CHAPTER 2: LIQUID VISCOSITY MEASUREMENT Objective Calculate viscosity (dynamic or absolute, and kinematic) and determine how this property varies with changes in temperature for a constant-composition multi-component hydrocarbon mixture. Introduction When shear stress is applied to a fluid it will suffer a continuous deformation, which is referred to as flow. Fluid viscosity is the property, which indicates the fluid s resistance to flow due to an unbalanced shearing force (see Figure 2-1). Fluids characterized as Newtonian fluids exhibit constant values for viscosity with variations in the shear rate, everything else being constant. As a first approximation, the liquids found in petroleum reservoirs (hydrocarbons and water) are considered to be Newtonian. Newton s Law of viscosity can be written as follows: v...(2-1) y Where is the shear stress applied to the fluid, is the absolute viscosity of a Newtonian liquid, and v / y is the velocity gradient (flow or deformation). The variation in viscosity of liquid petroleum is due to the variation in composition, which results from reductions in pressure and the resultant liberation of gases or vapors. A study of such variation is beyond the scope of this laboratory. However, changes in temperature are another important source of variation of viscosity for constant composition hydrocarbon mixtures. This is the objective of this study. Thus, the purpose of this laboratory exercise is to determine how the viscosity of a multi-component hydrocarbon varies with temperature. 2-1
Figure 2-1. Flow Between Two Plates (Modified from Bear, 1972) There is a multitude of methods for measuring viscosity. Each method often has its own relative viscosity scale, which is good only for that particular method. For instance, two such units are Saybolt (SSU) or the Redwood scale. Viscosity can be absolute (dynamic) or kinematic. Absolute viscosity is commonly used for equations describing the flow of fluids in porous media. It is related to the kinematic viscosity through density: *...(2-1) Where is kinematic viscosity of liquid and is density of the liquid. Common units for absolute viscosity are illustrated in Table 2-1. Table 2-1. Units of Dynamic (Absolute) Viscosity Units System S.I. c.g.s Field Units N*s/m 2 Dyne*s/cm 2 = Poise cp = Poise/100 The kinematic viscosity is usually given in centi-stokes (cst). Multiplying cst by density in g/cc, the dynamic (absolute) viscosity is obtained in cp. Kinematic Viscosity Measurement Using a Cannon-Fenske Viscometer 2-2
Figure 2-2 presents a Cannon-Fenske viscometer. This instrument is used to measure kinematic viscosity, which will be used (and density measurements) to compute dynamic (absolute) viscosity. Figure 2-2. Cannon-Fenske Viscometer The instructions for using this apparatus are (Cannon-Fenske Viscometer Instrument Manual): 1. The viscometer should be cleaned with a suitable solvent and dried in a stream of clean, filtered N 2. 2. The instrument should be periodically cleaned with chromic acid to remove any possible traces of organic deposits. 3. If a possibility of lint, dust, or other solid material is present in the liquid sample, this may be removed by filtering through sintered glass filter or fine mesh screen 4. To introduce sample into viscometer, invert viscometer, immerse tube A into liquid and apply suction to I, which causes the sample to rise to etched line E. Turn the viscometer to normal position and wipe tube A clean. 5. Insert the viscometer into a holder and place in constant temperature bath. Allow some minutes for viscometer to reach equilibrium at desired temperature (10 min for 100 o F; the higher the temperature the longer the waiting time). 6. Vertical alignment may be accomplished in bath by suspending a plumb bob in tube I. 2-3
7. Apply suction to tube A and bring sample into bulb B a short distance above mark C. 8. The efflux time is measured by allowing the sample to flow freely through mark C, measuring the time for the meniscus to pass from C to E. 9. To repeat efflux time measurement, repeat steps 7 and 8. 10. The kinematic viscosity is calculated by multiplying the efflux time by the viscometer constant. Every Cannon-Fenske tube has its own calibrated constant, which is multiplied by the recorded time in second to obtain kinematic viscosity in cst, that is: k *t where k is the viscometer constant in cst/seconds and t is time in seconds. It is particularly important to remember the following points: Use the correct viscometer size. The best results will be obtained operating near the center of the viscometer range. For instance, if the fluid has a viscosity of 100 cst, then use a tube size 300 (range 50-250). Table 2-2 shows the tube size with their respective viscosity ranges and approximated constants. Avoid overlapping the viscometer. Using either too much or too little sample will produce inaccurate values of viscosity. The volume that fills the tube from E to the inlet of the A tube will approximately half fill the bulb marked H. Use the correct calibration factor, k. The constant shown in Table 2-2 are only approximated. Each viscometer tube must be calibrated regularly with standard fluid to find the correct constant. After loading the sample, wait at least 10 minutes before making measurements. Some time is required to allow the sample to equilibrate at the temperature of the water bath and to allow air bubbles to segregate. 2-4
Use special methods for opaque samples. For high viscosity oil samples, which are sometimes quite opaque, it is necessary to use auxiliary illumination to improve judgments of the passage of the interface past the starting and ending marks. Table 2-2. Recommended Viscosity Ranges for The Cannon-Fenske Routine Viscometers (Cannon-Fenske Viscometer Instrument Manual) Size cst / Second Approximate constant Centistokes Range 25 0.002 0.5 to 2 50 0.004 0.8 to 4 75 0.008 1.6 to 8 100 0.015 3 to5 150 0.035 7 to 35 200 0.1 20 to 100 300 0.25 50 to 250 350 0.5 100 to 500 400 1.2 240 to 1200 450 2.5 500 to 2500 500 8 1600 to 8000 600 20 4000 to 20000 Laboratory Experiments Place fluid sample in a Cannon-Fenske tube as explained in this chapter and put it in a controlled temperature bath. Measure the time that takes to the sample to go from mark C to mark E for four or five different temperatures (same as those of density measurements). References 1. Cannon-Fenske Viscometer Instrument Manual. 2. Bear Jacob, Dynamics of Fluids in Porous Media, Dover Publications, INC. New York 1972. 2-5