Page 89 - Primer on Enhanced Oil Recovery

80 Primer on Enhanced Oil Recovery

side, if oil is too heavy then during forward burning the coke can start blocking
steam dispersion. For this reason for heavy oils preferred extraction by application
of reverse combustion method.
As it is clear from the above, the bitumen content is very important. It is accepted
that the lover limit for the sulfur containing bitumen content is at around 20 wt%.
The oil deposit (oil bearing zone) thickness is very important. As the rule up to
third of thermal energy can be lost to the surrounding formation if the oil deposit
has thickness between 10 and 6 m. Below 6 m thickness heat loss can be too high
for sustainable burning.
The best formation stratification arrangement for ISC is combination of oil con-
taining stratas of clay layers. Clay strata is very effective thermal insulator, which
reduces significantly vertical thermal losses. The clay provides then for higher tem-
perature and higher sweep figures.
During ISC there is tendency of upward burning front movement. This is mostly
gravity meditated process. If the oil containing strata is too thick (above 20 m) then
sweep efficiency can be significantly reduced by this process.
In summary, before ISC application one needs to consider formation depth, for-
mation thickness, oil gravity and oil composition. In the ideal case scenario labora-
tory trials with the formation cores need to be conducted in order to define
necessary amounts of air and water, especially for wet burning.

8.2.3 The implementation technology
The process starts with heater insertion into a well as this is widely accepted step.
Propagation of heated zone is assisted by injection of air, which spreads heat into
the formation. Later some water is added to the injected air.
Burning ignites either spontaneously or by special devises lowered into the for-
mation. Spontaneous ignition is possible in the cases when oil has high oxidation
activity. As soon as oil starts to oxidase the reaction process releases thermal
energy which increases temperature and stimulates speed of oxidation process fur-
ther. At certain stage temperature rises to the level of burning.
Forced ignition is produced by special electrical or flame burners. In some cases
easily oxidizing liquids (linseed oil) are injected into the proximity of injected air.
In the case of forced ignition the burners are lowered into a well on the steal
cable, tope or tube. The implementation is shown on Fig. 8.13.
At the beginning of the process it is possible to increase temperature at the sur-

rounding well formation by approximately 260 in 24 hours.
The most common method of initiating combustion of reservoir oil is carried out
utilizing downhole electric heaters, as it is less complex compared to flame burners.
For lowering electric heaters into the well reinforced electrical cable is used.
Usually the electric power is supplied from the field network but it is possible to
use mobile electric generators.
Oxygen in form of air needs to be supplied at the required high pressure and vol-
ume. Specialized compressor equipment is needed and has been developed. Field
experience in the implementation of in situ combustion shows that to carry out the