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In-situ upgrading (ISU) via downhole heating is an innovative and efficient technique for oil shale and ultra heavy oil recovery. However, low heat transfer in the ISU process can limit its application in large scale. Therefore, efficient air injection assisted ISU process (AAISU) was proposed. In this study, reservoir stimulation of the AAISU process was performed to investigate the effect of air injection on heat transfer, oil production performance and energy conversion efficiency, as compared to the original ISU process. The influences of important engineering aspects were investigated, including well pattern, well distance, heating temperature and air injection rate for the field application of the AAISU technique. The reservoir stimulation results indicate that heat transfer rate can be significantly accelerated by air injection, and the average reservoir temperature during the AAISU can be increased to 250 °C after one year, in comparison with that of 116 °C in the ISU process. The contribution of air injection on the total heat transfer can be approached to 60%. Air injection can also enhance oil recovery and improve the energy efficiency. Heating temperature and air injection rate also have significant impact on the performance of the AAISU process. For a 15 m × 15 m × 10 m geo-model, increase of the heating temperature from 250 °C to 350 °C and air injection rate from 500 m3/day to 1000 m3/day can result in improved energy efficiency. However, heating temperature above 350 °C and the air injection rate over 1000 m3/day can led to a decreased energy efficiency, due to higher energy consumption on electrical heating and high rate of air injection. • Air injection assisted in situ upgrading process (AAISU) of ultra heavy oils was proposed. • The effect of air injection on heat transfer was studied. • The influence of well pattern and well distance on the AAISU was investigated. • The influence of heating temperature and air injection rate on the AAISU process was evaluated. [ABSTRACT FROM AUTHOR]