Graphene synthesis on copper foils is experimentally studied at 1000°C under various methane partial pressures. A decrease in the carbon supply permits the formation of larger graphene flakes with fewer layers and crystalline defects. An influence of the copper foil confinement on graphene nucleation and growth is shown. Then, methane pyrolysis is simulated using the Fluent Ansys® code. The model shows that, at 1000°C, CH4 is decomposed to form gaseous saturated and unsaturated species. This means that two types of unsaturated species can co-exist during graphene synthesis, the first one arising in the reactor volume from methane pyrolysis, and the second one only forming in the vicinity of copper, thanks to its catalytic activity.