3.3.3. Permeability Ratio

Based on a previous study, here we considered the reservoir to be the positive stratigraphic layering case. The effect of the permeability ratio on the formation oil-wall and oil recovery was analyzed, considering the four different permeability ratios: 1, 5, 10, 100. Comparison of the production indicators (recovery degree, cumulative oil production, gas-oil ratio, and gas production rate) under different permeability ratio are shown in Figure 10.

The dynamic production indicators under different permeability ratios have been listed in Table 15. As can be found from Table 15, the greater the permeability ratio, the lower the recovery degree and the cumulative oil production, which means that the permeability difference was not conducive to in-situ combustion. The larger the permeability ratio was, the earlier the gas appeared in the production well. The gas-oil ratio also increased at the end of production.

**Figure 10.** Comparison of production indicators under different permeability ratio. (**a**) recovery degree; (**b**) cumulative oil production; (**c**) gas-oil ratio; and (**d**) gas production rate.


**Table 15.** Dynamic production indicators under different permeability ratio.

The characteristic parameters of the oil-wall with different permeability ratios have been listed in Table 16. As can be found from Table 16, the oil saturation peak, average oil saturation, and oil-wall migration length, oil-wall pressure gradient, oil-wall migration speed, and the oil-wall average temperature were the highest for the case with a permeability ratio equal to one. This means that the greater the permeability ratio (the difference in permeability), the more unstable the oil-wall was. When the permeability ratio was larger than 10, there was no oil-wall formed in the relatively low permeability layers. The oil layers were mainly formed in the middle and lower layers.

The characteristic parameters of the fire wall under different permeability ratios have been listed in Table 17. As can be found from Table 17, the greater the difference in permeability, the faster the rate of the fire wall propulsion. However, the breakthrough of the fire wall in one direction was not conducive to the formation of oil-walls and also damaged to the oil recovery.


**Table 16.** Characteristic parameters of oil-wall with different permeability ratio.

**Table 17.** Characteristic parameters of fire wall under different permeability ratio.


In summary, the permeability ratio had an important influence on the formation of the oil-wall. When the formation permeability was uniform, the condition for the oil-wall formation was the best.
