These fluids yield a straight-line relationship between shear stress and shear rate that does not pass through the origin. A finite shear stress is required to initiate flow. The value of this shear stress is called the "Yield Point" (Figure 6).
The Bingham Plastic Model is the most widely used mathematical rheological model in the oil field. All data are generated from the 600 and 300 readings on a VG Meter. The model assumes that the fluid evaluated acts in a linear manner on the shear rate - shear stress curve, but has a positive yield stress (Figures 7 and 8).
The equation for the Bingham Plastic model is:
Where
Plastic Viscosity (PV) = q600 - q300
Common terms associated with the Bingham plastic model are: Plastic Viscosity (PV), Apparent Viscosity (ma), Yield Point (YP) and gel strengths. Most drilling fluids, as seen in Figure 7, do not conform exactly to the Bingham plastic model or to any universal model, but drilling fluid behavior can usually be approximated with acceptable accuracy. The Bingham Plastic model assumes that the curve (Figure 9) is approximated by a straight line. This is seldom true for drilling fluids, especially at low shear rates found in the annulus, (Table 1).
Figure 9
Bingham Plastic Model Parameters
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