In an energy hungry world, fossil fuels are predicted to remain the dominant source of energy for a long time. Burning more fossil fuels will increase CO2 emissions in the atmosphere and will consequently increase the challenges of climate change mitigations. Carbon capture and storage (CCS) in deep saline aquifers is an important process for CO2 reduction on industrial scales but it is not economically attractive. Residual trapping of CO2 through capillary forces within the pore space of the reservoir is one of the most significant mechanisms for storage security and is also a factor determining the ultimate extent of CO2 migration within the reservoir. Carbon capture, utilization and storage (CCUS) in mature oil reservoirs can have a significant energy, economic and environmental benefits and is considered an important component in achieving the widespread commercial deployment of CCS technology. Residual trapping in mixed-wet systems, however, is assumed to be less efficient than in water-wet systems. In this study, we compare residual trapping efficiency in water-wet and mixed-wet carbonates systems on the same rock sample before and after wettability alteration by aging with crude oil. The observations were made at reservoir condition in a core-flooding system that included high precision pumps, temperature control, the ability to recirculate fluids for weeks at a time and an x-ray CT scanner for in situ saturation monitoring. The wetted parts of the flow-loop are made of anti-corrosive material that can handle co-circulation of CO2 and brine at reservoir conditions. We report the initial-residual CO2 saturation curve and the resulting parameterisation of hysteresis models for both water-wet and mixed-wet systems. A novel core-flooding approach was used, making use of the capillary end effect to create a large range in initial CO2 saturation in a single core-flood. Upon subsequent flooding with CO2-equilibriated brine, the observation of residual saturation corresponded to the wide range of initial saturations before flooding resulting in a rapid construction of the initial residual curve. Also, multiphase flow observations were made on a single carbonates core sample. It was made first on its original water-wet state, then were measured again after altering the wetting properties to a mixed-wet system. In particular, CO2 trapping was characterized before and after wetting alteration so that the impact of the wetting state of the rock is observed directly.