BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
As defined in the Fluids Testing Procedures Manual, the determination of PV and YP are obtained
from the dial readings at 600 rpm and 300 rpm. Substitution of the appropriate data into the equation
shows how these terms are derived.
θ 600 = YP + PV⎛
600 -------- ⎞ = YP + 2PV
⎝300
⎠
θ300 = YP + PV⎛
300 -------- ⎞ = YP + PV
⎝300
⎠
By subtracting θ 300 from θ 600 , we obtain,
or,
( θ 600 – θ 300 ) = ( YP + 2PV) – ( YP + PV) = ( YP – YP) + ( 2PV – PV ) = 0 + PV
PV = ( θ 600 – θ 300 )
By re-arranging the earlier equation for θ 300 , we have,
where,
YP = ( θ300 – PV)
θ 600 = 600 rpm dial reading
θ 300 = 300 rpm dial reading
Effective viscosity has been previously defined as the shear stress divided by the shear rate or the
slope of the line passing through the origin to the shear stress at some particular shear rate. From the
equations above, we see that PV can represent the effective viscosity if YP = 0, written as,
θ600 = 0 + 2( μ∞ )
The effective viscosity at a shear rate of 600 rpm on the V-G meter is distinguished from the effective
viscosity at other shear stress/shear rate data values by the term apparent viscosity. Therefore,
apparent viscosity is defined at a 600 rpm shear rate by,
θ 600
μ ∞ = ---------
2
Although plastic viscosity (PV) and yield point (YP) are two of the most recognized properties of
drilling fluids, these terms are simply constants in the Bingham Plastic Mathematical Model. Very
few drilling fluids follow this model, but the empirical significance of PV and YP is firmly entrenched
in drilling technology. In fact, drilling fluid systems such as the NEW-DRILL ® system and many
others deviate significantly from the Bingham Plastic Model and the terms PV and YP must be
interpreted with caution.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 1-15