In the oil industry, gas injection refers to pumping different sorts of gasses to the subsurface oil reservoirs in order to stimulate crude oil. The composition of the gas may be different such as carbon dioxide, nitrogen and so on.
Gas reinjection is the reinjection of natural gas into an underground reservoir, typically one already containing both natural gas and crude oil, in order to increase the pressure within the reservoir and thus induce the flow of crude oil or else sequester gas that cannot be exported.