Investigation of displacement efficiency of different gas flooding in extra-low permeability reservoirs

Extra-low permeability reservoirs have become increasingly prominent targets for oil exploitation. Compared with water flooding, gas flooding demonstrates superior potential for enhancing recovery in such reservoirs. This study aims to identify a more effective development method for the high water-cut, extra-low permeability reservoirs in the Yan-9 Yumen Oilfield. Slim-tube experiments determined the minimum miscibility pressures (MMPs) of CO2, CH4, and oxygen-reduced air. Long-core flooding experiments evaluated the enhanced oil recovery (EOR) potential of different displacement schemes. The experimental results indicated: (1) The MMP is 15.5 MPa for CO2, 36.5 MPa for CH4, and oxygen-reduced air cannot be miscible. Consequently, under these reservoir conditions, CO2 flooding can achieve miscibility, while CH4 and oxygen-reduced air cannot; (2) Among 4 pure mediums injection schemes, CO2 flooding exhibited superior performance with a displacement efficiency of 85.08%, followed by water flooding (55.75%), CH4 flooding (47.23%), and oxygen-reduced air flooding (36.30%); (3) After injected fluid breakthrough, gas flooding can further improve the displacement of the remaining oil, while water flooding cannot; (4) Water flooding + CO2 flooding effectively displaced the remaining oil in the core, achieving the highest displacement efficiency of 86.61% among the 5 flooding schemes evaluated. In conclusion, CO2 represents the optimal gas injection medium for maximizing the short-term development efficiency of extra-low permeability reservoirs with a low injected volume. Furthermore, water flooding + CO2 flooding as the flooding combination represents the optimal flooding scheme for maximizing the production of extra-low permeability reservoirs.