Thermal Behavior and Kinetic Triplets of Heavy Crude Oil and Its SARA Fractions during Combustion by High-Pressure Differential Scanning Calorimetry

In situ combustion (ISC) has been regarded as an efficient technique for the exploitation of heavy oil reserves. In this work, the thermal behavior of one heavy crude oil and the fractions of its saturates, aromatics, resins, and asphaltenes (SARA) during combustion was thoroughly investigated using high-pressure differential scanning calorimetry. Two typical isoconversional methods were adopted to determine the variation of activation energy (E) and frequency factor (Ar) versus conversion degree in the course of the reaction, followed by the evaluation of the reaction model, f(α), via the master plot method. The results indicated that the heavy oil encountered larger thermal release caused by low-temperature oxidation (LTO) reactions rather than high-temperature oxidation (HTO) reactions, suggesting that appreciable heat could be available within the low-temperature range. Saturates showed a notably apparent heat release in the LTO reactions. For aromatics, the exothermic effect at the LTO stage was apparently higher than that at the HTO stage, contrary to the results detected at atmospheric pressure. Saturates and asphaltenes gave the highest cumulative heat release in the LTO and HTO regions, respectively. The variation of kinetic parameters (E and Ar) versus conversion degree during combustion was quite different for the heavy oil and its SARA fractions, implying their varying reaction mechanisms and pathways. Saturates exhibited the lowest average value of E at the LTO stage, whereas aromatics and resins gave the lowest average value of E at the HTO stage. The most probable f(α) of the LTO interval for the oil and its SARA fractions followed power law reaction models P–0.6, P–0.3, P0.1, P0.05, and P0.2. The appropriate f(α) for the HTO interval of the oil, saturates, and aromatics was the chemical process or mechanism non-invoking equations F2.1, F0.8, and F0.6, respectively. The Sestak–Berggren reaction model SB(0.5,0.9) and Avrami–Erofeev reaction model A2 were regarded as the rational f(α) for the HTO region of resins. These observations could provide some guidance with regard to the numerical modeling of SARA fractions to simulate the ISC process