This thesis presents a pore-scale investigation into the mechanisms governing the interactions between water, oil and carbonate rocks using a series of core-flood experiments coupled with micro-CT imaging. Firstly, rock-fluid interactions leading to wettability alteration by seawater (injected in the secondary mode) and modified seawater (by addition or depletion of Ca2+ ion, injected in tertiary mode) are assessed using in-situ contact angle measurements. While the results have confirmed wettability alteration due to injection of the modified seawater (in tertiary mode), this change in wettability was found to be insufficient to fully explain the observed improvement of oil recovery. Secondly, a fluid-fluid interactions analysis methodology was developed and applied to the data generated as part of this thesis (Ca2+ enriched brine, Ca2+ depleted brine, SO42- enriched brine). The results show that emulsification is another mechanism responsible for enhancing oil recovery which is caused by reduction in interfacial tension. Data from multiple experiments (micro-CT imaging, stability analysis, surface tension measurements) also suggests that the amount of emulsion generated, and its stability depend on the type of ion added to seawater to modify the water chemistry. Specifically, Ca2+ depleted brine has shown to generate large amount of emulsion. As part of this thesis, a pore-scale direct flow simulation using the Volume-of-Fluid method was developed to model the mechanisms observed in the experimental studies, such as wettability alteration, viscous fingering and interfacial tension reduction. These mechanisms were grouped into two types, capillary-force affecting (contact angle and interfacial tension), and viscous-force affecting (viscous fingering reduction). When comparing the data between the two approaches (laboratory- and numerical experiments) the results show that capillary-force driven mechanisms were insufficient to predict the oil recovery due to development of flow stagnation zones in the sample. However, when the viscous forces were taken in account in estimation of sweep efficiency the displacement is significantly improved and the experimental data match well with that of numerical experiment. As a result, this thesis provides a first comprehensive pore-scale investigation of the physicochemical interactions between oil, water and carbonate rock related to the tertiary injections of modified seawater.