Chemical flooding is an important process for enhanced oil recovery. A substantial amount of remaining oil resides in U.S. reservoirs, especially in carbonate oil reservoirs that have low primary and waterflood oil recovery. In addition, many of these carbonate reservoirs are mixed-wet or oil-wet. Various enhanced oil recovery techniques have been tested on many different types of reservoirs. However, 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 fairly unknown. The purpose of this simulation study was to determine the effects of wettability and wettability alteration on surfactant/polymer flooding in carbonate reservoirs.This study used the multiphase, multicomponent, chemical flooding simulator called UTCHEM. Several modifications of UTCHEM have been performed over the past few decades. However, one modification has been developed recently that incorporates the effects of wettability alteration for surfactant-based processes. The first case study was to validate this modified UTCHEM code and develop an understanding for surfactant induced wettability alteration. Data from two surfactant spontaneous imbibition tests in carbonate cores were successfully matched with the modified UTCHEM. The results showed that surfactant diffusion, critical micelle concentration, capillary desaturation, and 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. The second simulation case study was a field-scale surfactant/polymer design optimization. The surfactant/polymer process is sensitive to parameters such as chemical slug size and concentrations, salinity, reservoir heterogeneity and surfactant adsorption among other key parameters. A sensitivity analysis of these key parameters was performed to optimize a surfactant/polymer design for a mixed-wet dolomite reservoir in the Permian Basin. The base case design was developed based on actual field data provided by the field operator. Surfactant and polymer properties were measured at the University of Texas at Austin. An optimum design was selected based on net present value calculated from discounted cash flow analysis. The results of this study showed that chemical flooding this mixed-wet dolomite reservoir is likely to be profitable. The last case study was to test the effects of wettability and wettability alteration by surfactant/polymer flooding. The same field scale model from the second study was used but changes in relative permeability, waterflood residual oil saturation, capillary pressure, and capillary desaturation curves were made to reflect different wettability conditions. Three simulations were run with water-wet, mixed-wet, and oil-wet properties to understand the differences in oil and surfactant breakthrough time, cumulative oil recovery, and economics. The study was then extended to simulating wettability alteration during the field scale surfactant/polymer flood using the modified UTCHEM simulator. The results of modeling the wettability alteration showed that significant differences in injectivity and oil recovery are caused by the changes in the mobility 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. This work focused on the effects of wettability alteration in an imbibition experiment and in a layered, non-fractured carbonate reservoir. There are also many oil-wet and mixed-wet naturally fractured reservoirs with significant amounts of remaining oil in place. Applying a similar simulation study to these naturally fractured reservoirs would be a good extension of this study.