Evolution of mass losses and evolved gases of crude oil and its SARA components during low-temperature oxidation by isothermal TG–FTIR analyses

A large amount of crude oils located in front of the combustion front experience low-temperature oxidation (LTO) reactions under relatively constant temperatures over a long period, as a result of the quite slow advancing speed of combustion front during an in situ combustion process. However, the isothermal LTO characteristics of crude oils are still less well understood. In this work, the evolution of mass losses and evolved gases of one crude oil and its saturates–aromatics–resins–asphaltenes (SARA) components during LTO under isothermal conditions was investigated using thermogravimetry connected with Fourier transform infrared spectroscopy. The results suggested that the mass loss at the LTO region was mostly caused by the evaporation of hydrocarbons. Almost no CO2 was emitted from 50 to 350 °C for saturates, aromatics, and resins, whereas the absorbance of CO2 was observed at 300 °C for asphaltenes. During LTO of the oil and its SARA components between 150 and 350 °C, the signal of compounds with C=O group was notably higher than that of compounds with C–O group. Additionally, the effect of the interactions between SARA components on the evolved gases and LTO reaction rate was analyzed. The interactions between SARA components promoted the formation of products with carbonyl group at 100 and 300 °C but inhibited the formation of these products at 200 °C. A better understanding to the gas products emitted by heavy oil LTO could be achieved based on this investigation on the SARA components.