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Polymer flooding has been traditionally applied to achieve favorable mobility ratios compared to water flooding, and thus increase formation sweep efficiency in oil displacement processes. Based on laboratory studies, several authors have reported lower residual oil saturations, Sorc, to the waterflood residual oil saturation, Sorw, when viscoelastic polymers were used during polymer flooding. The reported mechanisms contributing to the observed reduced residual oil saturation include mobility control, improved sweep efficiency, and improved microscopic displacement efficiency. The limitations of polymer flooding are obvious: high oil viscosity requires high concentration of polymer which increases the process cost, presence of clays which increases polymer adsorption, formation brine and water make-up salinity and type of ions which will affect the injected polymer viscosity, and extensively fractured formations which will cause sweep challenges. This review paper summarizes the most recent research findings, observations, conclusions, and reported physical mechanisms contributing to more efficient oil displacement processes and lower than Sorw residual oil saturation. The discussion focuses on viscoelastic polymer oil displacement processes and includes formation water and polymer solution salinity, resistance factor (RF), and polymer molecular weight effects. The use of nanoparticles (NPs) during polymer flooding is examined and results obtained are also reported in this review. More specifically, the polymer-NP system rheological behavior for several different types of NPs and surface modified silica NP are examined to find the most efficient system for oil displacement. Mechanisms contributing to more effective oil displacement processes when carefully screened NPs are added into the viscoelastic polymer solutions include interfacial tension reduction, formation wettability alteration, increased injected polymer solution viscosity, and prevention of asphaltene precipitation. |
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