Chemical flooding is an important process for enhanced oil recovery. A substantialamount of remaining oil resides in reservoirs especially in carbonate oil reservoirs thathave low primary and waterflood oil recovery. Most of the chemical flooding studies to date have been performed in water-wet sandstone reservoirs. As a result, the effects of heterogeneity and wettability of carbonates on chemical flooding efficiency are fairlyunknown. The purpose of this simulation study was to determine the effects of wettability and wettability alteration on Polysorbate20 surfactant flooding in carbonate reservoirs. This study used the multi-phase, multi-component, chemical flooding simulator called(UTCHEM). The results showed that surfactant diffusion, critical micelle concentration, capillary desaturation and interfacial tension (IFT) reduction were the most important parameters affecting imbibition of surfactant and wettability alteration. However, these results were dependent on the transport processes, such as gravity and buoyancy, controlling the oil mobilization and wettability alteration. Sensitivity analyses of key parameters such as chemical slug size and concentrations, salinity, reservoir heterogeneity and surfactant adsorption were performed to optimize a surfactant designfor a mixed-wet dolomite reservoir. The field scale model was used but changes in relative permeability, waterflood residual oil saturation, capillary pressure, and capillary desaturation curves were made to reflect different wettability conditions. The study was then extended to simulating wettability alteration during the field scale surfactant floodusing the UTCHEM simulator. The results of modeling the wettability alteration showed that significant differences in injectivity and oil recovery are caused by the changes in themobility of the injected fluid. As the use of chemical flooding spreads to new reservoirs,especially oil-wet and mixed-wet reservoirs, the importance of surfactant-based wettability alteration will become important.
Key word: Polysorbate20, surfactant, simulation, wettability, enhanced oil recovery.
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