A system (100) for injecting flue gas to a subterranean formation, wherein the system (100) is configured to receive an initial mixture of N2, CO2 and less than 2% other components and comprises a compressor (110) for obtaining and maintaining a predetermined downhole pressure. The system (100) has a control system (200) for maintaining the amount of CO2 in an injection mixture in the range 12-90% and can be configured for EOR.