In a polymer flooding process, the volume of polymer adsorbed in porous media directly affects the rate of polymer propagation through a reservoir, oil recovery and total cost. Polymer retention is an important factor that should be estimated before field application. A review of the polymer retention in literature reveals that polymer adsorption is strongly dependent on surface wettability. The retention of polymers on oil-wet surfaces may differ significantly from their retention on water-wet surfaces. Moreover, the retention of polymers on oil-wet surfaces is minimized while the adsorption of cationic polymers is still significant on such a surface. Although many reservoir rocks are mixed-wet, usually one adsorption isotherm curve is used for both water-wet and oil-wet fraction of rock. In some polymers there is a noticeable relationship between wettability and polymer retention which may affect the calculations significantly. On the other hand, the residual water saturation of porous media directly affects the distribution of wettability in mixed wet rocks and consequently the polymer retention volume. In this study, a 3-D pore-scale network model has been developed for a typical mixed-wet sandstone rock. The porosity and permeability data reported in literature for Bentheimer sandstone and reservoir rocks are used for model verification. The developed pore network was used to simulate the polymer flooding at different parts of reservoir above the OWC. For the same polymer (cationic or anionic), different experimental adsorption isotherms has been used in water-wet and oil-wet portions of reservoir. The results of pore network simulation were used to calculate the polymer retention in simulated volume. The volume of adsorbed polymer calculated by this method has been compared with the estimation that neglects wettability variation. The results show that there is a big difference in retention estimation if the wettability change is not taken into account. By assuming constant wettability (oil-wet or water-wet) throughout the reservoir, polymer retention is dependent only on polymer concentration and the predicted polymer retention is underestimated or overestimated.


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