Mechanisms of water flooding characteristic curve upwarping at high water-cut stage and influencing factors

Water flooding is widely used in oilfield development as a secondary recovery method owning to its effectiveness and economic feasibility. When the injected water breaks through the production well, water is produced with oil. The water flooding characteristic curve is an important method for predicting recoverable reserves, recovery rates, and oilfield development dynamics. It reflects the oil-water composition of oilfield-produced fluids. The linear relationship between the relative permeability ratio ( K ro/ K rw) and water saturation ( S w) in semi-log coordinates is a theoretical basis for the water flooding characteristic curve. However, many mine practices and core experiments show that the relationship deviates from a straight line at a high water-cut stage, and the water flooding characteristic curve shows an upward trend. The upwarping phenomenon of the water flooding characteristic curve is related to the flow state of the oil/water phase. The continuum description, Darcy’s law, fails to predict and describe the transport of scattered residual oil in pore spaces. Many previous studies tried to obtain a non-linear relationship between ln( K ro/ K rw) and S w at the high water-cut stage by different methods, including fitting the experimental data. Though there are many studies on the upwarping phenomenon of water flooding characteristic curve, few studies focus on the mechanisms of this phenomenon. In addition, the effects of important development parameters, such as wettability and water-oil viscosity ratio, on the upwarping phenomenon are unclear. In this study, the Navier-Stokes equation is coupled with the phase field method to describe the water flooding processes in porous media and to track the two phase interface. The effects of wall wettability and oil-water viscosity ratio on the flow patterns were analyzed by numerical simulations. In addition, the critical water cut when the relationship between ln( K ro/ K rw) and S w deviates from the straight line was investigated. The results show that residual oil is mainly scattered at the high water-cut stage, and the flow capacity of oil rapidly decreases, which leads to the upwarping phenomenon of the water flooding characteristic curve. The critical water cut is different under different wetting conditions. Due to the effect of capillary force, the flow capacity of oil is stronger, and more oil can be displaced out through the imbibition process at the high water-cut stage in water-wet porous media, and the critical water cut increases when wettability changes from oil-wet to water-wet. The flow patterns of oil and water are different at different oil-water viscosity ratios. At the low viscosity ratio, the displacement front is more uniform, and the removal of oil by water mainly depends on expanding the sweeping area at the high water-cut stage. For the high viscosity ratio case, clear fingers of injected water can be observed during the flooding process, and it is difficult for the injected water to enter the areas which were not previously swept at the high water-cut stage. The viscous fingering phenomenon enhances the ineffective displacement of the injected water. The relationship between ln( K ro/ K rw) and S w deviates from a straight line at a lower water cut when the oil-water viscosity ration is high.