Precise estimation of annular pressure losses is essential in drilling and well completion operations to control formation pressures and optimize drilling and completion fluids' hydraulic programs. Without accurate estimation of annular pressure losses it is possible to face serious problems that would yield interruption in drilling operation and sometimes lead to abandonment of the well. While annular pressure loss estimations are critical, determination of pressure losses associated with flow of Non-Newtonian fluids is challenging. Many investigators proposed different equivalent diameter concepts to provide a similarity between pipe and annular flow.

In this study we have investigated the coupling effects of rheological model and equivalent diameter definition on pressure losses using three rheological models (Bingham Plastic, Power Law and Yield Power Law) together with four different equivalent diameter definitions. Effects of inner/outer radius ratios and flow rate on pressure losses determined using different rheological model and equivalent diameter pairs are analyzed using an onshore well and an offshore well. The differences in pressure losses using different pairs for the cases of onshore and offshore wells are shown. The study provides detailed analysis on how severely the annular pressure losses varies once rheological model and equivalent diameter definitions are changed for different operating conditions.

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