Horizontal drilling technology has shown to improve the production and costeffectiveness of the well by generating multiple extraction points from a single vertical well. The efficiency of hole cleaning is reduced because of the solidcuttings accumulation in the annulus in cases of extendedreach drilling. It is difficult to study the complex flow behavior in a drilling annulus using the existing visualization techniques. In this study, experiments were carried out in the multiphase flowloop system consisting of a simulated drilling annulus using electrical resistance tomography (ERT) and a highspeed camera. Realtime tomographic images (quantitative visualization) of multiphase flow from ERT were compared to the actual photographs of the flow conditions in a drilling annulus. The quantitative analysis demonstrates that ERT has a wide potential application in studying the holecleaning issues in the drilling industry.