Abstract: To explore the mechanism of enhancing the oil recovery of tight oil reservoirs by surfactant combined with low-salinity water flooding, in a study case of a tight sandstone reservoir in Xinjiang Oilfield, the effects of low-salinity water flooding, surfactant flooding and their combination on the recovery efficiency at different injection rates and solvent ratios are studied with self-made test equipment. The result shows: The surfactant combined with low-salinity water flooding can effectively play the synergistic advantages to improve the recovery efficiency of tight oil reservoirs. When the injection rate is too low, the surfactant can effectively modify the pore throat interface, but the energy of water flooding is insufficient. When the injection rate is too high, it is easy to induce the coning of oil-water interface, and the effect of surfactant on modifying the pore throat interface is limited, leading to oil displacement efficiency increasing first and then decreasing with the increase of injection rate. At the injection rate of 0.3 mL/min, the highest oil displacement efficiency of 89.79% is achieved with a 7∶3 mass ratio of low-salinity water (0.1% NaCl mass fraction) to sodium dodecyl-benzene sulfonate anionic surfactant (0.4% mass fraction), which is at least 29.83% higher than that of single-fluid flooding. The field application shows that the surfactant combined with low-salinity water flooding can effectively enhance oil recovery and increase monthly production by about 47% in tight reservoirs where the production is severely depleted per well. The study results can be referred for efficient development of similar tight oil reservoirs.

Key words: low-salinity water, surfactant, tight oil reservoir, enhanced oil recovery, synergy