Horizontal drilling technology has shown to improve the production and cost–effectiveness of the well by generating multiple extraction points from a single vertical well. The efficiency of hole cleaning is reduced because of the solid–cuttings accumulation in the annulus in cases of extended–reach 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 flow–loop system consisting of a simulated drilling annulus using electrical resistance tomography (ERT) and a high–speed camera. Real–time 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 hole–cleaning issues in the drilling industry.