The present paper focuses on analysing selective properties of a membrane made of graphene sheets in terms of separation of methane-helium mixtures. The regular allocation of pores allows considering a representative fragment of space near the investigated membrane. Sampling of molecules with respect to velocity values is performed on the basis of the Maxwell distribution which is valid at a distance from the membrane. The frequency of molecular strikes against the surface of the membrane is determined by the gas state parameters and the size of the representative fragment. The particles are started from the input face of the representative fragment with a frequency which is characteristic for methane molecules and their accumulation inside and near the membrane is investigated. The mobile particles which are present in the carbon structure increase the potential barrier of graphene sheets. This new barrier is used in the wave model of helium atoms passing through porous graphene sheets.