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)