Geological storage of carbon dioxide in deep saline aquifers is one of the methods of reducing its concentration in the atmosphere. Deep saline aquifers are geological formations located at depths that range from 800 to 2500 metres. In Croatia, a research has recently been conducted based on capacity estimations. Therein created geological maps of specific storage capacities in the Western part of Sava depression are confined to the Poljana sandstones that were singled out as a potential storage unit. Since the porosity had been measured in only 20 wells, in this study, this parameter was extrapolated to 60 wells, using NCSS statistical software. After that, due to the only available data of the water composition from the observed area, well Žu-249 has been selected for further geochemical modelling of CO2(g) dissolution in brines. The modelling has been performed with the PHREEQC program implementing chemical equilibrium between formation water and dissolved CO2(g). The same model has been created for deep saline aquifer in Ketzin, Germany. The Ketzin models have been made with the available data from water analysis prior the injection, in 2008, and in 2014, six years after the injection had started. Finally, all the models were compared to obtain a general picture of the CO2 storage efficiency and safetey due to mineralogically different formations, different depths, temperatures, pressures and salinities as the key parameters. The most important findings are that it is relatively safe to store CO2(g) in this type of aquifers. CO2(g) changes pH of water and under the new circumstances certain minerals precipitate and CO2(g) is slowly being removed from the brine.