Water imbibition into the reservoir matrix is an important recovery mechanism especially for fractured reservoirs. In this study, we numerically investigate surfactant effects on water imbibition in carbonate reservoirs that exhibit oil-wet to intermediate-wet states. The objective is to understand and quantify the impact of various factors including interfacial tension (IFT) and wettability on capillary and gravity forces, and hence the eventual oil recovery. In this work, an open boundary core-scale imbibition model with 9×9×10 gridblocks was used in UTCHEM to simulate carbonate core plugs exposure to a vast water body. Water imbibition tests conducted at elevated temperatures using Amott cells were used to benchmark the simulation model. Those imbibition tests investigated surfactant effects in both secondary and tertiary modes. Capillary and gravity forces were captured by history matching the experiments. Through this history matching, the inputs for surfactant adsorption and diffusion, capillary pressure and relative permeability were calibrated. The simulation model was then used to investigate the effects of various parameters including the volume of the surrounding water body and the properties of the surfactant, oil, water and rock. Both simulation and experimental results confirm the importance of water imbibition and its significant contribution to oil recovery. In the case of tertiary application, water imbibition occurs along with wettability alteration, where the effect of capillary forces became stronger with decreasing wetting phase saturation. The oil recovery increases with the increase in the volume of surrounding water and finally approaches a constant. Furthermore, changes in contact angle and IFT had a clear effect on the fluid displacement. At high IFTs, positive capillary forces drove the oil out in all directions; while at low IFTs, negative capillary forces led to a vertically dominant flow. This study presents a mechanistic approach to better understand the water imbibition process, and provide better understanding of the relative impact of the various key factors including oil, rock and, surfactant properties on oil recovery from the matrices of fractured systems.