Foam in porous media can plug some pore channels and reduce gas mobility. Direct observation of foam flow patterns in porous media is difficult and therefore research on relationships between gas mobility and bubble size is scarce. The influence of bubble size on fluid resistance in porous media is simulated using the Navier-Stokes equation combined with the conservative level set method. The computational results reveal that the effect of bubble size on flow resistance is pronounced. The pore wall cannot constrain bubble deformation when size of the bubble is smaller than that of the pore throat. In that case

the flow resistance is steady in the flow and is the same as resistance of the single-phase liquid flow. Therefore

small bubbles cannot plug pore channels. The pore channel constrains bubble deformation when size of the bubble is larger than that of the pore throat; and the flow resistance is fluctuant in the flow. The maximum resistance increases linearly with the increase of bubble volume. When the flow resistance reaches the maximum

it decreases linearly with the increase of bubble volume. Alternation of the increase and decrease of the maximum resistance is periodic; and the period is unit pore-body volume.