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The absolute value is staying between 

LaTeX Math Inline
body f = 0.04

 for the very small flow rates (< 100 cmd) and 

LaTeX Math Inline
body f = 0.015

 for the very high flow rates  (> 1,000 cmd) and this should be taken into account in calculations. 

For complex well designs with varying pipe flow diameters and water source/stocks which may lead to substantial variation of flowrate the wellbore model can be split in segments each having a constant friction factor.

For the horizontal sections of wells and surface pipelines the value of friction-based pressure loss dominates over vanishing gravity-based pressure build up and requires more accurate calculations of Darcy friction factor with account of its variation along the flow.

Derivation

Consider a ration between friction-based pressure gradient 

LaTeX Math Inline
body --uriencoded--\displaystyle \left[ \frac%7Bdp%7D%7Bdl%7D \right]_f =\frac%7B\rho_s \, q_s%5e2 %7D%7B2 A%5e2 d%7D \, f_s

 and gravity-based pressure gradient in vertical well 

LaTeX Math Inline
body --uriencoded--\displaystyle \left[ \frac%7Bdp%7D%7Bdl%7D \right]_g= \rho_s \, g

:

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The decrease in water viscosity with growing temperature is partially compensated by decrease in response to growing pressure thus making viscosity staying within 10% along-hole in most practical cases (usually slightly decreasing with depth).

Providing that friction losses are only 3.3 % of the hydrostatic column the further variation of Darcy friction factor by 10% provides only 0.33 % error against pressure modelling with constant Darcy friction factor.

In case of slanted wells even a strong the inclination will not change the friction contribution by much (may increase by from 3 % up to 5 %).

See also

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Physics / Fluid Dynamics / Pipe Flow Dynamics / Darcy–Weisbach equation / Darcy friction factor 

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