Study on the Influence of Oxygen Concentration and Fracture Characteristics on In Situ Combustion Development of Low–Medium Maturity Shale Oil Reservoirs

In situ combustion (ISC) technology has been proven feasible in low-medium mature shale oil reservoirs. However, the impact of the oxygen concentration and fracture characteristics on development effectiveness is still unclear. Combustion tubes, gas chromatography (GC), and GC-mass spectrometry (GC-MS) are used to conduct ISC experiments on low-medium maturity shale oil. The impact of millimeter-scale fractures, oxygen concentration, and fracture characteristics on reservoir recovery is quantitatively analyzed using nuclear magnetic resonance (NMR) technology and a newly proposed shale oil recovery calculation method. Research has found that there is difficulty in ignition during ISC in millimeter-sized fractures. However, with the increase of the oxygen concentration and complexity of fractures, the ISC reaction becomes more complete, significantly improving the oil recovery of the reservoir. When the complexity of fractures and oxygen concentration reached 40%, the conversion of kerogen reached 90.6%, and the oil recovery of the reservoir increased to 60.6%. This indicated that high oxygen concentration can accelerate the oxidation reaction of organic matter and improve combustion stability, while fracture complexity increases the contact area between oxygen and organic matter, improves heating efficiency, and provides channels for the flow of modified oil. In addition, ISC can enhance the quality of shale oil and promote the flow and production of modified oil. The significance of this study lies in revealing the key influencing factors of oxygen concentration and fracture characteristics on ISC development of low-medium maturity shale oil reservoirs and proposing methods to improve the efficiency of successful ignition and combustion front advancement through chemical ignition agents and optimized fracture morphology. The research results provide a theoretical basis and practical guidance for optimizing the ISC process of low-medium maturity shale oil reservoirs.