



91

44.5) respectively, while pour point varies between

18 to 36 °C. The result data of the samples

understudy showed that Penara contains the highest

percentage of wax content followed by Bunga

Kekwa and Dulang, while Tapis had the lowest wax

percentage. The Wax Appearance Temperature

measurements were made following the wax app I

method (65.0oC – 5.0

oC / min), and the figure

varied from 27.87 to 55.70 (values in oC). There

was also a clear correlation between high density

and low API Gravity, while a qualitative correlation

confirmed higher pour point with higher wax

content amongst the analyzed samples.

Table I: Physiochemical properties of some

Malaysian oilfields

Table II. Physiochemical properties of some

Malaysian oilfields

Types of

Crude

Pour

point

oC

WAT

oC

Wax

content

% wt

Water

content

% vol.

Angsi 30 33.32 2.0 0

Bunga

Kekwa 36 46.49 20.2 0.2

Dulang 33 33.76 3.0 0.7

Penara 36 55.70 18.0 0.5

Tapis 18 27.87 1.0 0

IV. Empirical Correlation for Dead Crude Oil.

The deposition tendency of wax from crudes

can be predicted using a generalized empirical

correlation for crude rheological properties. The

assumption that fluid obeys the Arrhenius equation,

with exponential dependency of viscosity on

temperature was applied by [Elsharkawy et al,

2000] in their study of wax deposition for Middle

East crudes. Viscosity is an important empirical

parameter in wax deposition [Richard S. Fulford,

1975]. Yong Bio and Qiang Bai (2005) stated that

oil viscosity correlations use oil API gravity and

reservoir temperature to estimate dead oil viscosity

(μod). Many correlations have developed in

petroleum literatures to estimate or determine

viscosity and temperature, using empirical

correlation in lieu of laboratory data. [Yam and

Luo, 1987] investigated the effect of temperature on

the rheological properties of Daqing crude oils.

Specifically, they correlated apparent viscosity with

temperature. However, the correlations for viscosity

and temperature were derived specifically for the

case of conditions of dead crudes (no gas in

solution) and it is often estimated using empirical

Types of

Crude

Density

g/cm3

@ 15 oC

Viscosity

(cSt)

@ 40 oC

Viscosity

(cSt)

@70 oC

API

Gravity

Angsi 0.8124 7.714 2.878 42.6

Bunga

Kekwa 0.9255 14.52 4.119 21.3

Dulang 0.9814 30.56 3.817 12.6

Penara 0.9165 37.50 32.50 22.8

Tapis 0.8036 3.831 2.251 44.5



92

corrections developed by a number of investigators

including Baal (1946), Beggs and Robinson (1975),

Standing (1981), Glaso (1985), Khan (1987) and

Ahmed (1989).

The temperature dependence of viscosity is the

phenomenon by which liquid viscosity tends to

decrease as its temperature increases, vice versa.

Realizing that the viscosity of waxy-crude oil at low

temperature is non-Newtonian, and its behavior is

governed by changes in temperature. As the

temperature decreases, the waxy-crude oil becomes

more viscous, hence its viscosity depends on

temperature decrease. Therefore, as viscosity

increases in cold areas the flow resistance increases

and if the temperature becomes low enough, the

wax in crude may precipitate and deposit.

Consequently, organic deposition from reservoir oil

occurring in a specific flow regime should be

identifiable from the characteristic oil viscosity-

temperature correlation. In (1981), Standing

published empirical correlations for estimating

viscosity-temperature curves, using values of

viscosity, temperature and the crude gravities. The

Standing correlation for dead oil is expressed as:

(

) (

)

1

Where A = (

)

and

μod = viscosity of dead crude oil (cP).

The above correlation equation was developed

for determining the viscosity of the dead crude oil

as a function of temperature and API gravity of the

crude. [Kunal et al. 2000] suggests when waxy

crude is allowed to cool below the WAT,

precipitation of waxes continues, resulting in an

increase in the number and size of crystals. These

crystals, if undisturbed, tend to cohere together to

form a netlike structure trapping oil within. As a

result, the oil attains gel-like characteristics and the

viscosity increases. At certain temperature,

depending on the amount of wax precipitated and

the strength of the network, the oil may cease to

flow. Flow assurance studies for waxy systems

often require measurements of at least three crude

oil properties such as wax appearance temperature

(WAT), pour points and viscosity [Kunal et al.

2000]. Using the WAT and pour point, the

rheological and problematic behavior of waxy crude

can be mapped into three regions on a temperature

scale:

A region defined by temperature below the pour

point, where the fluid exhibits highly non-

Newtonian behavior and oil may gel under

quiescent conditions.

A region of mildly non- Newtonian behavior

defined by the temperature between the WAT and

pour point.

Generally, the wax appearance temperature (WAT)

and pour point measurements are performed on

crude oil samples and used as conservative



93

estimates for making flow-assurance-related

decisions.

V. V. Result and Discussion

VI. Wax Prediction in Malaysian Oilfields

Among the rheological properties of reservior

oil which affect the flow behaviour in the producing

well, the viscosity appears to be the most important.

This is due to the fact that other properties such as

ºAPI gravity and wax % wt are invariantly with

changing temperature of the well. Consequently,

organic deposition from reservoir oil occuring in a

specific flow regime could be identifiable from the

characteristic oil viscosity-temperature correlation.

The crude viscosities have been estimated using

Standing correlation model and then temperatures

were adjusted accordingly. Equation (1) was

employed to determine wax precipitation point

using viscosity-temperature relationship for dead

crude oil as a function of temperature and API

gravity of the crude. Precipitation of waxes results

in an increase in the number and size of crystals and

lead to higher viscosity. The rheological and

problematic behavior of waxy crude can be mapped

into regions on a temperature scale [Kunal et al,

2000]. Figures 1-5 shows the characteristic

viscosity-temperature plots for all crude samples

from Angsi, Bunga-Kekwa, Dulang, Penara and

Tapis fields in Malaysia.

Figure 1: Effect of wax precipitation on viscosity

of Angsi crude.


of Bunga Kekwa crude.

y = 0.0008x2 - 0.166x + 11.452 R² = 0.9985

0

2

4

6

8

10

0 50 100 150

Vis

cosi

ty (

cP)

Temperature (ºC)

Angsi crude

WAT

y = 0.0673x2 - 12.768x + 673.81 R² = 0.9946

0

100

200

300

400

500

0 50 100 150

Vis

cosi

ty (

cP)

Temperature (ºC)

Bunga Kekwa…

WAT

y = 19.61x2 - 3199.4x + 130300 R² = 0.9686

-20000

0

20000

40000

60000

80000

100000

0 50 100 150

Vis

cosi

ty (

cP)

Temperature (ºC)

Dulang crude

WAT



94


of Dulang crude


of Penara crude.

Figure 5: Effect of wax precipitation viscosity of

Tapis crude.

The viscosity-temperature curve exhibits

waxy crude behavior during the cooling process.

The points above the curve represent Newtonian

behavior of crude, while below the curves exhibits

non-Newtonian as viscosity changes with

temperature decrease. It’s evident from these plots

that these crudes differ in their rheological

properties due to differences in composition and

consequently their waxing tendencies will differ.

Figures 4 shows clearly that Penara crude indicates

the highest wax appearance temperature (WAT) and

exhibits a continuous increase of viscosity as the

temperature is decreasing. The higher the wax

appearance temperature of crude is the more waxing

problems would be expected from the well. A near

similar scenario was also observed for Bunga

Kekwa and Dulang crudes in Figures 2 and 3,

particular Dulang crude which exhibited highest

viscosity increase. As regards Angsi crude plots

presented in Figure 1 shows a lower wax

appearance temperature with minimum viscosity

increase as the temperature is declined. In contrast

to the above cases, Figure 5 shows that Tapis crude

indicated the least wax appearance temperature and

exhibits almost a near constant viscosity even as

temperature is decreasing. These conditions suggest

that this crude does not manifest enough wax

deposit tendencies. Working on the assumption

[Kunal et al. 2000] when a waxy crude is allowed to

cool below the WAT, precipitation of waxes

continues, resulting in an increase in the number

and size of crystals. These crystals, if undisturbed,

tend to cohere together to form a netlike structure

trapping oil within. As a result, the oil attains gel-

like characteristics and the viscosity increases. At

certain temperature, depending on the amount of

wax precipitated and the strength of the network,

the oil may cease to flow. However, the subsea

y = 0.0388x2 - 7.4973x + 408.33 R² = 0.9955

0

50

100

150

200

250

300

0 50 100 150

Vis

cosi

ty (

cP)

Temperature (ºC)

Penara crude

y = 0.0006x2 - 0.1371x + 9.5851 R² = 0.9986

0

2

4

6

8

0 50 100 150

Vis

cosi

ty (

cP)

Temperature (ºC)

Tapis…

WAT



95

temperature of the oilfields is averagely 27ºC

whereas the surface temperature fluctuates between

28 to 34ºC, which is below most of the crude’s wax

appearance temperature. The viscosity behaviors of

most of the crudes were very sensitive to

temperature change. Hence, the formed wax crystals

in the oil leads to increase in viscosity as the

temperature decreased. Based on the evaluated

samples and their viscosity varying by several

orders of magnitude at declining temperature,

majority of the crudes has exhibited propensity for

potential waxing problems during crude

productions.

Figure 6: Measured and predicted wax appearance

temperature (WAT).

A simple and generalized correlation has

been presented for predicting wax precipitation

point based on viscosity-temperature correlation.

Figure 6 presents the plot of measured and predicted

wax appearance temperature (WAT) of the

reference crudes. From the result of Figure 6, it’s

clear that wax appearance temperature predicted

from the empirical correlation model compares well

with the measured WAT of all the crude samples

with absolute average deviation (%AAD) of

11.71%.

Figure 4.35:Effect of temperature decrease on

crude properties for Angsi crude.

Figure 4.36: Effect of temperature decrease on

crude properties for Bunga Kekwa crude

y = 1.2012x - 4.6019 R² = 0.9164

20

30

40

50

60

70

20 30 40 50 60

Pre

dic

ted

WA

T ºC

Measured WAT ºC

0

5

10

15

20

25

30

0

20

40

60

80

100

120

0 2 4 6 8 10

Po

ur

po

int

(ºC

)

Tem

pe

ratu

re (ºC

) Viscosity (cP)

Angsi crude Tempt - Viscosity

Pour point - Viscosity

0

5

10

15

20

25

30

35

40

0

20

40

60

80

100

120

0 100 200 300 400 500

Po

ur

po

int

(ºC

)

Tem

pe

ratu

re (ºC

)

Viscosity (cP)

Bunga Kekwa crude

Tempt - Viscosity




96


crude properties for Dulang crude


crude properties for Penara crude


crude properties for Tapis crude

Figures 7 to 11 shows the referenced crude

rheological correlation, temperature – viscosity and

pour points plots for the entire samples understudy.

It’s evident from the result of Figures 7-11, that’s

these crudes differ in their rheological properties

due to differences in their composition and may

exhibits different degree of production problems.

[Kunal et al. 2000] suggest that flow assurance

studies for waxy systems often require

measurements of at least three crude oil properties

such as wax appearance temperature (WAT), pour

points and viscosity. Bunga Kekwa and Penara

crudes has exhibited high pour points, WAT and

viscosity as temperature declines, which in

comparison to the offshore environment of most

oilfields, these crudes indicate potential waxing

problems, particularly in the case of production

shutdown and subsea environment.

0

10

20

30

40

0

20

40

60

80

100

120

0 50000 100000

Po

ur

po

int

(ºC

)

Tem

pe

ratu

re (ºC

)

Viscosity (cP)

Dulang crude Tempt - ViscosityPour point - Viscosity

0

5

10

15

20

25

30

35

40

0

20

40

60

80

100

120

0 100 200 300

Po

ur

po

int

(ºC

)

Tem

pe

ratu

re (ºC

)

Viscosity (cP)

Penara crude

Tempt - Viscosity


0

5

10

15

20

0

20

40

60

80

100

120

0 2 4 6 8

Po

ur

po

int

(ºC

)

Tem

pe

ratu

re (ºC

)

Viscosity (cP)

Tapis crude Tempt - Viscosity




97

Angsi and Dulang crudes, their plots presented

in Figure 7 and 9 also showed an average possibility

of gelling and wax deposition in their respective

fields’ problems, given such scenario as described

above. Figure 11 shows that Tapis crude has the

least possibility of any paraffin wax deposition

potential, as the physical parameters studied

remained minimum even at low temperatures.

Therefore, the majority of oilfields crude samples

evaluated in this study clearly demonstrated the

possibility of wax becoming a problem, considering

their high pour point and viscosity varying by

several orders of magnitude at declining

temperature, which is an obvious indication for

potential wax problems during crude productions.

VII. Comparison of Standing’s Correlation

Model with Experimental Data

The application of dead oil viscosity correlation to

crude oils from different sources or fields is

necessary to evaluate a crude oil with respect to

viscosity changes with temperature. Since, for a

given crude oil, the slope changes with temperature

for different crude oil fractions from same natural

source. It is evident generally that as temperature

increases the viscosity of each oil decreases. The

comparative viscosity-temperature variation of the

experimental results and the simulated from

(Standing’s correlation model) of Malaysian crudes

are presented in Figures 12-16.

Figure 12: Variation of model correlation with

experimental results for Angsi crude.


experimental results for Bunga Kekwa crude.

0

5

10

15

0 50 100 150

Vis

cosi

ty (

cP)

Temperature (ºC)

Standing Model Experiment

0

100

200

300

400

500

0 50 100 150

Vis

cosi

ty (cP

)

Temperature (ºC)

Experiment Standing Model

0

20000

40000

60000

80000

100000

0 50 100 150

Vis

cosi

ty (

cP)

Temperature (ºC)




98


experimental results for Dulang crude


experimental results for Penara crude


experimental results for Tapis crude

The above (Figures 12 to 16) shows Standing’s

correlation equation almost a good agreement with

the experimental data for Angsi and Tapis crudes.

However, the reserve was the case for Bunga

Kekwa, Dulang and Penara crudes. The variation

exhibited by the later mentioned crudes could be

atrributed to the unique composition and properties

of each these crudes which strongly influence

rheological behaviour for individual crude. This

work did not consider the discrimination of heavy

and light crude oils with respect to the application

of Standing’s relationship for dead crudes, rather it

limited itself to simply correlation. It’s clear that

majority of the crude understudy, perticularly

crudes with higher viscosity did not prove suitable

comparison with the experimental results.

VIII. Conclusions

In this study, five stock-tank crude oil samples

from various oilfields in Malaysia have been

investigated to determine possible wax problems

that may affect crude production from these fields.

Physical characteristic of the oil from a field was

used to evaluate its waxing potentials. The

Standing’s empirical correlation for dead crudes

was employed to estimate precipitation point.

General rheological correlation of crude properties

yielded a good result and indicated crudes with

possible waxing problems.

References

[1] Abdel-Waly, A.A. 1997. New Correlation Estimates

Viscosity of Paraffinic Stocks. Oil Gas J. 95 (26):

61-65.

0

50

100

150

200

250

300

0 50 100 150

Vis

cosi

ty (

cP)

Temperature (ºC)


0

2

4

6

8

0 50 100 150

Vis

cosi

ty (

cP)

Temperature (ºC)




99

[2] A.M. Elsharkawy, T.A. Al-Shhaf, M.A. Fahim,

2000. Wax deposition from Middle East crudes.

[3] A.Nagar, V.K. Mangla, S.P Singh, and J. Kachari,

2006. Paraffin Deposition Problems of Mumbai

High.

[4] Alex Hunt, 1996, “Uncertainties Remain in

Predicting paraffin deposition”, Oil and gas Journal,

July 29, 1996, Texaco Ltd. London.

[[5] A.S. Abdulkareem and A.S. KOVO, 2006.

Simulation of the Viscosity of Different Nigerian

Crude Oil. Leonardo journal of Sciences, P.7-12.

[[6] Ding, J., Zhang, J., Li, H., Zhang, F., and Yang, X.

2006. Flow Behavior of Daqing Waxy Crude Oil

under Simulated Pipelining Conditions. Energy &

Fuels (20): 2531-2536

[[7] Escobar-Remolina J.C.M. 2006. Prediction of

Characteristics of Wax Precipitation in Synthetic

Mixtures and Fluids of Petroleum: A New Model.

Fluid Phase Equilib. 240: 197 – 203.

[[8] J.R. Becker, 1997. Crude Oil Waxes, Emulsions,

and Asphaltenes.

[[9] Ken, B. 2003. Understanding Paraffin and

Asphaltene Problems in Oil and Gas Wells.

[1[10] K.M. Barker, SPE, J.V. Breitigam, SPE, R.L.

Brotherton, L.L Goff, SPE, K.J. Hake, SPE, and

R.D. Schofield, BakerPetrolite, 2007. Crude Oils

of Kentucky and Tennessee: Characteristics,

Problems, and Solutions. SPE Eastern Regional

Meeting, Lexington, Kentucky, U.S.A, 17-19

October 2007.

[11] Kulkarni, V.B., University of Alaska, Fairbanks,

Alaska, Zhu, T., The Petroleum Institute, Abu

Dhabi, U.A.E., and Hveding F., BP Alaska,

Alaska. Determination and prediction of wax

deposition from Alaska North Slope Crude Oil.

International Petroleum Technology Conference,

Kuala Lumpur, Malaysia, 3-5 December 2008.

[12] Shell Method 1769-5. “Wax Content of Petroleum

Products”, Shell Method Series (1972).

[13] Victor A. Adewusi, 1997. Prediction of wax

deposition potential of hydrocarbon systems from

viscosity-pressure correlations. Fuel vol.76, No.

12, pp. 1079-1083, 1997.

[14] Wang, B. and Dong, L.: “Paraffin Characteristics of

Waxy Crude Oils in China and the methods of

paraffin removal and Inhibition” Paper SPE 29954

presented at the 1995 SPE International meeting on

Petroleum Engineering., Beijin, PR China, and

Nov. 14-17.

[15] Wardhaugh, L. T., D. V. Boger. 1991. Flow

Characteristics of Waxy Crude Oils: Application to

Pipeline Designs, AIChe Journal, June.

[16] Zulkania, A., B. Pramudono, H. B. Mat, A. K. Idris

and M. Manan. 2001. Malaysian Crude Oil

Emulsions: Physical and Chemical

Characterizations.

Ekeh Modesty is a diversified

professional in oil and gas

industry. He divides his time

lecturing Petroleum

Engineering at UCSI

University, Kuala Lumpur,

Malaysia and engages in active

research in oil industry related challenges. His

research involves organic solid deposition



100

management in offshore environment, Enhanced Oil

Recovery and Future Energy Sustainability.

Engr. Ekeh is a professional member of Society of

Petroleum Engineers and has received certification

from Society of Petroleum Engineers. And also

been awarded UTM excellent research award in

2008; young professional award and Shell excellent

student award, in 2009 and 2010 respectively. He

has authored several journals.

Documents

Prediction of wax Deposition Risk of Malaysian Crude