# Investigation of the Effect of Capillary Barrier on Water-Oil Movement in Water Flooding
## 3 Capillary Barrier Phenomenon  
### Remaining Oil Formation during the Water-Flooding Process  
Figure 2 shows the remaining oil distribution after water flooding of the water-wet porous structure initially filled with oil. The black line is the pore boundary, the white part in the pore is filled with water, and the black part is the remaining oil. Water flows in from the left and out from right. The capital letters (A-Y) in the figure indicate the locations of the oil-water interface. It can be seen that these oil-water interfaces stagnate at the pore-throat interface.  
Figure 1a shows the morphology of the oil-water interface in the throat channel. As to a pore space with the radius of \(R\), the resultant interfacial tension (capillary force) can be expressed by Laplace formula  
\[f=\frac{\sigma\cos(\theta)}{R} \tag{12}\]  
where \(\sigma\) is the interfacial tension coefficient and \(R\) is the pore channel radius.  
It can be known from Equation (12) that three factors determine the magnitude of the capillary force: interfacial tension coefficient \(\sigma\), contact angle \(\theta\), and channel radius \(R\). For a given oil reservoir, \(R\) is constant. Therefore, enhancing the oil recovery rate of low-salinity waterflooding depends largely on the influence of the salinity on interfacial tension \(\sigma\) and contact angle \(\theta\). It has been found that salinity has a significant influence on the contact angle \(\theta\) via measuring the interfacial tension \(\sigma\) and contact angle \(\theta\) of the fluid.  
Figure 2: Water (white color) and oil (black color) distribution after water flooding.  
Thus, the influence of the low-salinity waterflooding on the oil recovery rate is regulated via the contact angle \(\theta\).  
Under water-wet conditions, the contact angle \(\theta\) is an acute angle, and the direction of \(f\) is consistent with the flow direction of the water phase, which is the driving force in the water-flooding process. Figure 1b shows the morphology of the oil-water interface at the location of the pore-throat interface. After that, the oil-water interface will keep contact angle \(\theta\) with the pore wall, and the capillary force induced by the oil-water interface is  
\[f=\frac{\sigma\cos(\theta+\beta)}{R} \tag{13}\]  
Under water-wet conditions, the direction of the capillary force is pointing to the concave side of the interface when the interface advances in the throat channel, which is consistent with the oil-water flow direction, and the capillary force is the driving force; while when the liquid flows out of the throat channel, and when \(\theta+\beta>90^{o}\), the interface will reverse. At this time, the capillary force obtained by Equation (13) is negative, which is opposite to the direction of water-oil displacement and hinders the interface advancement. As the interface moves forward, \(R\) will gradually increase, and the blocking effect of the capillary force on oil-water interface will gradually decrease. When the displacement force cannot overcome the capillary barriers, the interface movement stops, resulting in oil surplus. The capillary resistance caused by the change of geometry is called the capillary barrier in this work. The capillary barrier is caused by the geometric structure; even in the water-wet conditions, capillary force can still present resistance characteristics. However, how the capillary barrier influences the oil-water movement needs to be further investigated.