3.1.2. Original Oil Saturation

From the simulation results, when the original oil saturation of the local layer was less than 30%, the formation will not form oil-wall. At the end of production, the maximum temperature of the formation was less than 340 ◦C, and the highest temperature zone existed at the injection well end. In addition, there was no tendency to advance to the production well. Therefore, no fire wall was generated. It can be found that if the original oil saturation of the formation was too low, which was not conducive to the formation of the oil-walls. The formation of oil-walls was closely related to the geological conditions of the reservoirs.

## *3.2. E*ff*ect of Production Parameters*

In this section, we analyzed the e ffect of the production parameters (hu ff and pu ff rounds, air injection speed, and air injection temperature) on the formation of oil-wall and the e fficiency of in-situ combustion. Here, we chose di fferent hu ff and pu ff rounds (1, 2, 3, 4, 5, 6, 7 rounds), di fferent air injection speeds (1000 m<sup>3</sup>/day, 2000 m<sup>3</sup>/day, 3000 m<sup>3</sup>/day, 5000 m<sup>3</sup>/day), di fferent air injection temperatures (20 ◦C, 60 ◦C, 80 ◦C, 100 ◦C). Based on the simulation results, optimal production parameters were obtained.

#### 3.2.1. Hu ff and Pu ff Rounds

Here, we only changed the air hu ff and pu ff rounds and kept other the conditions the same to analyze the e ffect of the hu ff and pu ff rounds (one to seven rounds). The bottom pressure of the injection well was kept as 20 MPa; the steam temperature was 270 ◦C (543.15 K); the steam dryness was 0.70; the injection air temperature was 20 ◦C (298.15 K); the numerical simulation was carried out for 20 years. Comparison of the production indicators (recovery degree, cumulative oil production, gas-oil ratio, and gas production rate) under di fferent hu ff and pu ff rounds are shown in Figure 2.

Dynamic production indicators under di fferent hu ff and pu ff rounds have been listed in Table 2. It can be seen that the recovery degree and the cumulative oil production increased with the increase of the hu ff and pu ff rounds. However, the relative increase in the yield after five rounds of hu ff and pu ff was the most obvious. The more hu ff and pu ff rounds, the later the time the inject air burns the formation, and the longer the gas breakthrough the formation. In addition, it could be found that the gas-to-oil ratio was proportional to the hu ff and pu ff rounds. For one round hu ff and pu ff, the gas-oil-ratio still rose with time and was unstable. However, for other rounds, the ratio of gas to oil increased to the maximum and then gradually decreased and became stable.

The characteristic parameters of the oil-walls under di fferent hu ff and pu ff rounds have been listed in Table 3. As can be found from Table 3, when the number of hu ff and pu ff rounds increased, the later the oil-wall was generated. The initial formation position was closer to the production well. The time when the thickness of the oil-wall reached a maximum was also delayed. As the hu ff and pu ff rounds increased, the formation water saturation increased and the width of the oil-wall gradually narrowed and the average saturation of the oil-wall decreased. In addition, it can be found that the pressure gradient in the oil-wall was generally high. The pressure gradient increased with the increase of the hu ff and pu ff rounds. The migration speed of the oil-wall also accelerated. This was because the oil-wall saturation was reduced and the resistance was reduced, which was conducive to the migration of the oil-wall.

**Figure 2.** Comparison of the production indicators under different huff and puff rounds. (**a**) Recovery degree; (**b**) cumulative oil production; (**c**) gas-oil ratio; and (**d**) gas production rate.


**Table 2.** Dynamic production indicators under different huff and puff (HP) rounds.

**Table 3.** Characteristic parameters of oil-wall under different huff and puff rounds.


The characteristic parameters of the fire-wall under different huff and puff rounds have been listed in Table 4. As can be found from Table 4, after five rounds of huff and puff, the combustion front had the highest temperature, which was up to 710 ◦C. In addition, the combustion under this condition had the best effect. The position of the fire-wall under different rounds was almost the same. Therefore, the huff and puff rounds will affect the temperature of the fire-wall.


**Table 4.** Characteristic parameters of the fire-wall under different huff and puff rounds.

The temperature change of the firing front and oil saturation change under different huff and puff rounds have been shown in Figure 3a,b, respectively. As can be found from previous figures and tables, after five rounds of huff-and-puff, the increase of the recovery degree and cumulative oil production were the most obvious. In addition, the temperature of combustion front was the highest, and the gas appearing time in the production well was also the latest. Therefore, for this study, five rounds of huff-and-puff was more conducive to in-situ combustion and the formation of oil-walls.

**Figure 3.** The temperature change of the firing front and oil saturation change under different huff and puff rounds. (**a**) Temperature change of the firing front; (**b**) oil saturation change.
