Viscous fingering is one of the main challenges that could reduce areal sweep efficiency during waterflooding in oil reservoirs.A series of waterflooding experiments were carried out in a Hele-Shaw cell at ambient temperature during which areal sweep efficiency was estimated and techniques to ease the fingering problem were examined.The onset and propagation of viscous fingers were monitored as a function of both injection rate and injection/production positions.Image processing techniques were utilized to quantitatively investigate the propagation of fingers.The experimental results show that,under specific conditions,increasing the number of finger branches could improve the areal sweep efficiency,whereas growth of a single narrow finger has a negative impact on oil displacement efficiency.According to the obtained results,increasing the injection rate improves the areal sweep efficiency up to a critical rate at which viscous fingers start to grow.The impact of heterogeneity of the medium on distributing the viscous fingers was also investigated by introducing two different arrangements of fractures in the model.The results show that fractures perpendicular to the direction of flow would distribute the displacing water more uniformly,while fractures in the direction of flow would amplify the unfavorable sweep efficiency.