Study on displacement mechanisms of auto-ignition after air injection in low-permeability reservoirs and its contribution to enhanced oil recovery

Air injection maintains pressure and enhances oil recovery in low-permeability reservoirs, yet the individual contributions of different recovery mechanisms after auto-ignition and their relationship with reservoir spatial evolution remain unclear. This study established and calibrated a high-temperature oxidation reaction model against combustion-tube data and incorporated the validated kinetics into a field-scale reservoir simulation. The target crude oil was represented by heavy- and light-component combustion, with activation energies of 68.6 and 65.8 kJ/mol, respectively. The simulated results were validated by comparing combustion-front temperature, oxygen consumption, carbon-oxide generation, and oil-recovery behavior with experimental observations. Gas-injection pressurization, gas dissolution and oil swelling, and thermal effects contributed 70.08%, 10.07%, and 19.85% of total oil recovery, respectively. Air injection achieved a 71.8% recovery factor over 30 years. Auto-ignition generated four characteristic zones: post-reaction, combustion, flue-gas, and thermally unaffected zones. The quantitative contribution of each mechanism was further linked to its spatial manifestation during combustion-front propagation. Pressure maintenance was dominant, while oxidation-generated gases and heat provided additional recovery. This study provides a theoretical basis for numerical-model selection and operational optimization of air injection in low-permeability reservoirs.