We studied the permeability of graphene sheets with designed nanopores using the classical molecular dynamics. To characterize the energy profile for transmission we calculated the potential of the mean force. A high selectivity for methane + butane mixture with the hydrogen-passivated pore diameter of 0.32 nm was found where the volume exclusion mechanism governs the separation process. In the case of a slightly larger pore diameter of 0.64 nm the same alkane mixture separates completely unexpectedly: a larger butane molecule permeates much faster than a small methane one. The blocking effect of the permeation path by a larger mixture component when it worked like a cork was also observed. This is a promising perspective for using graphene to design intelligent membranes which can maintain a constant composition of mixtures in the permeable area.