Reynold number is the important figure because it demonstrates the flow regimes of drilling mud as laminar, transition or turbulent flow in annulus of the wellbore. In order to correctly calculate the Reynolds number, you need to use the effective viscosity, µea, which we already discuss about it from the previous topic.
The following equation is for Reynolds number in the annulus.
Rea = Reynold Number in the annulus
Va = Annular velocity, ft/min
Dh = Diameter of wellbore, inch
Do = Outside Diameter of tubular, inch
W = mud weight, ppg
µea = effective viscosity in the annulus, centi-poise
na = power law constant