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In case of slanted wells even a strong inclination will not change the friction contribution by much (may see a slight increase by from 3 % up to 5 %).
For the horizontal sections of wells and surface pipelines the value of friction-based pressure loss dominates over vanishing gravity-based pressure build up which zooms the value of accurate calculation of Darcy friction factor with account of its variation along the flow. In the meantime, for strongly inclined/horizontalised horizontal pipelines the pressure/temperature variation along the pipe is usually very minor, so is the water viscosity, and Darcy friction factor again has very little variation along the flow.
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