Page 88 - Primer on Enhanced Oil Recovery

Thermal EOR 79

Figure 8.12 Screening parameters for in situ combustion.
First and probably the most important criteria is the formation depth. While it is
widely accepted that the depth limit is at around 3500 m, the guidance depth is not
the real limit. Mostly the challenge is related to the ability to inject air at the forma-
tion pressure and ability of crude oil to produce enough heat to sustain burning
front propagation.
Historically it is accepted that the formation pressure should be below 140 atm.
In reality this is not the limit. The challenge is always to be able to maintain
injected air volume at the necessary pressure. There are application examples when
the pressure was well above 140 atm.
The formation permeability is the third criteria. This is where steam injection
differs from ISC. ISC can be used in the formations with lover permeability, as
compared for stem injection. Challenge is to provide sufficient amount of injected
air at the formation pressure. Amount of air is calculated on the basis of oil and for-
mation properties. The permeability should be just enough to sustain air delivery to
the sustained burning zone.
For the ISC application the formation can contain medium to heavy oil. The

method can be used for oils with API between 35 and 10 . As the rule, operating
in this range does not require additional fuel infection into the formation.
The existence of high API degree limit is only surprising at the first glance. In
reality, if the oil is too light, almost all oil evaporates and coke does not form.
Absence of coke then does not allow to sustain continuous burning. On the other