Comparative evaluation on the thermal behaviors and kinetics of combustion of heavy crude oil and its SARA fractions

In situ combustion (ISC) plays a significant role in the exploitation of heavy crude oil. In this study, the thermal behaviors and kinetics of combustion of heavy crude oil and its saturates-aromatics-reins-asphaltenes (SARA) fractions were comparatively evaluated using thermogravimetry (TG) and differential scanning calorimeter (DSC) techniques. The results indicated that saturates and aromatics were vulnerable to severe mass loss at the low-temperature oxidation (LTO) stage; however, resins and asphaltenes encountered appreciable mass loss in the fuel deposition (FD) interval to form a great amount of coke, which consequently contributed to high-temperature oxidation (HTO) reactions. Saturates made an inconsequential thermal release to the HTO reactions. Aromatics showed apparent exothermic effect both at the LTO and HTO stages, which was consistent with crude oil. Compared with saturates and aromatics, resins and asphaltenes gave considerably higher heat release in the HTO region, especially asphaltenes. By comparing the DSC curve of heavy crude oil with fitting curve of SARA superposition based on their proportion in crude oil, apparent inhibiting (promoting) effect among SARA fractions at the LTO stage (FD and HTO stages) were detected in terms of heat release, which should be of much significance for using SARA fractions to model ISC process. The kinetic parameters determined by Ozawa-Flynn-Wall (OFW) method were almost identical to those obtained by distributed activation energy model (DAEM). The activation energies of crude oil fluctuated and varied in the range of 45-95 kJ/mol during combustion, disclosing its intricate combustion processes and multiple reaction mechanisms. Unlike other three fractions, the FD and HTO activation energies of asphaltenes descended consecutively with conversion degree, which is favorable for boosting the sustainability of combustion front.