During the late stage of water flooding, most reservoirs will enter the high water cut stage. The remaining oil is mainly concentrated in the high structural part. At this time, the water flooding effect is no longer ideal. Based on this problem, nitrogen foam-enhanced EOR research is carried out. Through twodimensional visualization experiments, we observed the migration and diffusion of nitrogen foam and summarized the mechanism of nitrogen foam flooding system in enhancing oil recovery in heavy oil reservoirs with bottom water. The physical displacement experiment of core tube is carried out under simulated reservoir conditions, the gas−liquid ratio parameters of nitrogen foam are optimized, and the influence of injection and production methods on recovery factor is studied. The results show that nitrogen foam flooding has certain washing ability and can increase the sweep efficiency. The plugging effect of foam can effectively improve the oil displacement effect. Foam foaming is used to form frothing solution with emulsifying and viscosity-reducing function. The crude oil is produced in the form of foam oil and emulsion under the action of emulsification and viscosity reduction, and the recovery factor is increased. In visual experiments and core tube experiments, the recovery rates of foam flooding stage are 22.4 and 34.9%, respectively. The gas− liquid ratio of 3:1 is the best proportion, and the injection method is the best effect of nitrogen foam flooding and foam huff and puff. The recovery rate of nitrogen foam flooding-foam injection method is 40.7%, which is 5.8% higher than that of nitrogen foam flooding injection mode.

To improve the oil recovery of a block in the Wutonggou Formation of the Changji Oilfield, viscosity reducing and foaming agent was optimized to improve the development effect of the water flooding reservoir. The core flooding experiment and microscopic visual experiment were conducted to investigate the production characteristics and EOR mechanism of nitrogen foam flooding. The results show that the 0.5 wt% viscosity reducing and foaming agent DXY-03 was optimized. In the process of microscopic oil displacement by nitrogen foam, nitrogen foam continuously expands and spreads, improves oil displacement efficiency, and greatly improves oil recovery through emulsification and viscosity reduction, squeezing action, dragging action, and Jamin effect. The core flooding experiment shows that on the basis of the water flooding recovery rate of 20.3%, the nitrogen foam huff and puff is increased by 9.2%. The viscosity reducing and foaming agent flooding is increased by 7.8%, and the nitrogen foam flooding is increased by 12.9%. The main EOR mechanism of the viscosity reducing and foaming agent is that it can reduce the interfacial tension between oil and water and can promote heavy oil emulsification and dispersion, thereby forming an oil/water- (O/W-) type emulsion. The reduction in the viscosity of heavy oil makes crude oil easier to extract, realizing the synergistic viscosity reducing and efficiency enhancing effect of nitrogen and viscosity reducing and foaming agents. This study is helpful to provide reference for the development of low-permeability and high-viscosity medium-deep heavy oil reservoirs by chemical agents combined with cold production.