Investigation into the key factors influencing the establishment and propagation of combustion front in ultra-deep high-temperature heavy oil reservoirs

The in-situ combustion technique is currently on the exploratory stage for ultra-deep high-temperature heavy oil reservoirs. In this work, the oxidation experiments of heavy oil were conducted using thermogravimetry, high-pressure differential scanning calorimetry, and combustion tube. Based on these experimental results, a reaction kinetics model was developed, which was then integrated into the Computer Modeling Group (CMG) STARS reservoir simulation software by adjusting stoichiometric numbers, kinetic parameters, and reaction enthalpy to match the experimental data obtained. Furthermore, a corrected reservoir simulation approach was employed, combining the variance analysis and a Pareto chart, to identify the key factors influencing the establishment and propagation of combustion front in the absence of electrical ignition. The results indicate that a reservoir temperature of 148 °C leads to the establishment of combustion front approximately 7 m away from the injection well, with a delay time of around 25.21 days. The Pareto chart reveals that reservoir temperature and water saturation are the primary controlling factors for combustion occurrence. Specifically, a reservoir temperature above 148 °C and water saturation below 60% increase the likelihood of combustion front formation. Among the examined factors, the reservoir temperature has the most significant effect on combustion front propagation. Once the reservoir temperature reaches 208 °C, a stable combustion front is formed, and the average temperature of combustion front remains around 400 °C throughout 8 years of model operation. When the water saturation was 50%, a consistently stable combustion front can be established within the first 6 years of model operation. However, beyond the 6th year, the average front temperature decreases below 400 °C. Therefore, it is recommended to introduce a certain amount of oxidation catalysts into the oil reservoirs after 6 years of air injection to enhance the combustion reactions. Increasing the air injection rate or pre-exponential factor does not have a significant effect on the average front temperature after 8 years of model operation, which remains below 350 °C. This study can provide significant references for future assessments and decision-making processes related to the application of ISC in ultra-deep high-temperature heavy oil reservoirs.