Annular Pressure Losses
Hydraulic calculations continue with a determination of the amount of pressure lost in the annulus. Assuming laminar flow, the Power Law Model "pressure loss" equation is:
where: k = Consistency index
L = Length of section (feet)
V = Annular Velocity in Section (ft/min)
n = Power Index
G = Geometric Factor
Fluid velocity in the annulus is determined by using:
where: D1 = hole or casing I.D. (inches) D2 = pipe or collar O.D. (inches)
A "geometric factor" is used to take into account the friction factor (a ratio of the actual shear stress imposed on the borehole wall to the dynamic pressure imposed on the system) and is determined by calculating two dimensionless variables (y and z).
'G" can then be calculated: G
If flow in the annulus is transitional, common in the drill collar/open hole annulus, then the pressure losses are determined using a friction factor (f), calculated using:
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