BAKER HUGHES - Drilling Fluids Reference Manual

HYDRAULICS
model is generally accepted as the best available for describing the flow properties of drilling
fluids.
Robertson-Stiff Model
Figure 9 - 8 Idealized Herschel-Bulkley Fluid
The Robertson-Stiff equation, like the Herschel-Bulkley model, is a three variable model. Like the
Herschel Bulkley model it has been found to provide very close approximations for pressure losses
in the circulating system in most drilling situations. The Newtonian, Bingham Plastic and power
law models are specific cases of the Roberson-Stiff model.
In ADVANTAGE SM a least squares method is used to determine the rheological parameters for the
Robertson-Stiff model also. This considers all the available Fann readings to determine the
rheological parameters for the Robertson-Stiff model. If Plastic Viscosity and Yield Point are
entered instead of Fann readings, the Fann readings at 300 and 600 [RPM] can be recalculated.
Additionally, the 10sec gel strength can be used to approximate the Fann reading at 3 [RPM].
where:
τ = K×
n
( γ + ) ...........................................................................................(5)
0
γ
τ = Shear stress
K = Consistency index
γ 0 = Shear rate intercept
γ = Shear rate
n = Power law index
Rheological Models in ADVANTAGE
In ADVANTAGE Reporting it is possible, and recommended, to input at least Fann readings for
600, 300, 200, 100, 6 and 3 rpm. The 10 second, 10 minute and 30 minute gel strengths are also
entered. From these numbers the Plastic Viscosity and Yield Point are derived along with n and k
for the power law model.
The hydraulics equations in ADVANTAGE SM Reporting are API 13-D equations and utilize the
power law. These equations do not account for the effect of temperature and pressure on fluid
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 9-8