Abstract: To address the issue of poor geological conditions of the Class Ⅱ reservoirs in Daqing Oilfield,the complex development law of polymer flooding and the difficulty in determining the timing of polymer flooding,a typical geological model of the second-class reservoirs was established by numerical simulation method.Taking the water content index commonly used in oilfield development as the control parameter,the relationship between water content and polymer flooding timing and its influence on the development effect of polymer flooding were studied,and then the relationship between water content and polymer flooding timing was established.The method of determining the reasonable timing of polymer flooding in the Class Ⅱ reservoirs blocks is given.The research shows that the earlier the polymer injection timing is,the better it is for the Class Ⅱ reservoir blocks,but the timing of stopping polymer injection should generally be selected when the water content of the block reaches 96.00% to 97.00%.The application results show that the established chart is practical and effective for the actual block,and can clearly show the development law of polymer flooding.This chart has important predictive guiding significance for determining a reasonable polymer flooding timing for polymer flooding or the blocks that are about to be polymer flooding.

Key words: Class Ⅱ oil reservoirs, water cut, timing of polymer flooding, influence patterns, correlation chart