EPR as a complementary tool for the analysis of low-temperature oxidation reactions of crude oils

Air injection is a promising method for enhanced oil recovery (EOR) in both conventional and unconventional oil sources. It is widely accepted that oxidation reactions between oil and injected air determine the success of an air injection process. In this study, electron paramagnetic resonance (EPR) was introduced as a new route to investigate the occurrence of low-temperature oxidation (LTO) and its behavior by monitoring the signals of free radicals. The EPR experiments were conducted for the different crude oil samples (light, medium and heavy) heated in both static air and flow air (air-bubbling) conditions under different temperatures from 25 degrees C to 180 degrees C. The results showed that the free-radical concentrations exhibited a good correspondence on the heating temperature. Furthermore, nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) experiments were carried out to help to analyze the oxidation process and verify the EPR results. It turned out that the EPR results can be well supported by NMR, FTIR and DSC data, which indicates that proposed EPR monitoring method can be applied as a fast and low-cost technique to investigate LTO under mild reaction conditions. Simultaneously, the combination of EPR, NMR, FTIR and DSC can help to better understand the LTO mechanism and to monitor the application of in-situ combustion technique in the field.