Carbon dioxide geological storage involves injecting captured CO₂ streams into a suitable reservoir. Subsequent mineral trapping of the CO₂ as carbonate minerals is one of the most secure forms of trapping. Injection of CO₂ dissolved in water or co-injection of CO₂ with water may enhance trapping mechanisms. Produced waters are already re-injected into reservoirs worldwide, and their co-injection with CO₂ could enhance mineral trapping in low reactivity rock by providing a source of cations. Sandstone drill core from a reservoir proposed for CO₂ storage was experimentally reacted with supercritical CO₂ and a synthetic produced water. Micro computed tomography (CT), QEMSCAN, and SEM were performed before and after the reaction. The sandstone sample was predominantly quartz with minor illite/muscovite and kaolinite. The sandstone sub-plug micro-CT porosity was 11.1% and 11.4% after the reaction. Dissolved Ca, Mg, and Sr decreased during the reaction. After the reaction with CO₂ and synthetic produced water, precipitation of crystalline carbonate minerals calcite and dolomite was observed in the pore space and on the rock surface. In addition, the movement of pore filling and bridging clays, as well as grains was observed. Co-injection of CO₂ with produced waters into suitable reservoirs has the potential to encourage CO₂ mineral trapping.