Many experimental works have investigated smart water and low salinity water flooding and observed significant incremental oil recovery following changes in the injected brine composition. The commonway approach to model such EOR mechanisms is by shifting the input relative permeability curves, particularly including a reduction of the residual oil saturation. Cores that originally display oil-wetness may retain much oil at the outlet of the flooded core due to capillary pressure being zero at a high oil saturation. This end effect is difficult to overcome in high permeable cores at typical lab rates. Injecting a brine that changes the wetting state to less oil-wet conditions (represented by zero capillary pressure at a lower oil saturation) will lead to a release of oil previously trapped at the outlet. Although this is chemically induced incremental oil, it represents a reduction of remaining oil saturation, not necessarily of residual oil saturation.
This paper illustrates the mentioned issues of interpreting the difference in remaining and residual oil saturation during chemical EOR and hence the evaluation of potential smart water effects. We present a mathematical model representing core flooding accounting for wettability changes due to changes in the injected composition. For purpose of illustration, this is performed in terms of adsorption of a wettability alteration component coupled to shifting of relative permeability and capillary pressure curves. The model is parameterized in accordance with experimental data by matching brine-dependent saturation functions to experiments where wettability alteration takes place dynamically due to changing one chemical component. It is seen that several effects can give an apparent smart water effect without having any real reduction of the residual oil saturation, including 1) changes in the mobility ratio, where the oil already flowing is pushed more efficiently, and 2) the magnitude of capillary end effects can be reduced due to increased water-wetness or due to reduction in water relative permeability giving a greater viscous drag on the oil.