Objective The low permeability reservoirs have poor physical properties and strong interlayer heterogeneity, resulting in CO₂ flooding which is prone to gas channeling, and the effect of enhancing oil recovery is not good. As a method combining the advantages of CO₂ flooding and water flooding, CO₂-water alternating flooding has high applicability. In order to further improve the development effect of CO₂-water alternating flooding, CO₂-low interfacial tension viscoelastic fluid synergistic flooding was studied.

Methods The basic performance of viscoelastic fluid with low interfacial tension was evaluated by interfacial tension and wettability tests, and the oil displacement effect of different displacement methods and the "synergism" mechanism of CO₂-viscoelastic fluid with low interfacial tension in synergistic oil displacement were explored by microscopic visualization oil displacement experiment and core oil displacement experiment.

Results Viscoelastic fluid with low interfacial tension has good interfacial activity and the ability to change the wettability of rock surface. After water flooding, the recovery efficiency of CO₂ flooding, low interfacial tension viscoelastic fluid flooding and CO₂-low interfacial tension viscoelastic fluid alternating synergistic flooding increased by 0.91%, 10.66% and 16.25% respectively on the basis of water flooding. The mechanism of enhanced oil recovery includes reduction of interfacial tension, improvement of fluidity ratio, change of wettability of rock surface and synergistic effect of emulsification.

Conclusions CO₂-low interfacial tension viscoelastic fluid synergistic flooding can not only effectively enhance gas flow control during CO₂ injection in heterogeneous ultra-low permeability sandstone reservoirs, but also reduce the influence of light hydrocarbon extraction on heavy component deposition, which has synergistic synergistic effect.