Severe viscous fingering during water flooding of heavy oil leaves a large amount of oil untouched in the reservoir. Improving sweep efficiency is vital for increasing heavy oil recovery. Previous researches have proved that foam flooding can increase the sweep efficiency and control gas mobility, and thus increase oil recovery. However, researches on foam flooding were always focusing on the effect of high gas/liquid ratio (The gas/liquid ratio is usually larger than 1:1). Little research was conducted on foam flooding with low gas/liquid ratio.
This paper presents the results of a laboratory study of polymer enhanced foam flooding with low gas/liquid ratio for heavy oil in Shengli oilfield. For polymer enhanced foam (PEF), the displacement medium is N2 mixed with surfactant as foaming agent and HPAM. The experimental investigation evaluated the performance of foaming agent and studied the use of polymer to enhance foam properties. Polymer enhanced foam flooding were performed in cores, sandpacks and 3D physical model to study the factors affecting displacement efficiency such as gas/liquid ratio, injection slug size, and the viscosity of crude oil. The results of sandpack flood tests showed that the tertiary oil recovery of polymer enhanced foam flooding with the gas/liquid ratio of 0.2:1 could reach 39% IOIP for heavy oil with the viscosity of 325mPa·s at 55, which was 11% higher than the alkali/surfactant/polymer flooding by injecting a 0.3 pore volume (PV) of the same chemical slug. However, the tertiary oil recovery decreased with the increasing viscosity of crude oil. The result obtained in 3D physical simulation indicated that polymer enhanced foam system had excellent profile modification effect to increase sweep area and thus increase oil recovery. The above results demonstrate that polymer enhanced foam flooding with low gas/liquid ratio is a promising economical method for improving heavy oil recovery after water flooding.