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Yield Point & Viscosity
Date of experiment: December 17th
Mr. Pshtiwan Jaf
Aim of experiment:
The aim of this experiment is to find the properties such as
plastic viscosity, apparent viscosity and yield point using the Fann V-
The most common instrument used for measuring actual rheological
parameters (rather than ranking slurries, for example, by flow time from a
funnel) is the Fann viscometer. The Fann viscometer (sometimes referred to
as a rheometer) is a co-axial cylinders viscometer specially designed for
testing oil well drilling fluids. Two general versions of the instrument arc
available: an electrically driven instrument and a hand cranked instrument. All
versions of the instrument can be operated at 600 and 300 rpm and have a
hand wheel so that the outer cylinder (the sleeve) can be rotated slowly for
gel strength measurements. Some have additional speeds of 3, 6, 100 and 200
rpm. For all versions of the instrument there is a central bob connected to a
torque measuring system and outer rotating sleeve. (Doran & Cather, 2013)
A wide range of shear rates is made possible through selective gearing
and by interchangeable rotors and bobs of various diameters. The instrument
may be operated with open end rotor sleeves, which permit a gentle
recirculation of material through the annulus, thereby minimizing settling of
heavy particles. Optional closed end rotor cups are available for testing of
smaller sample volumes. The torsion springs are designed for ease of
interchangeability, which permits the shear stress range of the instrument
and, hence, the viscosity measuring range to be optimized for a given testing
problem. (Newman & Choo, 2003)
A Fann viscometer (rheometer) is a laboratory device used to
measure the way in which a liquid, suspension or slurry flows in
response to applied forces. It is used for those fluids which cannot be
defined by a single value of viscosity and therefore require more
parameters to be set and measured than is the case for a viscometer.
It measures the rheology of the fluid.
The following apparatuses and materials are used in this
Mud balance Electronic balance Beaker
Syringe Electric mixer & steel vessels
1. Weigh 27.065 gm of bentonite on the electric balance.
2. Pour 489.1 cc of water into a 500 ml beaker.
3. Pour the water into two vessels and stir the water with the mixer.
4. Slowly add the bentonite to the stirring water.
5. Leave the mixer running for few minutes till we have a good mixed mud.
6. Mix the mud in one of the vessels with the mud in the other.
7. Place the mud balance base (preferably in carrying case) on a flat level
8. Fill the clean, dry cup to the top with the freshly obtained mud sample
to be weighted.
9. Place the lid on the cup and set it with a gentle twisting motion. Be sure
that some mud is expelled through the hole in the cap as this will ensure
the cup is full and also will free any trapped air or gas.
10. Cover the hole in the lid with a finger and wash all mud from
outside of the cup and arm. Then thoroughly dry the entire balance.
11. Place the balance on the knife edge and move the rider along the
outside of the arm and leave it on 8.58 ppg (the expected mud density)
and wait till the cup and arm are balanced as indicated by the bubble.
12. Place a recently agitated sample in the cup, tilt back the upper
housing of the rheometer, locate the cup under the sleeve (the pins on
the bottom of the cup fit into the holes in the base plate), and lower the
upper housing to its normal position.
13. Turn the knurled knob between the rear support posts to raise or
lower the rotor sleeve until it is immersed in the sample to the scribed
14. Stir the sample for about 5 seconds at 600 RPM, then select the
RPM desired for the best.
15. Wait for the dial reading to stabilize (the time depends on the
16. Record the dial reading and RPM.
Q1) before the test begins, why we have to make sure that the mud
doesn’t contain large particles and what should be done in that case?
A1) because the gap between the bob and the sleeve is very small
thus any large particles may cause the sleeve to get stuck, that’s why
any mud with large particles should be screened first.
Q2) what does the drilling mud viscosity affects?
A2) it affects Hole cleaning, drilling rate, hole stability, cuttings
settling rate, circulating pressure.
Q3) what are the materials used for enhancing viscosity?
A3) bentonite, asbestos, attapulgite and polymer (such as starch and
Q4) what are the materials used for reducing viscosity?
A4) commonly used viscosity reducers are phosphates, tannates and
Q5) what does the yield point show?
A5) Shows a minimum level of stress must be provided before mud
Doran, D. & Cather, B., 2013. Construction
Materials Reference Book. NewYork: Routledge.
Newman, J. & Choo, B., 2003. Advanced Concrete
Technology 3: Processes. Burlington: Butterworth-