Numerical Simulation and Parameter Optimization of Air-Foam Flooding in Ultra-Low-Permeability Reservoirs

Air-foam flooding combines gas-assisted displacement, foam mobility control, and surfactant-induced interfacial tension reduction in ultra-low-permeability reservoirs. A three-dimensional mechanistic model informed by a field-scale reservoir model was used to compare six displacement cases, reservoir-property responses, and injection schedules. The final recovery factors for baseline waterflooding, gas-assisted flooding, surfactant-background flooding, foam mobility control, interfacial tension reduction, and integrated air-foam flooding were 38.14%, 40.50%, 40.81%, 42.03%, 41.44%, and 47.79%, respectively. Relative to the surfactant-background case, activating foam mobility control alone increased recovery by 1.22%, whereas activating interfacial tension reduction alone increased it by 0.63%. Activating both increased recovery by 6.98%, including a 5.13% non-additive term that suggests a potential interaction between foam plugging and surfactant-enhanced oil displacement under the modeled conditions. Within the tested ranges, the normalized sensitivity weights for permeability contrast, the vertical-to-horizontal permeability ratio, and average permeability were 41.13%, 34.79%, and 24.08%, respectively. Within the parameter ranges investigated in this model, the single-factor results indicated favorable recovery performance at permeability contrasts not exceeding 10, vertical-to-horizontal permeability ratios of at least 0.4, and average permeabilities of at least 4 mD. A gas-to-liquid ratio of 1:1 produced the highest final recovery among the ratios tested. With the same prescribed total injection volume, shortening the alternating injection interval from 24 to 3 months increased final recovery from 47.60% to 48.40%. Switching at water cuts no higher than 85% avoided the lower recovery associated with the latest switching case. These findings provide a quantitative basis for reservoir screening and injection scheme design of air-foam flooding.