Research on Enhanced Gas Recovery by CO2/N2 Injection in Tight Sandstone Gas Reservoirs

CO2-enhanced gas recovery (CO2-EGR) is a crucial technology for achieving both natural gas production increase and CO2 geological storage. While pure CO2 flooding demonstrates favorable recovery performance, the technical challenges and high costs associated with purifying CO2 remain significant. CO2 purification from exhaust gas incurs prohibitive costs, while direct injection of an unpurified CO2–N2 mixture can greatly cut engineering expenditure. Nitrogen also provides synergistic pressure support, working with CO2 to drive natural gas displacement. Therefore, from an economic and practical standpoint, employing impure CO2 mixtures (e.g., CO2–N2) for flooding presents a more advantageous approach. To clarify the factors influencing the recovery enhancement in tight sandstone gas reservoirs using CO2–N2 mixtures, long-core flooding experiments were conducted at 100 °C. This study systematically investigates the impact patterns of three key factors—injection timing, injection rate, and injection gas composition—on the enhanced recovery of tight sandstone gas reservoirs. The experimental results indicate that: (1) Advancing the injection timing significantly improves the recovery performance for both CO2 and N2 flooding. However, the cumulative recovery factor (sum of the depletion recovery and the incremental recovery from gas injection) shows a declining trend. (2) The enhanced recovery effect exhibits a trend of first increasing and then decreasing with the increase in injection rate. When the injection rate exceeds 0.05 mL/min, it tends to cause premature breakthrough of the injected gas, thereby reducing the displacement efficiency. (3) As the proportion of CO2 in the injected gas increases, the enhanced recovery effect shows a nonlinear rise. The highest incremental recovery (17.02%) was achieved with pure CO2 flooding, while pure N2 flooding yielded the lowest result (14.64%). The research findings, from a macroscopic perspective, elucidate the influence patterns of three distinct factors on enhancing gas recovery in tight sandstone reservoirs, thereby providing theoretical foundation and scientific guidance for the development of such reservoirs. In summary, the injection timing, injection rate and CO2 proportion in injected gas are the key controlling factors for gas flooding enhanced recovery in tight sandstone reservoirs. This study clarifies the macroscopic influence law of each factor, and the optimized development parameters proposed can provide direct theoretical support and technical guidance for the on-site application of gas flooding in tight sandstone reservoirs.