Polymer flooding is a widely used chemical enhanced oil recovery technique in sandstones, but it has limited applications in carbonates. This is due to the harsh conditions of carbonates including high temperature, high salinity, low permeability, and high heterogeneity. Polymer flooding improves volumetric sweep efficiency through improving the adverse mobility ratio by increasing water viscosity. Therefore, the effect of trapping number on oil recovery is usually neglected. This paper numerically investigates the effect of trapping number on oil recovery by polymer flooding in carbonates under harsh conditions. The trapping number effect was predicted by running several 1D simulations using measured reservoir rock and fluid data. A previously history matched biopolymer rheological behavior was considered. This study aims to validate the flow behavior of this biopolymer through history matching recently conducted coreflooding experiments. Sensitivity studies were performed on different uncertain parameters to history match the oil recovery observed in the lab. These parameters include inaccessible pore volume (IPV), polymer adsorption, permeability reduction, shear rate coefficient, hardness of makeup water, and trapping number.
The results showed that IPV, polymer adsorption, permeability reduction, shear rate coefficient, and hardness of makeup water have negligible effect on oil recovery by biopolymer flooding. Also, history matching of oil recovery data was not possible when these parameters were varied within their typical range of values. Interestingly, when trapping number effect was applied through capillary desaturation curve (CDC), residual oil saturation was reduced to 10% as opposed to 20% without considering its effect. The pronounced effect of trapping number in this case is due to the increase in water viscosity post-polymer flooding compared to pre-polymer flooding, which is about two orders of magnitude. Therefore, the common practice of neglecting trapping number effect during polymer flooding might be misleading, particularly in carbonates that are characterized by a shallow CDC. Trapping number effect on oil recovery by polymer flooding depends on rock type and the targeted polymer viscosity, so the results might vary. The results indicate as well the effect of polymer flooding in improving both displacement and volumetric sweep efficiencies. The effect incorporated in this study is the elastic effect without considering polymer viscoelasticity, which might require further investigation. The study at the laboratory-scale is considered as a basis for field-scale predictions.