Experimental Investigation of Enhanced Oil Recovery Mechanisms of Air Injection under a Low-Temperature Oxidation Process: Thermal Effect and Residual Oil Recovery Efficiency

The air injection process (AIP) has been applied as an enhanced oil recovery (EOR) method in the light oil reservoir for decades. When high-temperature combustion reactions cannot be achieved, the low-temperature oxidation (LTO) reactions will dominate the AIP. The exothermic intensity of LTO reactions is much milder than that of high-temperature combustion reactions, which caused the thermal effect of the LTO reactions to be underestimated by researchers for a long time. Besides the thermal effect, questions such as whether LTO reactions could improve recovery efficiency and whether the LTO reactions could produce residual oil from the reservoir need to be answered. In this study, a series of isothermal core flooding tests under different environmental temperature conditions were applied to study the thermal effect on oil recovery. In addition, alternate injection of nitrogen and air was performed to study the LTO effect on oil recovery besides the thermal effect. The experimental results showed that the thermal effect can play a significant role on recovery performance because a higher temperature results in a higher oil recovery factor, where a temperature increment of 40 °C by the LTO reactions can result in a 10% recovery factor increase. On the other side, the LTO effect on producing residual oil was not observed in this study. Moreover, despite the thermal effect, the LTO-generated oxygenated compounds will increase the viscosity of the crude oil, which will decrease the recovery efficiency. Therefore, because the thermal effect of LTO works against the viscosity increment effect of LTO, the AIP is recommended only if the thermal effect is more significant compared to the increased viscosity effect in terms of recovery efficiency.