Feasibility of spontaneous combustion in ultra-deep high-pressure shale oil reservoirs during air injection

There is a lack of research on the spontaneous combustion phenomenon and its main influencing factors in ultra-deep high-pressure shale oil reservoirs with additional fracturing. In this study, we examine the exothermic characteristics of shale oil oxidation using high-pressure differential scanning calorimetry (HP-DSC) and accelerating rate calorimetry (ARC). Subsequently, a reaction kinetics model is built by integrating the HP-DSC and ARC data. Furthermore, the main factors affecting spontaneous combustion are identified by combining the simulation results with range and variance analyses. The HP-DSC and ARC results indicate a positive potential for achieving spontaneous combustion in shale oil. The developed reaction kinetics model successfully captures the exothermic characteristics of shale oil oxidation. The simulation results demonstrate that spontaneous combustion occurs approximately 5 m away from the injection well, with a delay time of 10.74 days. The critical factors determining the occurrence of spontaneous combustion are O₂ concentration and oil saturation.