..”’ SPE SY 3V

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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 resewoir temperature.This information can be obtained from a standard labora-tory PVT analysis that is mn 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 me 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 Beal 1for dead oil and Chew andConnally’ 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 obse~wed. 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 de-scription of the data used, which were obtained from CoreLaboratories, Inc., is given in Table 1.

The correlation for dead oil viscosity was developed bypIotting log10(T) vs logIOlog,0 (WD + 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 equation developed is

*D= lox- 1, . . . . . . . . . . . ...!... . . .(1)where

-x =Y T-1.163

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 log10WD and lOg,OPfora particular value of dissolved gas, Rs. Live oil viscositymay be calculated from

/L= A~DB, . . . . . . . . . . . . . . . . . . . . . .(2)

TABLE1 — DESCRIPTIONOFDATAUSED

Variable RangeSolution GOR, scf/STB 20 to 2,070Oil gravity, “API 16 to 58Pressure, psig Oto 5,250Temperature, “F 70 to 295

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

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TABLE2 — STATISTICALCOMPARISONOFCORRELATIONSAverage Error Standard Deviation

(percent) of Percent Errors

Dead oil data used todevelop this correlation

Beal -19,64 21.86This study -0.64 13.53

Dead oil datacollected fromliterature (93 cases)

Beal 378.46 1,568.09This study 114.27 530.00

Liveoil data used todevelop this correlation

Chew and Connally 25.35 “ 35.70This study - 1.83 27.25

whereA = 10.715 (RS + 100)-0”515B = 5.44 (Z?~+ 150)-0”338

Development of these correlations neglec~ the depen-dence of oil viscosity on composition, since oils of widelyvarying compositions can have the same gravity. Viscos-ity 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 dis-solved 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

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

NomenclatureR,= dissolved GOR, scf/STBT = temperature, ‘F

~~ = viscosity of gas-free oil at T, cpp = viscosity of gas-saturated oil at T, cp

% = oil gravity, “API

References1.

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3.

4.

5.

Beal, C.. “Viscosity of Air, Water, Natural Gas, Crude Oil and ItsAssociated Gases at 011-FieldTempemnm?s and Pressures,” Trans.,AIME (1946) 165, 94-115.Chew, J. and Connally, C. A.: “A Viscosity Correlation for Gas-Saturated Crude Oils,” Trans., AIME (1959) 216, 23-25.Lohrenz, J., Bray, B. G., and Clark, C. R.:’‘CalculatingViscoshksof Reservoir Fluids From Their Compositions,’. J. Pet. Tech. (Oct.1964) 1171-1176 Tram., AIME, 231.Houpemt,A. H. andllrelliez, M. B.: “PredictingtheViscosityofHydrocarbon Liquid Phases From Their Composition,.’ paper SPE5057 presented at the SPE-AIME 49th Annual Fall Meeting, Hous-ton, Oct. 6-9, 1974.Little. J. E. and Kennedy, H. T.: “A Correlation of the Viscosity ofHydr&arbon Systems With Pressure, Temperature and Composi-tion,” Sot. Pet. Errg. J. (June 1968) 157-162;Trans., AIME, 243.

H. D. Beggs, SPE-AIME

J. R. Robinson, *sPE-AIMEU. of TulsaTulsa, Okla.

“NowwithMarathonOilCo.,Littleton,Colo.

Solids Control in Weighted Drilling Fluids

“IntroductionThe need for solids control’for drilling mud systems hasbeen widely discussed in the literaturel-e and is gainingacceptance in drilling operations. tie of the major ad-vantages of reduced drill-solids contei<in drilling mudsare lower plastic viscosity, improved filtir.cakes, longerbit bearing and pump life, and improved pertqtration rate.

Increases in the drill-solids content of mu&.generallylead to higher filtration ratm and increased falter-cakethickness. TMck falter cakes aggravate the proble of

\differential-pressure sticking and cause increases in’

60 rpm. K en water was the circulating fluid, the bear-

2ings his 36 hours. The addition of formation cuttingsdecre ed the bearing life to 26 hours. When the mud wastreated with caustic and quebracho, the bearing life wasreduced to 12 hours. When the same experiment wasdonducted using bentonite instead of formation clays, thebearing life increased from 36 to 38 hours. Additions ofcaustic and quebracho to tlds system did not reduce thebearing life, but when drill solids were added to this mudthe bearing life decreased to 15 hours.

torque and ‘drag on the &lMring. Field observations\indicate that filter-cake properties can be cont@lled by ~ew Mud-Treating System (Mud Cleaner)controlling the amount and types of solids in the mud. To ‘b ercome many of the limitations of current solids-

Surprisingly, drill solids cause a drastic reduction in Lremov lmiques, a new mud-treating system was de-bit-bearing life. King’ performed a series of experiments vekqxd f~ eighted muds. This new system, called theusing an 8%-in. blt with 50,000 lb weight and rotating at \Mud Cleaner, moves most of the drilled solids while

SEPTEMBER, 1975\

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