Wet PVT Lab PETE 310
Lab # 6
Determination of Viscosity for oil samples
Learning Objectives
Gain a basic knowledge of the techniques used to measure viscosity
Learn to use a rotational and a glass capillary viscometer.
Understand the basics of rheological modeling.
General Skills
Use Excel spread sheets and graphs.
Work with different systems of units and convert one set to another.
Background Material
Rheology is the study of the change in form and flow of matter in terms of elasticity, viscosity and plasticity. A clear understanding of the rheological properties of fluids is vital in many fields of science and engineering. The purpose of this lab is to introduce you to the different techniques and approaches to measure viscosity experimentally.
Viscosity is the measure of the internal friction of fluid. This internal friction is caused when a layer of fluid moves in relation to another layer. The greater the friction, the greater the amount of force required to cause this movement. This movement is known as shear.
∆ v
V 1
∆ x
V
2
Figure 1. Deformation of a liquid under the action of a tangential force.
To define viscosity more precisely, let’s take a look at figure 1. Two parallel planes of fluid of equal area “A” are separated by a distance dx and are moving at different speeds

V
1
, V
2
. The force required to maintain the difference in speed is proportional to the difference in speed through the liquid.
F
= µ dv
,
A dx where:
µ is known as the viscosity, usually in units of centipoises or Pa.s.
(1) dv
is the shear rate. Describes the shearing the fluid experiences when the layers move dx with respect of each other. Units in reciprocal second, sec -1 .
F
is the force per unit area required for the shearing. This is known as the shear stress
A and it has units of pressure.
Therefore, we can define viscosity as µ =
Shear Stress
Shear Rate
Newtonian fluids
A Newtonian fluid is characterized by having a constant viscosity at a given temperature.
This is normally the case for water and most oils. A plot of shear rate versus shear stress would show a constant slope, (figure 2). This is the simplest and easiest fluids to measure in the lab.
Ne wto nia n
Figure 2. Shear rate versus Shear stress for a newtonian fluid
Non Newtonian fluids
A non-newtonian fluid is characterized by not having an unique value for viscosity. That is, the relationship stress rate/shear rate is not constant. The viscosity of these fluids will depend on the shear rate applied.

There are several types of non-newtonian fluid behavior that we can observe in the lab.
The most common are shown in figure 3.
Pseudo plastic fluids : these are fluids like paints and emulsions, there is a decrease in viscosity as the shear rate increases. Also known as shear thinning fluids.
Dilatant fluids: these are fluids that increase their viscosity as the shear rate increases.
Examples are cement slurries, candy mixtures, corn startch in water. Also known as shear thickening fluids.
Plastic fluids: These fluids will behave like solids under static conditions. They will start to flow only when certain amount of pressure is applied. Examples are tomato catsup and silly putty. ke nin g
Sh ea r T hic
Sh ea r t hin nin g
P P la st ic
Figure 3. Shear rate versus Shear Stress for different types of fluids
Instruments to measure rheological properties
Most instruments designed to measure viscosity can be classified in two general categories: tube type and rotational type. Figure 4 shows the different types of instruments available. The selection of a particular instrument must be based on the type of analysis required and the characteristics of the fluid to be tested. For example, rotational methods are generally more appropriate for non-newtonian fluids, while glass capillary viscometers are only suitable for Newtonian fluids.

Figure 4. Classification of rheological instruments.
In this lab, we will use three different instruments to measure viscosity: Cannon Fenske viscometers, the Ruska Rolling Ball viscometer and the rotational Cone and Plate viscometer.
Cannon-Fenske Viscometer
The Cannon Fenske tube viscometers are very useful and easy to use to collect rheological data. This device uses gravity to create flow and its use is limited to
Newtonian fluids with viscosities in the range 0.4 to 20000 cP.
The viscosity obtained from this instrument is typically reported as kinematic viscosity.
The kinematic viscosity can be defined as the viscosity µ of the fluid divided by its density ρ .
ν =
µ
ρ
, (2) where ν is the kinematic viscosity, in mm 2 /s, also known as cSt (centistokes).
Procedure
1Clean the viscometer using a suitable solvent and dry with clean air
2To charge the sample into the viscometer, invert the instrument and apply suction to tube G immersing tube A in the liquid sample. Draw liquid to mark E. Wipe clean arm A, and turn the instrument to its normal vertical position.
3Place the viscometer into the holder, and insert it inside the constant temperature bath. Align the viscometer vertically in the bath.
4Allow approximately 10 minutes for the sample to come to the bath temperature at 40 ° C and 15 minutes at 100 ° C.

5Apply suction to tube A or pressure to tube G and draw the liquid above mark C.
6To measure the efflux time, allow the liquid sample to flow freely down past mark C, measuring the time for the meniscus to pass from mark C to mark E.
7Repeat at least once your measurement.
8Calculate the kinematic viscosity in centistokes of the sample by multiplying the efflux time in seconds by the viscometer constant.
ν =
C
t
Where:
ν : kinematic viscosity, mm 2 /s.
C: viscometer constant, mm 2 /s 2 . t: flow time, s
Figure 5. Cannon-Fenske viscometer
Cone and Plate Viscometer
This method of determining Viscosity uses a cup to contain the sample as the plate and a spindle with specific dimensions as the cone to determine viscosity. This test has some very specific advantages over other tests. First it uses a very small sample, usually from
0.5-2 ml of fluid. Second with the correct installation and equipment samples can be tested at numerous shear rates and temperatures.
The basics of the test are that a spindle which is a flat round plate is attached to the devices called a rheometer. This round plate has specific properties such as a crosssectional area. This plate is then rotated while in the sample to be measured. The space between the bottom plate and the bottom of the device is strictly controlled. The plate is then rotated at a constant speed (rpm). While rotating a strain gauge attached to the shaft

the spindle is mounted on measures the drag produced between the plate and the sample being tested. This drag force along with the dimensions of the plate are used to determine the shear stress. These are measured in dynes/cm 2 . The physical characteristics of the spindle are inputted into the device so that it will convert them into the proper shear stress units.
Figure 6. Cone and Plate diagram
Figure 7. Picture of a small and large spindle and the cone (cup) in which they are installed
.
To determine the viscosity of any sample the following criteria need to be known:
1.
Test temperature
2.
Sample Container size (or sample spindle/chamber geometry)
3.
Sample volume required
4.
Rheometer specifications
5.
Characteristics of the spindle to be installed in the unit
6.
Range of shear rates or speeds to be used
7.
Time required for each test
The above described list is the criteria as required by Brookfield for their unit.
When determining viscosity it helps if the rheological model of the fluid is known. There are generally four models or fluid types. These are listed below:

Newtonian- Viscosity is not dependent on shear rate. A sample tested at 50 rpm will have the same viscosity as one tested at 500 rpm.
Non-Newtonian- These fluids will not have the same viscosity at different shear rates.
The general rule is that as shear rate increases the viscosity will decrease. The general description for this type of fluid is “shear thinning.”
Time Dependent- This category has two types, one in which the fluid will continue to become less viscous as the test progresses at the same shear rate. This fluid would be considered Thixotropic. A second category is pseudoplastic and the fluid will “thin” as rate increases but will return to their original viscosity as the shear rate is decreased.
Of the above described fluid type we will primarily deal with Newtonian fluids in this experiment. The fluid will have the same viscosity throughout the range of shear rates and will only change with the temperature of the sample.
The discussion above was borrowed extensively from the Brookfield Engineering Manual for the instrument.
Test Criteria
1.
Test Temperature
The test of the oil sample in this experiment will begin at room temperature. The temperature will be increased at some rate until it reaches a temperature near bottom-hole conditions. Periodically the sample will be tested at different temperatures. The final temperature will depend on the well the sample came from and in some cases may not be reached. From the data collected a predicted bottom-hole viscosity of the fluid can be found from the data.
2.
Sample Container Size
For this test the sample size is dependent on the spindle. It can vary from 0.5-2.0 ml. In this case we will probably be using 0.5. The volume is not a part of the analysis, but in the testing procedure.
3.
Sample Volume
This item is keyed or related to item 2. This will be determined by the instructor as part of the setup of the experiment.
4.
Rheometer Model

For this experiment we are using a Brookfield LDV-III cone and plate viscometer.
5.
Spindle Used
This is determined by the instructor before the lab. The size of the spindle is inversely proportional to the viscosity. The larger the spindle the lower the range of viscosity measurements. A better discussion of this will be seen in the attachments to this lab.
6.
Test Speed
This will be determined as we run the test. The greater the speed the more accurate the test. In this case it is dependent on the spindle used as will as the sample and the range of temperatures to be tested.
7.
Time Required for Each Test Point
For this item the manual asked for a minimum of 5 revolutions before data is taken.
Hopefully the shear rate will be great enough to limit test time to 1-3 minutes.
Test Procedure
Spindle Selection
To select spindle some idea of the fluid viscosity must be known. For most conventional crude oils a viscosity range of 100 or less cp can be used. For heavy oils such as found in
Kern River or Venezuela a viscosity range of 10000cp could be used. In this case we will probably use a range of 100 cp or less.
The following table gives the spindle and the range of shear rates and expected viscosities for each:
Table 1. Spindle specifications for Brookfield Cone and Plate viscometer
Shear Rate
Spindle
Calibration of Instrument

The instrument will be calibrated before the test but the following criteria must be met.
The measured viscosity of the fluid must be within a range of the viscosity of the standard +/- 1% of the standards viscosity and 1% +/- the full scale viscosity of that spindle at the test speed.
The following equation defines the full scale viscosity:
Cp = TK * SMC * 10,000/(test rpm)
TK and SMC are both specific for the spindle and rheometer used.
Table 2. Spindle constants for Cone and Plate viscometer
Spindle TK SMC SRC
CP-40 0.09373 0.327
CP-41 0.09373 1.228
CP-42 0.09373 0.64
CP-51 0.09373 5.12
CP-52 0.09373 9.83
Calculate allowable range of viscosities for the unit:
7.5
2
3.8
3.84
2
RPM 12
Spindle CP-41
Viscosity of standard at 25C 100.2 cp
From the equation we find the full scale viscosity to be: 95.92 cp. The accuracy variation for this test is +/- 1% or 0.9562 cp.
The accuracy of the viscosity standard is also +/- 1%. For this fluid the accuracy would be 1.002 cp. By combining these terms the measured accuracy of the fluid during calibration will be 100.2 cp +/- (0.9562+1.002) or 1.9582 cp.
As long as the unit returns a value within the calculated range for that spindle and fluid it can be considered calibrated.
Sample Testing Procedure
The unit will be setup and calibrated before the experiment. This will allow more time for testing and the elimination of problems before the test is conducted.
1.
Take a sample of the appropriate volume as requested by the instructor and place it in the cup.
2.
Return the cup to the unit and allow the unit to stabilize at temperature. This should only take a few minutes.

3.
Set the unit to an rpm/shear rate that will be above 10% of the allowable torque on the unit. Let it run for at least a minute or a minimum of 5 revolutions.
4.
Note the temperature of the test, the shear rate in 1/sec and the shear stress in dynes/cm2.
5.
Change the shear rate if possible and take a second reading.
6.
Adjust upwards the temperature at least 10C and repeat 3-5.
At the conclusion of the test the data can be used to determine the viscosity of the fluid.
For each temperature the viscosity can be determined by the following equation:
Viscosity (poise) = shear stress/shear rate
Rolling Ball Viscometer
Figure 8. Ruska falling ball viscometer
The Ruska rolling ball viscosimeter is used to determine the viscosity of bottom hole and surface samples at elevated temperatures and pressures, up to 10,000 psi and 300 ° F. This instrument operates on the rolling ball principle, where the roll time of a ¼ inch diameter ball is used to obtain viscosity data.
The viscosity is calculated as
µ =
Kt
(
ρ ball
− ρ fluid
)
(4)

where
µ: viscosity
K: constant
ρ
ball
: Density of the ball
ρ fliud
: Density of the fluid t: roll back time
The driving force in this instrument is the difference in density between the fluid and the ball. At a fixed temperature, the difference in ball and fluid density will be constant. The viscosity will be directly proportional to the roll back time. The constant of the viscometer must be determined by previous calibration using a liquid of known viscosity.
Operating sequence:
1) choose the correct ball size. If the fluid viscosity is estimated to be between 0 and
5 cP, a 0.252 or 0.248 inch diameter ball should be used. Above 25 cP, the 0.234 inch diameter ball will be appropriate
2) Clean the test assemblywith kerosene and vent air to ensure the chamber is free of dust.
3) Place the ball in the bottom of the empty measuring barrel
4) Evacuate the test assembly. This is done by opening the vacuum pump valve at the lower end of the unit and closing the charging valve
5) Charge the test sample fluid in the viscometer. The vacuum valve shold be closed while the high pressure charging valve is reopened.
6) Rock the test assembly to obtain a single phase sample. The presence of gas bubbles inside the chamber can prevent the ball from moving freely and stop the experiment completely.
7) Set the temperature of the viscosimeter to the desired value. Allow 3 hours for the temperature to estabilize.
8) Bring the ball to the hold position, by rotating the test unit 180 degrees.
9) Turn on the coil and switch to HOLD. The yellow light must be on
10) Rotate the assembly to the desired angle (70°, 45°, or 23°), this will depend on how viscous the fluid is.
11) Switch to FALL. The green light must be on. The ball is released and the time to travel is displayed. When the ball hits the bottom, a sound alarm will be triggered.
12) Calculate the viscosity by using equation 4. with the appropriate values for the constant. These values will be provided to you in the lab.
Required
1.
Measure the kinematic viscosity of water, kerosene and motor oil at three different temperatures using the Canon Fenske viscometer. Justify your selection of tubes.
2.
Using the Cone and Plate viscometer, construct a plot of viscosity versus temperature for a sample of oil (77 to ~150 °F). Construct the shear stress versus

shear rate plot for each fixed temperature in your experiment. Discuss your observations.
3.
Using the Ruska rolling ball viscometer, construct a plot of viscosity as a function of pressure and temperature for a mixture of n-decane-methane. Start at 500 psia,
77 °F. Increase the pressure to 700 psia and 1000 psia. Repeat the same measurements for 100°F and 200°F. Do each measurement three times, at least at two different angles.
General Guidelines for the Report
Follow the format for lab reports (check Useful Notes in the Technical Presentations course as needed).
Abstract: What important lessons did you learn about these viscosity measurements?
Why are they valuable for petroleum engineers to know?
Conclusions: Evaluations, estimations, generalizations…..
Introduction: Why is it important to measure rheological properties.
Discussion: You may want to include a Methods section, or you might want to introduce your method for each part of this exercise as you discuss the various topic areas. Do not regurgitate the lab manual; instead, tell us how you met its standards. If you made any other decisions about how to proceed, explain what you did. Remember that we know the procedure, but we gave you some opportunities to make choices, and we need to know what choices you made. You have at least a couple of different figures here. Be sure to have a Discussion section to handle each of them.
End Matter: If you used any symbols, be sure to include a Nomenclature; if you cited your textbook or other resources for the Introduction, be sure to include a References list. (Remember that lab notes/class notes cannot be cited.)
Appendix: The table you formatted to make your graphs should not be included in the body of your report. Instead, include it (or any other extraneous materials) in an
Appendix.