Study on the fire front propagation characteristics during unstable combustion processes

Stability of the fire front is the key to the success of the field test, however, the mechanisms of combustion state and oil recovery variation are not clear and need to be further studied. The characteristics of fire front propagation under different ignition temperature, oil saturation and air flux are investigated by the combustion tube experimental device combining with the heavy oil sample obtained from the Xinjiang oilfield. The results show that a successful ignition process cannot be achieved with an ignition temperature of 400 °C, and a higher ignition temperature is prone to a stable fire front propagation with an initiation of high temperature oxidation (HTO) at earlier time. The fuel supply was insufficient, the amount of coke formed was limited, and the combustion reaction intensity was reduced when the oil saturation was excessively low, which could easily lead to fire front attenuation or even extinction. The HTO process will become unstable due to insufficient oxygen supply when a low air injection flow rate combined with high oil saturation is selected. The air flux is recognized as the most sensitive to the stability of the combustion state, and a greater air flux is prone to more stable fire front propagation combing with more intense HTO process over the range of 0.29–0.88 L/(min·cm2). However, an excessive higher air flux with the value 1.17 L/(min·cm2) would be prone to the air breakthrough, resulting in a significant reduction in the oil recovery. A judgment matrix for identifying unstable combustion process in combustion tube is constructed, and an unstable state can be inferred when any parameter exceeds its threshold in this matrix, The screening criteria proposed in this work in optimization the parameters of in-situ combustion (ISC) process can be treated as a reference.