Chemical flooding agents, especially surfactant-based flooding oil agents, have been widely studied due to their excellent emulsification ability. Nevertheless, there are many evaluation methods for chemical flooding agents, and the focus of each test is different. How to combine routine testing with reservoir conditions is an urgent problem to be solved. In this study, different kinds of chemical flooding agents were evaluated for interfacial tension (IFT), minimum emulsification speed, emulsification state, droplet size distribution, viscoelasticity, apparent viscosity in porous media, stability, and flooding efficiency. The influence of their characteristics on the pressure dynamics during flooding is analyzed, and the mechanism in the different stages of chemical flooding is clarified. A comprehensive analysis of IFT and the minimum emulsification speed is needed to evaluate the emulsification ability. The apparent viscosity when W/O emulsion flows through porous media is more accurate to evaluate the viscosity reduction of the chemical flooding agents than the traditional viscosity test. The chemical flooding dynamics is mainly divided into three stages: pressure increase (emulsification ability), pressure decrease (fluidity of formed oil in water (O/W) emulsion), and pressure stabilization (flooding efficiency). The stronger the emulsification ability, the faster the pressure increase and the wider the main flow channel. The easier the O/W emulsion is formed, the smaller the droplet size, the stronger the elastic deformation ability, and the higher the oil flooding efficiency. The smaller the droplet size and the stronger the viscoelasticity of O/W emulsion, the lower the maximum pressure in the flooding process and the slower the pressure decrease in the middle stage. Therefore, the evaluation of chemical flooding agents should mainly consider their emulsification ability and the formed O/W emulsion droplet size, viscoelasticity, and stability. This study clarified the evaluation focus of chemical flooding, and laid a foundation for the research and development and screening of chemical flooding agents, and the dynamic analysis of chemical flooding.

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This study carried out interfacial tension (IFT) test, sand surface element analysis and scanning electron microscope (SEM) image, rock-oil-emulsification system interaction test, microstructure, droplet size distribution and stability of oil in water (O/W) emulsion to clarify the porous media flooding mechanism of hydrophilic Nano-SiO2 enhanced emulsification system. Results shows that by adding small amount of Nano-SiO2 (0.01 wt.%) into anionic surfactant fatty alcohol polyoxyethylene ether sodium hydroxypropyl sulfonate (AEOSHS) solution (0.5 wt.%), the IFT of oil-water effectively reduced, the adsorption loss of AEOSHS on the formation sand surface was reduced by more than 70%, the droplet size of formed O/W emulsion reduced by 50%. This greatly improves the effective concentration of AEOSHS and emulsify heavy oil ability in formation away from injection well. Moreover, the spreading ability of oil on core surface is greatly reduced, and the width of diffusion zone is narrowed. Meanwhile, a very clear dividing line of oil can be seen which shows that the wettability of core has changed to water wet. The stability of formed O/W emulsion was further enhanced, and the coalescence and migration process of droplet is extremely slow. The oil recovery of AEOSHS+ Nano-SiO2 system can effectively increase 21.95% of original oil in place (OOIP). Both the sand-packed tube experiment and the microscopic visual oil flooding experiment show that the system can not only expand the sweep volume, but also improve the oil displacement efficiency, which means that the combination system can significantly improve the oil displacement effect.