Thermo-oxidative characteristics, kinetic triplets, and auto-ignition potential of shale oil during air injection

The air injection technology is promising for shale oil production due primarily to its capacity to facilitate heat release through oxidation reactions between oil and oxygen. In this work, the thermo-oxidative characteristics of shale oil, which is heavy oil in terms of viscosity, were investigated by differential scanning calorimetry (DSC) and high-pressure DSC, followed by the determination of kinetic triplets by two typical iso-conversional methods and an integral master plots method. The auto-ignition potential of shale oil under varying oxygen concentrations was then evaluated using accelerated rate calorimetry. The results indicated that the thermal release caused by oxidation reactions was obviously increased while increasing from 0.1 to 5 MPa, particularly the lowtemperature oxidation (LTO) stage, which was conducive to establishing a fast combustion front. The pressure played a significant role in the kinetic parameters for shale oil oxidation. The activation energy at 0.1 MPa decreased with the increased conversion rate, whereas the opposite was true at 5 MPa. During LTO, the activation energy at 5 MPa was significantly lower than that at 0.1 MPa. However, during the high-temperature oxidation (HTO) stage, the activation energy at 5 MPa was considerably greater than that at 0.1 MPa. The most likely reaction mechanism function for shale oil oxidation at 0.1 MPa was D3 (three-dimensional diffusion, spherical symmetry), while that at 5 MPa was D4 (four-dimensional diffusion). The increase in oxygen partial pressure enhanced the mass transfer rate and accelerated the oxidation rate. Within the range of oxygen concentrations studied (16%-26%), the oxidation induction period was shortened, and the combustion intensity was increased with increasing oxygen concentration. If the oxygen partial pressure of the reaction zone during highpressure air injection (HPAI) was not lower than that in this experiment and the oxygen could fully contact the shale oil sample, the HPAI process also had great potential for auto-ignition of the targeted shale oil reservoir.