Abstract: A method and system enable injection of gas into an underground formation, enhanced by the simultaneous injection of liquid. Mixing of the gas and the liquid that have been separately injected into a well comprising two coaxial cylinders can occur at different levels in the well in order to facilitate the flow and to limit the gas pressure necessary for reinjection at the wellhead. Thus, the system and the method require less energy and the associated gas compression equipment is less expensive. The method and system can be applied to geothermal energy applications, underground gas storage, notably CO2, or to enhanced recovery in petroleum reservoirs.