Engineered Waterflooding has gained increased attention in the past few decades as an emerging enhanced oil recovery (EOR) method. Besides the well-known waterflooding high displacement efficiency and reliable injectivity into hydrocarbon-bearing formations, the technique is very attractive because of its relatively low cost. The method consists of controlling the injected brine chemical composition aiming improved oil recovery. Laboratory and field studies indicated contradictory results, showing significant, minor, or no benefit on oil production even for similar experimental conditions. Although it is a consensus that wettability alteration is the leading cause for the improved recovery, modeling its underlying mechanisms is fundamental for successful project design and deployment. The dissertation presented here provides a comprehensive investigation of engineered waterflooding as an improved oil recovery method. It is hypothesized that changes in wettability are based on rock-fluid interactions through geochemical reactions and interfacial phenomena. Thus, a multi-mechanistic model that combines surface complexation and disjoining pressure calculations to predict contact angle and zeta-potential measurements is improved and validated. A method for determining surface complexation equilibrium constants is presented to honor several zeta-potential measurements for different ion concentrations (Na⁺, Ca²⁺, Mg²⁺, SO₄²⁻ and H⁺). In addition, a new data-driven wettability correlation is proposed to calculate contact angle based on surface complexation modeling and compared with the disjoining pressure approach. After the fundamental investigation of wettability in pore-scale, the modeling is coupled with UTCOMP-IPhreeqc, a reservoir simulator developed at The University of Texas at Austin, to study wettability alteration in core and field scale. Reactive-transport simulations are performed to history-match capillary pressure and relative permeability curves from spontaneous imbibition and coreflood experiments under controlled conditions and different lithologies (sandstone, carbonate, and shale). Geochemical calculations are conducted to study simultaneous wettability alteration mechanisms (e.g., electric double-layer expansion, multi-ion exchange, local pH increase). In addition, several physical phenomena related to changes in injected brine composition are modeled, such as clay swelling and scale deposition. Wettability and geochemical modeling are applied to investigate the synergy between engineered waterflooding and other EOR methods during reactive-transport simulations. An adsorption model for cationic surfactant is proposed based on surface complexation reactions and combined with contact angle calculations. The method is employed to match laboratory data and explore the combined effect of brine dilution and surfactant in field scale. In another study, the dissolution phenomenon, its impact on petrophysical properties, and well injectivity is investigated during water-alternating-CO₂ gas. A predictive model is proposed and validated against published coreflood data. Besides the effect of brine composition, other reservoir conditions are investigated during the water-alternating-CO₂ flood in field scale, such as mineralogy type, heterogeneity, and gravity segregation. Reactive-transport modeling has been improved significantly with recent advances in geochemical and petrophysical characterization. However, detailed simulations are usually associated with an increase in computational cost. A method is proposed to decrease the computational time during reactive-transport simulations in porous media. It is hypothesized that, if geochemical calculations can be ignored for regions under equilibrium conditions (far from the saturation front), then the computational performance would be improved without compromising simulation results. The validity of the approach is tested for three simulation cases considering different complexities (e.g., heterogeneity, wettability alteration, CO₂ solubilization in water, and dissolution)