1.1.2 Yield Point (Lb/100ft2)
The yield stress extrapolated to a shear rate of zero is known as yield point. It is also an important parameter of the Bingham plastic model, which is used to evaluate the ability of a mud to lift cuttings and debris out of the annulus. On a Bingham plastic fluid plot of shear rate versus shear stress, the yield point is the zero shear rate intercept. The yield point is calculated using the 300 rpm shear rate reading and the 600 rpm shear rate reading from the viscometer [3]. A non-Newtonian fluid such as a drilling mud system which has a high yield point would transport drill cuttings better than another fluid of a lower yield point but a similar density. In order to lower the yield point, deflocculants are added to clay based mud drilling systems while to increase it, freshly dispersed flocculants such as lime are added to the drilling mud system. The equation used to calculate the yield point is as shown below.
\begin{equation} \text{Yield\ Point\ }\left(\text{YP}\right)\nonumber \\ \end{equation}
\(=\ Reading\ at\ 300\ rpm-\ Plastic\ Viscosity\ \left(\text{PV}\right)\)(2)