Plastic Viscosity (PV) is the resistance of fluid to flow. According to the Bingham plastic model, the PV is the slope of shear stress and shear rate. In the field, we can get the PV from a viscometer. Typically, the viscometer is utilized to measure shear rates at 600, 300, 200, 100, 6, and 3 revolutions per minute (rpm).
We can calculate the PV from the difference between the 600 and 300 rpm reading.
The formula looks like this: Plastic Viscosity (PV) = Reading at 600 rpm – Reading at 300 rpm
The unit of PV is Centi Poise (CP)
For example, you have these values from a viscometer.
Reading at 600 rpm = 56
Reading at 300 rpm = 35
Plastic Viscosity (PV) = 56 – 35 = 21 CP
Any increase in solid content in drilling mud as barite, drill solid, lost circulation material, etc will result in higher the plastic viscosity. In order to lower the PV, you must reduce the solid content that can be achieved by using solid control equipment and/or diluting drilling mud with base fluid. With increasing temperate while drilling deeper, the plastic viscosity of the drilling mud will decrease because the viscosity of the base fluid decreases.
Normally, the higher mud weight, the higher PV will be. However, if you have an increasing trend of PV with constant mud weight, it means that there is an increase in ultra-fine drill solid content in the mud system. Moreover, if you use oil base mud, please keep in mind that emulsified water in oil base drilling fluid will act like a solid, and it will increase the viscosity dramatically.
Several impacts of PV on drilling operation are as follows;
Equivalent Circulating Density (ECD) – The more PV you have, the higher ECD will be.
Surge and Swab Pressure – The PV has the same effect as ECD. If the PV increases, surge and swab pressure will increase.
Differential Sticking – A chance for differential sticking will increase, especially in water base mud, when the plastic viscosity increases because of increases in solid content.
Rate of Penetration (ROP) – The ROP will be directly affected by the plastic viscosity. If you drill a well with the high PV drilling mud, you will drill at slower ROP than low PV mud.