Plastic Viscosity centipoise (cps)
The plastic viscosity (PV) is calculated by measuring the shear rate and stress of the fluid. These values are derived by using a Fann viscometer, which is a rotating-sleeve viscometer, and may be a simple hand operated two speed model or a more complex variable speed electric model. The two speed model operates at 300 and 600 rpm.
The Fann viscometer consists of an outer rotating sleeve and an inner bob. When the outer sleeve is rotated at a known speed, torque is transmitted through the mud to the bob. The bob is connected to a spring and dial, where the torque is measured. The shear rate is the rotational speed of the sleeve and the shear stress is the stress (torque) applied to the bob, measured as deflection units on the instrument dial. These measurement values are not true units and need to be converted.
Shear rate is the rate of change as the fluid layers move past one another per unit distance, and is measured in reciprocal seconds (i.e. (ft/sec)/ft) and is usually written as seconds-1. To convert the dial reading to shear stress, the dial reading is multiplied by 1.067 to give a reading in lb/100ft2.
The units of viscosity are poise or centipoise (1/100 poise) and is derived as follows:
Viscosity (poise) = (F/A) / (V/H)
where: F = Force (dynes)
A = Area (cm2)
V = Velocity (cm/cc)
H = Distance (cm)
This produces viscosity as Dynes (sec/cm2) or poise.
The Fann viscometer reading is therefore multiplied by 1.067 to obtain shear stress in lb/100ft2; or multiplied by 478.8, and divided by the shear rate in second-1 to get Dynes/cm2.
Viscosity then becomes:
511 x dial reading / shear rate (sec-1)
since 511 sec-1 = 300 rpm
or (300 x dial reading) / Fann shear rpm
The viscometer is designed to give the viscosity of a Newtonian fluid when used at 300 rpm.
For Non-Newtonian fluids, the ratio of shear-stress to shear-rate is not constant and varies for each shear rate. With a Bingham plastic fluid, a finite force is required to initiate a constant rate of increase of shear-stress with shear-rate. To obtain a value for this constant rate of increase, readings are taken with a viscometer at 511 sec-1 and 1022 sec-1 (300 and 600 rpm). The 600 dial reading minus the 300 dial reading gives the slope of the shear-stress/shear-rate curve. This is the Plastic Viscosity. The “apparent viscosity” is given by the 600 reading divided by 2. This is a measure of that part of resistance to flow caused by mechanical friction between solids in the mud, solids and liquids and the shearing layers of the mud itself.
We can see that control of the solids will give us control over our PV! This leads to “Why are we controlling the solids?” Since the viscosity of the mud is one of the principal factors contributing to the carrying capacity of the mud, the suspension of weighting materials, and pressure surges applied to the formation through frictional pressures in the annulus, it is obvious that increased solids will increase these annular pressures (and may increase the mud density), so a balance must be found in which the correct mud density and carrying capacity are maintained without exerting unnecessary pressures on the annulus.
In the mud system, we have solids that are an integral part of the mud (bentonite, starch, CMC, etc.) and solids that are undesirable (sand, limestone, dolomite, etc.). As the mud density is increased, by the addition of barite or hematite (more solids), the PV will automatically increase. The PV is also a function of the viscosity of the fluid phase of the mud (as temperature rises, the viscosity of water decreases, and the PV will decrease).
Several methods of lowering the solids content of the mud are available, all of which will lower the plastic viscosity and apparent viscosity, as well.
1. Dilution; add water and lower the concentration of solids.
2. Shaker Screens; using the finest screens possible without “blinding” to remove solids. Avoid hosing water on the screens as this washes fine solids through the screens.
3. Centrifuge; these separate the solids by size and mass, reducing total solids concentration.
4. Desander/Desilter; these mechanically remove the sand/silt sized particles from the mud.
To increase the viscosity of a mud system, various “mud chemicals” can be added. These are mostly types of bentonite, but attapulgite clays, asbestos and gums (Guar or Xanthan) are also used.
The polymer viscosities such as XC polymer, consist of these gums. Most polymers provide a mud with a shear thinning effect. This is desirable as it allows viscosity to be maintained while circulating pressures are reduced.
The plastic viscosity (PV) is calculated by measuring the shear rate and stress of the fluid. These values are derived by using a Fann viscometer, which is a rotating-sleeve viscometer, and may be a simple hand operated two speed model or a more complex variable speed electric model. The two speed model operates at 300 and 600 rpm.
The Fann viscometer consists of an outer rotating sleeve and an inner bob. When the outer sleeve is rotated at a known speed, torque is transmitted through the mud to the bob. The bob is connected to a spring and dial, where the torque is measured. The shear rate is the rotational speed of the sleeve and the shear stress is the stress (torque) applied to the bob, measured as deflection units on the instrument dial. These measurement values are not true units and need to be converted.
Shear rate is the rate of change as the fluid layers move past one another per unit distance, and is measured in reciprocal seconds (i.e. (ft/sec)/ft) and is usually written as seconds-1. To convert the dial reading to shear stress, the dial reading is multiplied by 1.067 to give a reading in lb/100ft2.
The units of viscosity are poise or centipoise (1/100 poise) and is derived as follows:
Viscosity (poise) = (F/A) / (V/H)
where: F = Force (dynes)
A = Area (cm2)
V = Velocity (cm/cc)
H = Distance (cm)
This produces viscosity as Dynes (sec/cm2) or poise.
The Fann viscometer reading is therefore multiplied by 1.067 to obtain shear stress in lb/100ft2; or multiplied by 478.8, and divided by the shear rate in second-1 to get Dynes/cm2.
Viscosity then becomes:
511 x dial reading / shear rate (sec-1)
since 511 sec-1 = 300 rpm
or (300 x dial reading) / Fann shear rpm
The viscometer is designed to give the viscosity of a Newtonian fluid when used at 300 rpm.
For Non-Newtonian fluids, the ratio of shear-stress to shear-rate is not constant and varies for each shear rate. With a Bingham plastic fluid, a finite force is required to initiate a constant rate of increase of shear-stress with shear-rate. To obtain a value for this constant rate of increase, readings are taken with a viscometer at 511 sec-1 and 1022 sec-1 (300 and 600 rpm). The 600 dial reading minus the 300 dial reading gives the slope of the shear-stress/shear-rate curve. This is the Plastic Viscosity. The “apparent viscosity” is given by the 600 reading divided by 2. This is a measure of that part of resistance to flow caused by mechanical friction between solids in the mud, solids and liquids and the shearing layers of the mud itself.
We can see that control of the solids will give us control over our PV! This leads to “Why are we controlling the solids?” Since the viscosity of the mud is one of the principal factors contributing to the carrying capacity of the mud, the suspension of weighting materials, and pressure surges applied to the formation through frictional pressures in the annulus, it is obvious that increased solids will increase these annular pressures (and may increase the mud density), so a balance must be found in which the correct mud density and carrying capacity are maintained without exerting unnecessary pressures on the annulus.
In the mud system, we have solids that are an integral part of the mud (bentonite, starch, CMC, etc.) and solids that are undesirable (sand, limestone, dolomite, etc.). As the mud density is increased, by the addition of barite or hematite (more solids), the PV will automatically increase. The PV is also a function of the viscosity of the fluid phase of the mud (as temperature rises, the viscosity of water decreases, and the PV will decrease).
Several methods of lowering the solids content of the mud are available, all of which will lower the plastic viscosity and apparent viscosity, as well.
1. Dilution; add water and lower the concentration of solids.
2. Shaker Screens; using the finest screens possible without “blinding” to remove solids. Avoid hosing water on the screens as this washes fine solids through the screens.
3. Centrifuge; these separate the solids by size and mass, reducing total solids concentration.
4. Desander/Desilter; these mechanically remove the sand/silt sized particles from the mud.
To increase the viscosity of a mud system, various “mud chemicals” can be added. These are mostly types of bentonite, but attapulgite clays, asbestos and gums (Guar or Xanthan) are also used.
The polymer viscosities such as XC polymer, consist of these gums. Most polymers provide a mud with a shear thinning effect. This is desirable as it allows viscosity to be maintained while circulating pressures are reduced.
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