In view of the problems existing in the development of the second⁃class reservoirs in sandstone reservoirs,such as low utilization degree,high water⁃bearing and low oil recovery rate,a set of well pattern, multiple sets of layers,polymer flooding,and the method of reverse⁃exploiting second⁃class reservoirs layer by layer was proposed.The parameters such as pore volume,injection rate, polymer injection volume and slug combination were optimized by numerical simulation in Pu I1-2 layers of Daqing Xing Twelve area.The results showed that when the PV number was 1 PV,the injection rate was 0.16 PV/a,the polymer injection volume was 1 200 mg/L⋅PV,the high⁃concentration polymer slug was injected first, then the low⁃concentration polymer slug was injected,and the recovery rate reached 50.11%.Compared with the predicted water flooding recovery rate,it was improved by 10.1%.

Firstly, the molecular coil size of 4 kinds of polymers, whose molecular weight polymers is 8 million, 12 million, 16 million and 20 million respectively, was measured by the light blockage counter. Then pore cumulative distribution curve of field cores was measured by kerosene mass method. Finally the relationship curves between polymer molecular coil size and inaccessible pore volume of field cores were plotted. On the basis of this study, the water saturation value is assigned to the volume of the pore volume, and the polymer molecular weight with better compatibility with rock pores is optimized according to the relationship curve drawn. This method is based on the micro channel of underground rock. It has reasonable value and simple steps. And there are some guidances for the selection of the polymer in the oil field.