Based on analyzing dominant internal and external factors affecting water-oil displacement efficiency, the variation of oil displacement efficiency is studied by means of experimental and theoretical analyses. Internal factors include reservoir heterogeneity (highlighted micro-pore structure heterogeneity) and wettability. External factors are waterflooding conditions, including pore volume multiplier of water injected, injection rate, and oil-water viscosity ratio. The variation mechanism is analyzed based on the definition, laboratory determination and field application of water displacement efficiency. A viewpoint is proposed that the changes of wettability and pore structure in a high water-cut stage would reduce the critical capillary number and the residual oil saturation, leading to improvement of water displacement efficiency. More attention should be paid to water-oil displacement efficiency when water-flooding oilfield enters a high water-cut stage. Research on water-oil displacement efficiency must be deepened from three levels as follows: the first is the definition of laboratory values, the second is to study economic and effective conditions for getting (reaching) a certain oil displacement efficiency, the third is how to develop cost-effective techniques to improve the displacement efficiency by increasing capillary number.%通过实验和理论分析,从影响水驱油效率的主要内在和外在因素出发,分析高含水阶段水驱油效率变化的可能性.内在因素主要包括储集层的非均质性(尤其是微观孔隙结构的非均质性)、润湿性等；外在因素主要为水驱条件,包括注水孔隙体积倍数、注入速度及油水黏度比等.根据水驱油效率的定义,实验室测定及矿场应用情况,尝试分析水驱油效率变化的机理,提出了高含水阶段储集层润湿性及孔隙结构变化将导致临界毛管数的降低,使残余油饱和度减小,进而提高水驱油效率.水驱油田进入高含水开发阶段后应加强对水驱油效率的关注,从3个层次加深对水驱油效率的研究,首先是水驱油效率的实验取值,其次要研究不同储集层水驱油效率的经济界限,最后要从毛管数出发,着眼于经济有效地提高水驱油效率的技术研究方向.
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