Page 85 - Primer on Enhanced Oil Recovery

76 Primer on Enhanced Oil Recovery

Figure 8.9 Zoning for dry forward burning.

Zone 1 is fully burned volume. It is area between injection well and burning
zone. In an ideal situation it is the formation zone completely dry (oil free) rock.
Injected air is heated in this zone. The temperature rises as we proceed from injec-
tion well towards the burning zone.
Zone 2 is burning zine. The burning processes with consumption of oxygen, car-
bohydrates and coke. Temperature in this zone depends on the formation geology
and reactants concentration/composition.
Zone 3 is coke formation volume. This zone significantly alters hydrocarbon
content and composition. In this zone the lighter reservoir hydrocarbons either
evaporate (become gaseous) and all other mobile hydrocarbon species are pushed
towards production well. Unmovable hydrocarbons are pyrolysed and have very
low mobility.
Zone 4 is characterized by gradual temperature decrease by the distance from
Zone 3. There are no chemical alterations to the hydrocarbon composition. The
zone is filled by combustion products (oxides and water steam), gaseous hydrocar-
bons and high mobility liquid hydrocarbons. The following needs to be noticed:
1. On the border with zone 3 we have continuous evaporation of light oil fractions and par-
tial condensation of water steam
2. In the further removed zone we have continuous and, at the zone edge, full water vapor
condensation. Additional, compared to original connate, water creates so named wall-
water bank. This additional water effectively displaces mobile oil from the pores. In the
case of very immobile oil this water ingression can plug the formation.

8.2.1.1.1 Wet forward combustion
During dry forward temperature of the formation still rises in zone 1. This happens
due to low heat capacity of air (injected oxidants). At around 80% of generated
heat energy remains in zone 1, does not contribute to the oil extraction and is