Abstract Hydraulic modelling is an integral part of drilling operations, thus its importance for the efficient delivery of a wellbore cannot be over emphasized. The standard API methods for drilling fluid hydraulics assume either Power law or Bingham Plastic, these models provide a simple way of estimating required parameters for efficient drilling operations, but for conventional wells. These models are sometimes not accurate when used to model the hydraulics of more difficult wells. When drilling fluid is circulated, pressure drop takes place due to friction between the fluid and the surface of the stand-pipe, rotary hose, swivel, Kelly, drill-pipe, drill-collar, drill-bit, and the annulus between the drill-string and the open hole or casing. This generates a total frictional pressure drop in the hydraulic circuit called the Stand Pipe Pressure (SPP). The Equivalent Circulating density (ECD), which is generated from annular pressure losses, needs to be predicted correctly to ensure that the fracture gradient of the formation to be drilled is not exceeded, as there will be losses of the drilling fluid into the formation and potential wellbore instability problems. The paper discusses seven (7) rheological correlations/equations from studies (Robertson-Stiff, Casson, Carreau, Sisko&Willamson, Hershel Buckley, Cross and Modified Bingham Plastic) which have been used in different conditions to simulate and predict the parameters required for efficient hole cleaning operations. It then highlights the magnitude of failure that occurs when simple hydraulic correlations/models are used for complex wells and very viscoplastic fluids. The paper compares and discusses the variation with actual results when the Bingham Plastic, Power, Hershel Buckley and Robertson Stiff models are used to model fluid hydraulics. It proposes that the correct hydraulics of a particular well depends on the nature of the reservoir and bottomhole conditions, as the data from an offset well might not be representative of a target well. Thus it is required that an engineer should run different simulations using different hydraulic models and then compare with the actual data during operations. The difference between the simulated and actual can be used as the fudge factor to further optimize the hydraulics for other hole sections or wells with similar configurations, if the best fit hydraulic model is unavailable in the simulator.