Field application proves that polymer flooding is an effective technology for improving development effect of heterogeneous reservoirs. Previously, single slug injection is performed. However, there are problems of profile reverse, non-effective circulation of polymer solution in high permeability zone, over much polymer dosage and so on. Thus a new technique of alternative injection for improving oil displacement effect of polymer flooding is developed. Some oil displacing experiments with different injection patterns are performed by using automatic oil displacement equipment and physical model of heterogeneous reservoir so as to study oil displacement efficiency of different alternative injection patterns. A series of polymer solution slugs with different molecular weight, concentration, viscosity and size are injected alternatively in the experiments and then reasonable alternative injection pattern is optimized and difference of displacement mechanism between single slug injection and alternative injection is illustrated. Meanwhile an evaluation method for the improvement effect of fluid absorption profile is introduced for the first time. Experimental results show that oil displacing effect of alternative injection is better than that of single slug injection and the incremental oil recovery is 2% more than that of single slug injection. Based on basic law of displacement mechanics and combined with experimental data, displacement mechanism of profile reverse and that of inhibiting profile reverse are demonstrated. The profile reverse is mainly due to different tendency of transient resistance factors in high and low permeability zones. Alternative injection can make this tendency the same and this is the main cause of alternative injection inhibiting profile reverse and increasing injection volume in low permeability zone so as to improve oil displacement effect. When alternative injection was applied in pilot tests of Daqing Oilfield, oil production is increased, while water cut increasing rate and polymer powder dosage are decreased. Application result shows that alternative injection is a new injection type of polymer flooding and is a effective measure to increase oil displacement efficiency and reduce polymer dosage.

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