A viscous unstable displacement can lead to viscous fingering and to enhanced channeling in rock heterogeneities. The viscous stability of the CO2 -brine primary drainage process is therefore of major importance for CO2 sequestration in saline aquifers and determines the spread of the CO2 plume in the target aquifer and consequently the initial utilization of the pore space for storage. In previous papers [1,2] we presented some experimental results of CO2 -brine displacement in sandstone and a stability analysis of the experimental situation. In the present study we investigate the onset of viscous fingering in a parameter range relevant for sandstone aquifers, thereby studying under which conditions CO2 -brine displacement remains stable on the experimental and field scales. Our findings are not limited to CO2 -brine systems, but can be applied to most two-phase flow problems in reservoir engineering.