3.2.3. Air Injection Temperature

Here, we set the huff and puff rounds as five and the air injection speed was 3000 m<sup>3</sup>/day based on a previous study. We changed the air injection temperature to 20 ◦C (298.15 K), 60 ◦C (333.15 K), 80 ◦C (353.15 K), and 100 ◦C (373.15 K) for comparative analysis. Comparison of production indicators (recovery degree, cumulative oil production, gas-oil ratio, and gas production rate) under different air injection temperatures are shown in Figure 6.

**Figure 6.** Comparison of production indicators under different air injection temperatures. (**a**) Recovery degree; (**b**) cumulative oil production; (**c**) gas-oil ratio; and (**d**) gas production rate.

The dynamic production index under different air injection temperatures have been listed in Table 8. As can be found from Table 8, the air injection temperature had little effect on the recovery degree and the cumulative oil production. The gas appearance time in the production well and the gas-oil ratio were almost the same at different air injection temperatures.


**Table 8.** Dynamic production index under different air injection temperatures.

The characteristic parameters of the oil-wall under different air injection temperatures have been listed in Table 9. As it can be found from Table 9, the air injection temperature had no effect on the formation time of the oil-wall. The oil saturation at the initial moment of the oil-wall formation was almost the same. When the air injection temperature was 80 ◦C, the peak of oil saturation was the highest, and the pressure gradient of the corresponding oil-wall was the largest. The effect of the air injection temperature on the migration length and speed of oil-wall was also negligible.


**Table 9.** Characteristic parameters of oil-wall under di fferent air injection temperatures.

The characteristic parameters of the fire wall under di fferent air injection temperatures have been listed in Table 10. As can be found from Table 10, the temperature at the front of the combustion increased if the air injection temperature increased. When the temperature increase was small, the variation of the fire wall migration velocity was also small.

**Table 10.** Characteristic parameters of the fire wall under di fferent air injection temperatures.


The temperature change of the firing front and the oil saturation change under di fferent air injection temperatures have been shown in Figure 7a,b, respectively. As can be found from previous figures and tables, when the hu ff and pu ff rounds was five and the air injection speed was 3000 m<sup>3</sup>/day, the features of the oil-wall were obvious and the fire-flooding e fficiency was good. The temperature of the injected air had little e ffect on the in-situ combustion and oil recovery.

**Figure 7.** The temperature change of the firing front and oil saturation change under di fferent air injection temperatures. (**a**) Temperature change of the firing front; (**b**) oil saturation change.

#### *3.3. E*ff*ect of Geological Parameters on oil-walls*

## 3.3.1. Bottom Water Thickness

When the bottom water was present in the reservoir, di fferent bottom water thicknesses (0 m, 100 m, 200 m, 360 m) were simulated to analyze the influence of the bottom water on the characteristic parameters of the oil-wall. Comparison of the production indicators (recovery degree, cumulative oil

production, gas-oil ratio, and gas production rate) under different bottom water thickness are shown in Figure 8.

**Figure 8.** Comparison of the production indicators under different bottom water thickness. (**a**) recovery degree; (**b**) cumulative oil production; (**c**) gas-oil ratio; and (**d**) gas production rate.

The dynamic production indicators under different bottom water thicknesses have been listed in Table 11. As can be found from Table 11, with the increase of the thickness of the bottom water, the oil recovery degree and the accumulated oil production were increasing; the gas appearance time in production well was almost the same; and the peak of gas-oil ratio decreased.


**Table 11.** Dynamic production indicators under different bottom water thicknesses.

The characteristic parameters of oil-wall with different bottom water thickness have been listed in Table 12. As can be found from Table 12, when the thickness of the bottom water was 360 m, the characteristics of the oil-wall were the most obvious: The average oil saturation, oil saturation peak, oil-wall average width, oil-wall migration length, oil-wall pressure gradient, oil-wall migration speed, and oil-wall average temperature were the highest. This was because the bottom water could provide the driving force and assisted the migration of the oil-wall.


**Table 12.** Characteristic parameters of oil-wall under di fferent bottom water thickness.

The characteristic parameters of the fire wall under di fferent bottom water thickness fire walls have been listed in Table 13. As can be found from Table 13, when the thickness of the bottom water was 200 m, the temperature of the combustion front reached 672 ◦C, and the average pressure of the fire wall was the highest. Therefore, the fire wall advanced faster and promoted the formation of the oil-wall.

**Table 13.** Characteristic parameters of the fire wall under di fferent bottom water thickness.


In summary, the presence of the bottom water in the formation will increase the e fficiency of in-situ combustion and improve oil recovery. The greater the thickness of the bottom water, the stronger the force of water flooding is.
