*2.1.1 Mechanism*

The main effect of the polymer is the enhancement of the water-oil mobility ratio to be unity or less, the mobility ratio is defined as the ratio of the mobility of displacing phase to the mobility of displaced phase which is calculated from the following equation [6].

$$\mathcal{M}\_{w-o} = \frac{\mathcal{M}\_w}{\mathcal{M}\_o} = \frac{K\_w}{K\_o} \ast \frac{\mu\_o}{\mu\_w} = \frac{K\_{rw}}{K\_{ro}} \ast \frac{\mu\_o}{\mu\_{w^o}} \tag{1}$$

where

Mw−o:the water − oil mobility ratio Mw:the water mobility Mo:the oil mobility Kw:the effective permeability to water, mD Ko:the effective mobility to oil,mD μo:the oil viscosity, cP μw:the water viscosity, cP Krw:the relative permeability to water Kro:the relative permeability to water

As per this equation, it is clear that in order to drive the mobility ration to be unity or less, the water viscosity is increased by adding the water-soluble polymers to the injected water as shown in **Figure 2**, when the displacing fluid (water) viscosity is lower than the oil, the recovery efficiency decreases as the remaining oil after this flooding is about 45% of the OOIP at 0.1 viscosity ratio. On the other hand, once the viscosity ration reached to 1 (polymer added to water) the remaining oil after the flooding will be reduced to 20% of the OOIP. As summary, the highest viscosity ratio is the highest oil recovery [9].

## *2.1.2 Polymer flooding advantages*

The advantages of polymer flooding could be summarized as following [10]:


Polymer flooding limitations are:


A summary statistical data for field projects of polymer flooding as shown in **Table 2**.
