# Investigation of the Effect of Capillary Barrier on Water-Oil Movement in Water Flooding
## 5 Conclusions  
In this paper, detailed theoretical analysis of the capillary barrier phenomenon as well as its occurrence conditions are identified under different wetting regimes, and the oil-water two-phase flow dynamics in the porous media model are investigated using direct numerical simulations (DNS) coupled with the volume of fluid method at different capillary numbers. The main conclusions drawn from this study are as follows:  
1. The capillary barrier effect is largely responsible for the formation of the residual oil in the reservoir rock. The interplay between the capillary force and viscous force determines the oil-water two-phase flow in the porous media; and the capillary barrier effect is caused by a geometric structure; even in the water-wet conditions, capillary force can still present resistance characteristics. The negative values of capillary force indicate the occurrence of the capillary barrier phenomenon. The capillary force is a type of driving force when the interface enters the throat section if the condition \(|\beta-\theta|<90^{\circ}\) is satisfied. The capillary force presents resistance characteristics if the condition \(\theta-\beta>90^{\circ}\) is satisfied. When \(\theta+\beta>90^{\circ}\), the oil-water interface will reverse, and the direction of the capillary force will point to the side of the water phase. 2. The occurrence of capillary barrier phenomena under different wettability regimes are mainly dominated by the contact angle and the opening angle during the oil displacement process. Under water-wet conditions, the capillary force is positive in the process of the liquid entering the throat section. The capillary force is negative, and  
Figure 23: At the opening angle of \(45^{\circ}\): (**a**) The change of oil displacement efficiency with the capillary numbers; (**b**) The distribution of oil and water in the porous media model when the optimal oil displacement efficiency is reached. the occurrence of the capillary barrier phenomenon can be observed in the process of the liquid leaving the throat section when \(\beta+\theta>90^{\circ}\); while under oil-wet conditions, the capillary force is negative when the liquid is leaving the throat section, and there is a positive peak capillary pressure when the oil-water interface enters the throat from the pore once the condition \(\theta-\beta<90^{\circ}\) is satisfied. 3. The highest oil displacement efficiency under water-wet conditions is obtained at the intermediate capillary number of \(1.8\times 10^{-3}\) and obvious fluctuations of the pressure difference before and after the breakthrough of the water phase are observed; while the highest oil displacement efficiency under oil-wet conditions is obtained at the large capillary number of \(1.8\times 10^{-2}\) and the occurrence of the capillary barrier effect facilitates the mobilization of the fluid phase and the increase of the sweep area when \(\theta-\beta\leq 90^{\circ}\). 4. Under oil-wet conditions, the larger the capillary number, the higher the oil displacement efficiency; while under water-wet conditions, the optimal oil displacement efficiency is obtained under intermediate capillary numbers and at the contact angles at which the capillary barrier phenomenon cannot be observed. However, the established porous media model in the present study can preliminarily illustrate the influence of the capillary barrier effect on the oil-water two-phase flow to some extent; the model is relatively simple, which is limited in characterizing the realistic structure of the oil reservoir rock. Besides, the role of surface roughness, the advancing and receding of the contact angle, and the gradual change of the geometrical structure have not been considered in this work, which do have a far-reaching impact on the oil-water two-phase flow in the porous media. However, it is difficult to obtain analytical results when considering the effects of these factors. Numerical methods or microfluidics techniques may be better options. Data curation, B.H. and C.Z. ; Formal analysis, B.H.