New understanding on water-oil displacement efficiency in a high water-cut stage

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 (PV) 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 oil 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.