2-D pore-scale oil recovery mechanisms of the anionic and nonionic surfactants

Surfactants are widely used in enhanced oil recovery (EOR) processes due to their amphiphilic properties, specifically oil enhancement effects including reducing interfacial tension, changing wettability, and emulsifi-cation. As the industrial application of EOR is refined, it is necessary to reveal the dominant EOR mechanism of different surfactants in multiphase flow accurately. However, current research on surfactant is usually mixed with multiple agents in the displacement process, so the dominant mechanism is difficult to be clarified. Thus, three commonly used surfactants (amphoteric-alcohol ether sulfate (AES); biological-lipopeptide (ZT); anionic- petroleum sulfonate (PS)) were selected for the dominant EOR mechanism study. The result of conventional interfacial behavior experiments showed that the three surfactant types have similar ability to reduce oil-water interfacial tension, indicating that the difference of the three agents is not the interfacial tension. The contact angle experiments showed that the three agents reduced the solid-liquid contact angle by 36 degrees, 37 degrees, and 16 degrees, respectively, showing that both AES and ZT have a stronger ability to change the wettability than PS. However, the difference between AES and ZT is still unclear. The microscopic oil displacement experiments showed that PS have no emulsification effect, while both AES and ZT can emulsify the remaining oil into oil droplets. However, it can be observed that the oil droplet size of AES and ZT are small (5-100 mu m) and large (20-100 mu m), respectively. Therefore, the two oil droplet types can cause the "bridging" effect and "Jamin" effect, respectively, and make the fluid flow perpendicular to the direction of the flow field. This is the main reason for the two surfactant types can improve the sweep volume. The overall recovery improved by the three surfactant types is 29.5% (AES), 28.1% (ZT), and 20.1% (PS), respectively. The dominant EOR mechanism of PS is to reduce the oil-water interfacial tension, while AES and ZT are emulsification.