Anionic-zwitterionic viscoelastic surfactant strengthened air foams for heterogeneous reservoirs

Foam flooding technology can effectively enhance oil recovery (EOR) and has conformance control in heterogeneous reservoirs. However, the stability of foam migrating through porous media is still a challenge with individual surfactant. Herein, we developed viscoelastic surfactant (VES) via the synergisms of anionic (sodium dodecyl sulfate, SDS) and zwitterionic (erucamidopropyl hydroxysulfobetaine, EHSB) surfactants to strengthen the air foams stability. First, the VES formulation was optimized by investigating the initial foaming volume (V-0), drainage half-life (t(d)), and foam half-life (t(f)) for VES solutions. Then, the oil-water interfacial tension (IFT) and emulsification of the optimum VES system were systematically studied. Particularly, emphasizes were placed on the anti-gas channeling ability, EOR performance, and mechanisms using the core-flooding and microfluidic experiments. The results showed that the V-0, t(d), t(f) of air forms could be greatly improved to 400 ml, 56.6 min, and 36 h, respectively, with 0.3 wt. % SDS:EHSB at mass ratio of 1:1.3. This was attributed to the synergisms of anionic and zwitterionic surfactants through electrostatic attraction, which promoted the formation of entangled wormlike micelles and increase in the solution viscosity. In addition, the oil-water IFT could be lowered to 0.1 mN/m and thus rapidly emulsifying the crude oil. Finally, the VES strengthened air foams exhibited excellent anti-gas channeling ability and great EOR efficiency of 20.44% after water-flooding process in heterogeneous reservoirs. The EOR mechanisms involving emulsification, stripping the residual oil, and jamming effect were identified from pore-scale during the microfluidics experiments.