SPE 5434

Ijpt forum

Estimating the Viscosity of Crude Oil Systems

Viscosity values of crude oils and crude oils containingdissolved natural gas are required in various petroleumengineering calculations. In evaluation of fluid flow in areservoir, the viscosity of the liquid is required at variousvalues of reservoir pressure and at reservoir temperature.This information can be obtained from a standard labora­tory PVT analysis that is run at reservoir temperature.There are cases, however, when the viscosity is needed atother temperatures. The most common situation requiringviscosities at various pressures and temperatures occurs inthe calculation of two-phase, gas-liquid flowing pressuretraverses. These pressure traverses are required intubing-string design, gas-lift design, and pipeline design.Calculation of these pressure traverses involves dividingthe flow string into a number of length increments andcalculating the pressure gradient at average conditions ofpressure and temperature in the increment. Calculation ofpressure gradients requires knowledge of oil viscosity. Inmany cases, the only information available on the fluidproperties are the separator gas gravity and stock-tank oilgravity; therefore, correlations requiring a knowledge ofcrude oil composition are not applicable.

The most popular methods presently used for predictingoil viscosity are those of Beall for dead oil and Chew·andConnally2 for live or saturated oil. Beal correlated dead oilviscosity as a function of API gravity and temperature.Chew and Connally presented a correlation for the effectof dissolved gas on the oil viscosity. The dead oil viscosityand the amount of dissolved gas at the temperature andpressure of interest must be known.

When these correlations were applied to data collectedfor a study of dissolved gas and formation volume factor,considerable errors and scatter were observed. Thesedata, therefore, were used to develop new empirical corre-

lations for dead or gas-free crude oil as a function of APIgravity and temperature, and for live oil viscosity as afunction of dissolved gas and dead oil viscosity. A de­scription of the data used, which were obtained from CoreLaboratories, Inc., is given in Table 1.

The correlation for dead oil viscosity was developed byplotting IOglo (T) vs loglo log 10 (/LaD + 1) on cartesiancoordinates. The plots revealed a series of straight lines ofconstant slope. It was found that each line represented oilsof a particul~ API gravity. The equati9n developed is

/LaD = lOx - 1, ~ . . .(1)where

x = Y T-l.l63

Y = 10z

Z = 3.0324 - 0.02023 'Yo.

The correction of the dead oil viscosity for dissolvedgas was developed by taking advantage of the fact that alinear relationship exists between IOglO /LaD and IOglOJ.L fora particular value of dissolved gas, Rs . Live oil viscosity·may be calculated from

J.L = A /LaDB, (2)

TABLE 1 - DESCRIPTION OF DATA USEDVariable Range

Solution GOR, scf/STB 20 to 2,070Oil gravity, °API 16 to 58Pressure, psig °to 5,250Temperature, OF 70 to 295

Number of oil systems = 600Number of dead oil observations = 460Number of live oil observations = 2,073

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1140 JOURNAL OF PETROLEUM TECHNOLOGY

TABLE 2 - STATISTICAL COMPARISON OF CORRELATIONSAverage Error Standard Deviation

(percent) of Percent Errors

Development of these correlations neglects the depen­dence of oil viscosity on composition, since oils of widelyvarying compositions can have the same gravity. Viscos­ity does depend on composition, and if the composition isavailable other correlations3-5 exist that should be used forgreater accuracy. However, the correlations presentedhere are easy to use and give fair accuracy and precisionover a wide range of oil gravity, temperature, and dis­solved gas. As is the case with any empirical study,extrapolation outside the range of the data used to developthe correlations should be done with caution. Table 2

A = 10.715 (Rs + 100)-0.515B = 5.44 (Rs + 150)-0.338

Dead oil data used todevelop this correlation

SealThis study

Dead oil datacollected fromliterature (93 cases)

SealThis study

Live oil data used todevelop this correlation

Chew and ConnallyThis study

where

-19.64- 0.64

378.46114.27

25.35- 1.83

21.8613.53

1,568.09530.00

35.7027.25

presents a statistical comparison of the correlationsof Beal, Chew and Connally, and our findings.

NomenclatureRs = dissolved GOR, scf/STBT = temperature, OF

ILoD = viscosity of gas-free oil at T, cpJ.L = viscosity of gas-saturated oil at T, cp

'Yo = oil gravity, °API

References1. Beal, C.: "Viscosity of Air, Water, Natural Gas, Crude Oil and Its

Associated Gases at Oil-Field Temperatures and Pressures, " Trans.,AIME (1946) 165, 94-115.

2. Chew, J. and Connally, C. A.: "A Viscosity Correlation for Gas­Saturated Crude Oils," Trans., AIME (1959) 216, 23-25.

3. Lohrenz, J., Bray, B. G., and Clark, C. R.: "Calculating Viscositiesof Reservoir Fluids From Their Compositions," J. Pet. Tech. (Oct.1964) 1171-1176; Trans., AIME, 231. .

4. Houpeurt, A. H. and Thelliez, M. B.: "Predicting the Viscosity ofHydrocarbon Liquid Phases From Their Composition," paper SPE5057 presented at the SPE-AIME 49th Annual Fall Meeting, Hous­ton, Oct. 6-9, 1974.

5. Little, J. E. and Kennedy, H. T.: "A Correlation of the Viscosity ofHydrocarbon Systems With Pressure, Temperature and Composi­tion," Soc. Pet. Eng. J. (June 1968) 157-162; Trans., AIME,243.

H. D. Beggs, SPE-AIMEJ. R. Robinson, *SPE-AIME

U. of TulsaTulsa, Okla.

*Now with Marathon Oil Co., Littleton, Colo.

SEPTEMBER,1975 1141