Coiled tubing is extensively used in oil industry to clean the wellbore to increase the productivity of oil/gas well by removing the fill/sand downhole. Well cleanup operation for low bottomhole pressure is problematic and common cleanout fluids are not effective as a circulation fluid due to severe pressure losses and low suspension capability. The use of foam as cleaning agent has become more popular due to its low density and high viscosity. The objective of this paper is to investigate the effect of foam quality and velocity on fill concentration and pressure loss during horizontal wellbore cleanup operation at different CT/annulus diameter ratios using Herschel-Buckley viscosity model. Results showed that foam quality and velocity are two deciding factors for the fill transport. Present study also showed that for all size of fill particles, lower foam quality removes fill more efficiently than higher foam quality. It is noticed that diameter ratio has high effect on particle removal when foam quality is 70. Surprisingly, it is found that the effect of diameter ratio on fill concentration decreases when foam quality is 90. Also, it is found that there is linear increase in pressure drop with increases of foam velocity. Copyright © 2015 Inderscience Enterprises Ltd.