Plastic viscosity (PV) can be defined as the resistance offered by a fluid to flow freely. This resistance is a result of friction between the liquid undergoing deformation under shear stress and the solids and liquids present in the drilling mud. PV is a parameter of the Bingham plastic model and is the slope of the shear stress/ shear rate line above the yield point.
PV is an important rheological characteristic that affects properties of drilling fluid.
Plastic viscosity or PV indicates the viscosity of the drilling mud when it is extrapolated to infinite shear rate based on the Bingham model. When PV is low it indicates that the mud will drill rapidly due to the low viscosity of mud that is exiting the bit. High PV is a result of a base fluid that is viscous and by the presence of excessive colloidal solids. To reduce PV, the solid contents can be reduced by diluting the mud.
PV is measured using a viscometer by measuring the shear rate at 600, 300, 200, 100, 6 and 3 rounds per minute (rpm). PV value is obtained by subtracting the 600 rpm reading from the 300 rpm reading.
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