Study on the construction of anionic-cationic surfactant binary mixed system and its wettability alteration performance in tight reservoirs

To solve the problem of reducing reservoir permeability in tight reservoirs due to higher capillary forces, a method is proposed to change the reservoir wettability by using surfactants and then increase oil and gas recovery rates. Mixed systems of anionic and cationic surfactants are less used in wettability alteration due to strong aggregation behavior. However, it has been shown that anionic-cationic surfactants exhibit excellent synergistic effects at appropriate concentrations and ratios, which have good theoretical value for practical applications. In this paper, firstly, we investigated the synergistic effects of mixed system DTAB/SDS and DTAB/AES in reducing the surface tensions. It was found that there was a safe zone between the precipitation boundary and the critical micelle concentration for anionic and cationic surfactant mixed systems, which indicated that the mixed systems had a better synergistic effect without aggregation at a certain concentration and ratio. The DTAB/AES mixed system could reduce the surface tension to a low value of 18.67mN/m. Further, the wettability alteration of the mixed system was studied. Results showed that a contact angle of the sandstone altered from 10.75 degrees to 71.8 degrees after treating in the DTAB/SDS mixed system at a low concentration, which was significantly better than a single surfactant on performance. The total concentration of the optimum mixed system was 5 x 10(-5) mol/L, and the molar ratio was 0.7:0.3. The optimum mixed system had excellent wettability alteration performance, it could significantly reduce capillary forces and relieve aqueous phase trapping damage, further, the oil and gas flow channels could be restored.