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Accurate estimation of the pressure losses for non-Newtonian drilling fluids inside annulus is quite important to determine pump rates and select mud pump systems during wellbore drilling operation. The aim of this study is to simulate non-Newtonian (power law and Herschel-Bulkly) foam flow in underbalanced drilling condition through wellbore annulus using finite volume method. The effect of various operational parameters on pressure loss such as fluid rheology, foam fluid velocity, foam quality, drillpipe rotation and wellbore eccentricity, have been considered. Simulation results were compared with the previously published experimental data. The agreement was close with a relative error less than 5%. The results of numerical method are closer to experimental data for Herschel Bulkly model for foam fluid. Also, the results of numerical method, showed that pressure drop increases with increasing the foam fluid velocity and quality and it decreases with increasing eccentricity, but drillpipe rotation don’t have noticeable effect on pressure drop.