3.2.2. Air Injection Speed

Based on previous analysis, here we set the huff-and-puff round as five in this section of study. We chose different air injection speeds for comparison, which were 1000 m<sup>3</sup>/d, 2000 m<sup>3</sup>/d, 3000 m<sup>3</sup>/d, and 5000 m<sup>3</sup>/d, respectively. Comparison of production indicators (recovery degree, cumulative oil production, gas-oil ratio, and gas production rate) under different air injection speeds are shown in Figure 4.

The dynamic production index under different air injection speeds have been listed in Table 5. As can be found from Table 5, at the end of the huff and puff, the recovery degree was approximately the same. At the end of the fire-flooding, the recovery degree and cumulative oil production had no significant difference for the four air injection conditions. When the daily air injection volume was 3000 m3, the recovery degree was the best (40.33%). The cumulative oil production was also the highest (4422.87 m3). The higher the air injection speed, the larger the air injection amount, the shorter the gas breakthrough time, and the higher the gas-oil ratio was.

**Figure 4.** Comparison of production indicators under different air injection speeds. (**a**) recovery degree; (**b**) cumulative oil production; (**c**) gas-oil ratio; and (**d**) gas production rate.


**Table 5.** Dynamic production index under different air injection speeds.

The characteristic parameters of oil-wall under different air injection speeds have been listed in Table 6. As can be found from Table 6, when the air injection speed increased, the formation time of oil-wall was shortened and the oil saturation was easy to reach the peak. When the air injection speed was 3000 m<sup>3</sup>/day, the average oil saturation value was the highest, and the peak value was higher than other situations. The air injection speed had almost no effect on the migration length of the oil-wall. However, the migration speed of the oil-wall was proportional to the air injection speed.

The characteristic parameters of fire wall under different air injection speeds have been list in Table 7. As can be found from Table 7, the daily air injection has no significant effect on the temperature of the fire wall. The temperature of the combustion front for all situations is about 700 ◦C.


**Table 6.** Characteristic parameters of oil-wall with different air injection speed.

**Table 7.** Characteristic parameters of different air injection velocity fire wall.


The temperature change of the firing front and oil saturation change under different air injection speeds have been shown in Figure 5a,b respectively. As can be found from previous figures and tables, we can conclude that the air injection speed had little effect on the fire wall. However, it had significant effect on the oil-wall. The daily injection volume of air was 3000 m3, which was conducive to the in-situ combustion and formation of oil-wall.

**Figure 5.** The temperature change of the firing front and oil saturation change under different air injection speeds. (**a**) temperature change of the firing front; (**b**) oil saturation change.
