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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 pipelines the pressure/temperature variation along the pipe is very minor, so is the water viscosity, and Darcy friction factor agains again has very little variation along the flow.
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