Injection of water and gas are considered to be effective approaches in improving recovery from sandstone oil reservoirs. Different combinations of water and gas injection (such as continuous gas injection after water-flooding, simultaneous water and gas injection, and carbonated water injection) are used in oil fields to increase the recovery. The selection of the injection method and optimization of practical parameters, such as the water/gas ratio, improves the performance of this approach. Comprehensive experimental modeling techniques help specialists in the oil industry to design the most effective gas/water based enhanced oil recovery methods to produce more oil in a cheaper way. In the first stage of this research, different experiments (such as core flooding) were carried out to study the performance of CO2-water EOR methods. Other parameters, such as gas solubility in oil and brine, oil swelling factor, oil viscosity reduction, gas diffusion coefficient, and gas utilization factor were measured and analyzed to investigate the mechanisms. At the next stage, gas/water injection in porous media was modeled. The model was in good agreement with experimental data. The confirmed model was used to analyze the main parameters of fluid flow in porous media, such as relative permeability. From the model, the optimized gas-based EOR method was selected to achieve the highest oil recovery. In our study, oil recovery reached about 50% through water-flooding. Hence, a successful EOR approach is essential for a case with this amount of residual oil. Our experiments and models showed that simultaneous injection of water and gas with a water/gas ratio of 2.4:1 had the highest oil recovery factor (36%) in the studied sandstone formation, which makes the total recovery more than 85%. Compared to gas flooding, the gas/water injection schemes have a lower gas utilization factor, which reduces the operational cost in the fields. Our comprehensive simulation studies, based on different experiments at core scale, provide an accurate and reliable approach to selecting the best gas injection scenario in the porous media. This methodology helps the industry to optimize the process from a technical and an economic point of view.