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24 Part I Liquid Drilling Systems

Figure 2.3 Fann VG meter 35A used for rheological property measurements. (Images
by author.)

widely employed in the drilling industry. It can measure the rheological
properties of all types of fluids quickly at six speeds. For a new fluid,
however, it is important to first run the instrument at various speeds to
get a complete set of shear rate and shear stress data. Plotting the data
will help to identify the type of fluid. Then the rheological properties
associated with the fluid model can be determined.

Newtonian Fluids
The viscosity of Newtonian fluids can be calculated using the following
formula:
μ = 300 θ N (2.5)
N
where
Ν = rotary speed of Fann VG meter, rpm
θ N = dial reading of Fann VG meter at rotary speed N

If the rotary speed of the Fann VG meter is set at 300 rpm, Eq. (2.5)
degenerates to μ = θ 300 , where θ 300 is the dial reading of the Fann VG
meter at the rotary speed of 300 rpm.