The urgency in demand for global warming alleviation has necessitated more research into carbon-dioxide diffusion (CO2) both for sequestration and enhanced oil recovery. Captured atmospheric CO2 can be stored underground to leave a haven for the populace. CO2 diffusivity in oil-brine-rock systems at various pressures and temperatures and also solely in brine with different ions but at a fixed temperature and pressure are studied. Increases in pressure and temperature leads to increase in diffusion coefficient of CO2 in oil-brine-rock systems. On the other hand, in CO2-brine system, the variations of ions affect CO2 diffusion while Ca ion appears to result in the highest diffusivity of CO2 in brine thereby, suggesting saline aquifers with Ca ion as the best choice for CO2 sequestration. © 2020 Author(s).