Optimization on self-assembled ultra-micro foam system for carbon dioxide flooding.

• Carbon dioxide foaming system was selected through comparative analysis: 0.50 wt% EAPMAC-12 + 0.20 wt% SDS + 0.09 wt% TEOA + 0.08 wt% EDTA. • The viscosity, foaming volume, foam half-life and foam composite index are 18.8 mpa·s, 531 mL, 25 min and 9956 mL·min. • The foam solution could form worm-like micelles by self-assembled with viscosity in the range of 5–20 mpa·s and the ultra-micro foam size was 5–31 μm. • The experiment indicated that the foam had good injection and oil displacement effects in ultra-low permeability reservoirs. Given the problem that the conventional foam injection and salt resistance are poor, the oil displacement effect is not obvious, and it is difficult to enhance oil recovery in the ultra-low permeability and high salinity reservoirs of Yanchang Oilfield, it is imperative to study a new foam flooding agent. Based on the surfactant's self-assembled properties, foaming agent, stabilizer and complexing agent were screened in the absence of polymer and non-alkali. Three sets of self-assembly worm-like micelle systems were screened, and then a set of carbon dioxide foaming system was selected through comparative analysis: 0.50 wt% EAPMAC-12 + 0.20 wt% SDS + 0.09 wt% TEOA + 0.08 wt% EDTA. Under this condition, its viscosity, foaming volume, foam half-life and foam composite index are 18.8 mpa·s, 531 mL, 25 min and 9956 mL·min, respectively. Under the condition of the ratio of gas to liquid 3:1, the ultra-micro foam was obtained by gas–liquid co-injection in the sand pack, and the foam was round rather than the traditional pentagonal or hexagonal shape and the size was 5–31 um, indicating that the foam had good injection. Oil displacement experiment showed the average enhanced recovery rate was 12.84 % in ultra-low permeability cores, indicating that the foam had good oil displacement effect in ultra-low permeability reservoirs. [ABSTRACT FROM AUTHOR]