This study examines the effect of pore geometry on the viscous instability in surfactant-polymer flooding in an initially preferential water-wet porous medium filled with heavy oil and brine. Two approaches are used to characterize the effect of viscous instability. In both approaches, the dynamic mean pore-scale capillary number N-c1 is considered as the foremost criterion to characterize the effect of viscous instability. In the first approach, which is a pore network approach, the effect of viscous instability on N-c1 is neglected. In the second approach, the viscous instability effect is included by using the viscous instability theory to determine N-c1. The comparison between N-c1 from the port network approach and viscous instability theory is used to show the effect of viscous instability. The change in N-c1 was found to be an appropriate tool to characterize the effect of viscous instability in surfactant-polymer flooding.