Abstract: It is important to study the law of water flooding in fractured-vuggy medium to determine the development mechanism of fractured-vuggy reservoir.Based on a typical connecting type of fractures and vugs,a series models with different fracture aperture were made from synthetic glass.Physical simulation experiments were carried out by using those models,and the distribution and origin of residual oil were analyzed.The influences of flow velocity,fracture aperture and capillary force on oil recovery and water cut were studied.The critical fracture aperture in different vug heights was further calculated using similarity criteria.The results indicated that the residual oil after water flooding mainly lay in the not connected bypassed vugs and fractures.The residual oil after surfactant flooding mainly lay in the top of the not connected vugs.When the differential pressure,which was caused by the interface of oil and water rising in the vugs,could overcome the capillary force,water would flow into the bottom of the fracture.The oil recovery of the model increased along with the decreasing of flow velocity,increasing of fracture aperture and decreasing of capillary force.The critical fracture aperture decreased along with the increasing of vug height.The oil recovery of this vug-fracture-vug medium can be effectively enhanced by surfactant flooding.