1.1.1 Plastic Viscosity (Centi Poise (CP))
The gradient of the slope of a shear stress versus shear rate curve, which are parameters of the Bingham plastic model above the yield point is known as the plastic viscosity. This is also represented when the viscosity curve of a sample of drilling mud is extrapolated to infinite shear rate on the foundation of the arithmetic of the Bingham model [3]. In the field and in this paper, it is measured using a viscometer, which basically measured various shear rates at 600 rpm, 300 rpm, 200 rpm, 100 rpm, 6 rpm and 3rpm. After the various shear rates have been measured, it is possible to calculate the plastic viscosity by subtracting the 300 rpm shear rate from the 600 rpm shear rate. The equation for this is as shown below.
\begin{equation} \text{Plastic\ Viscosity\ }\left(\text{PV}\right)\nonumber \\ \end{equation}
\(=Reading\ at\ 600\ rpm\ -\ \ Reading\ at\ 300\ rpm\) (1)
A drilling mud system that permits rapid drilling due to the low viscosity of the mud exiting at the bit is said to have a low plastic viscosity while a presence of colloidal solids and viscous base fluids can lead to a high plastic viscosity drilling mud system. Reduction of the plastic viscosity value can be achieved by diluting the drilling mud system.\(\ \)