Ultra-low interfacial tension Anionic/Cationic surfactants system with excellent emulsification ability for enhanced oil recovery

Droplet size and stability of the O/W emulsion formed by heavy oil determine its fluidity in formation porous media. Hence, an anionic/cationic surfactants system of with excellent emulsification ability, ultra-small droplet size, and reduced oil–water interfacial tension (IFT) to ultra-low level was developed in this study. The AEOSHS/DTAC system can reduce oil–water IFT to ultra-low level within the molar ratio of 4:6 to 6:4. Due to the hydrophilicity of sulfonic acid group and ether group, the precipitation-dissolution balance of AEOSHS/DTAC system is well improved, and its solubility in is enhanced, and transparent solutions can be formed in a molar ratio of 10:0 to 0:10. The droplet size of O/W emulsion formed can as low as 0.6 μm when the molar ratio is 5:5. Moreover, the stability of O/W emulsion is greatly improved. The coalescence and migration process of O/W emulsion is very slow, and the stability is excellent. The dehydration rate after 24 h is only 6%, with the dehydration speed of only 0.25 %·h−1. AEOSHS/DTAC can enhance the heavy oil recovery by 22.05%. Because the droplet size of W/O emulsion is much smaller than porous diameter, the pressure does not increase during the system injection and subsequent water flooding. The AEOSHS/DTAC system is composed by anionic and cationic surfactant, which can attract each other at the interface to form a compound molecular layer. So, the repulsion between molecules on the oil–water interface is greatly reduced, making the arrangement of molecular is tighter, and the adsorption layer is more stable (The occupied area of a single molecule is only 0.06 nm2). The surface activity and interfacial activity are higher, and the droplet size is smaller, and the stability is stronger.