Objective The use of present surfactants for flooding in low-permeability and high-salt reservoirs in Yanchang Oilfield cannot increase the swept volume, but only improve the oil washing efficiency, resulting in low utilization of crude oil in low permeability areas and limited improvement in oil recovery. Therefore, it is necessary to carry out research on the technology of profile control and flooding in low-permeability and high-salt reservoirs.

Method Based on the capillary bundle theory, a low viscosity and ultra-low interfacial tension surfactant (OBU-3) system was developed as a large plug for oil displacement, and combined with a high viscosity and ultra-low interfacial tension surfactant (ACS-2) system formed by self-assembling counterions as a small plug for oil displacement control to form a multi-stage plug, achieving deep profile control and flooding.

Result The injection mass fractions of the large and small plugs were 0.5 wt% and 2.7 wt%, respectively. After 10 adsorption cycles, the interfacial tension reached an ultra-low value, demonstrating good resistance to adsorption loss. The emulsifying and wetting properties of OBU-3 were good, which was conducive to improving the efficiency of oil washing, while ACS-2 had good rheological property, viscoelasticity, and viscosity-temperature property, which were conducive to expanding the swept volume and adjusting the liquid absorption profile. The combination of multi-segment plug surfactants (0.1 PV small segment plug flooding + 0.2 PV large segment plug flooding+subsequent water flooding) had a good oil recovery effect, with an average increase of 14.8 percentage points, which was twice as much as using only large segment plug flooding, and had the oil flooding effect of polymer/surface secondary flooding.

Conclusion The combination of multi-stage plug surfactants profile control and flooding can effectively reduce ineffective water circulation, improve sweep efficiency, and not contain strong or weak alkalies, avoiding corrosion and scaling. It can significantly improve economic benefits and provide guidance for improving oil recovery in low-permeability and high-salt reservoirs.