# Investigation of the Effect of Capillary Barrier on Water-Oil Movement in Water Flooding
## 4 Effect of the Capillary Barrier on Oil-Water Two-Phase Flow in Porous Media Model
### Results and Discussion
#### 4.3.3 The Change of Oil Displacement Efficiency  
Seven different values of the contact angle (\(\theta_{1-7}\): \(15^{\circ}\), \(30^{\circ}\), \(45^{\circ}\), \(60^{\circ}\), \(90^{\circ}\), \(120^{\circ}\), \(165^{\circ}\)) are chosen for the porous media model with an opening angle of \(45^{\circ}\) in order to investigate the influence of the capillary barrier effect on the macroscopic oil displacement efficiency. Numerical simulations are performed to investigate the change of the oil displacement efficiency with contact angles and the influence of the capillary barrier effect on the change of the distribution of the oil and water in the porous media model based on the aforementioned oil-water two-phase flow dynamic model. The conditions of optimal oil displacement efficiency in the porous media model with capillary barrier correspond to different opening angles. In the case of the intermediate capillary number, that is, the viscous force and capillary force are at play, and the optimal oil displacement efficiency is obtained when the minimum conditions of the occurrence of the capillary barrier phenomenon are satisfied.  
Figure 21: The change of oil and water distribution with time at the capillary number of \(1.8\times 10^{-4}\) under oil-wet conditions (the contact angle is \(120^{\circ}\)): (a) 0.0333 s; (b) 0.0356 s; (c) 0.0576 s; (d) 0.0665 s. (The italic letters (a–j) in the figure indicate the locations of the oil–water interface.)Figure 23 shows the change of oil displacement efficiency with the capillary numbers and the distribution of oil and water in the porous media model when the optimal oil displacement efficiency is obtained. It can be seen from Figure 23 that 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 that the capillary barrier phenomenon cannot be observed. In addition, the sweep area of water flooding in the intermediate region of the porous media model is increased by the capillary barrier effect, and the oil displacement efficiency is higher on both sides of the intermediate region along the diagonal line. At the contact angle of 45\({}^{\circ}\), the effect of the dynamic mobilization of the water phase is stronger than that at the contact angle of 30\({}^{\circ}\) without considering the process of the liquid leaving the throat, and the displacement force prompts the advancing of the displacement front on both sides.  
Figure 22: The distribution of oil and water in the porous media model with different capillary numbers under oil-wet conditions and at the contact angle of 120\({}^{\circ}\): (**a**) Ca = 1.8 \(\times\) 10\({}^{-4}\); (**b**) Ca = 1.8 \(\times\) 10\({}^{-3}\); (**c**) Ca = 1.8 \(\times\) 10\({}^{-2}\).  
There is a significant difference in the oil displacement efficiency before and after the occurrence of the capillary barrier phenomenon. A stronger capillary barrier effect will impede the advancing of the displacement front, thus reducing the sweep area of the water phase. However, the capillary barrier effect can be used to increase the sweep area of the displacing phase especially in the case that the capillary barrier effect and the viscous force are at play in the waterflooding process. The weaker capillary barrier effect will prevent the displacement front from advancing along a path with the minimum resistance, which will result in the formation of a stable channel to discharge the water phase. To increase the displacement pressure will not work and the displacement front will switch to other routes instead of advancing along a path with large resistance, which will result in the formation of a pause and breakthrough of the blocking, thus improving the oil displacement efficiency to a large extent.