Page 77 - Primer on Enhanced Oil Recovery

68 Primer on Enhanced Oil Recovery

salt content and water viscosity to mention few. The steam has much smaller den-
sity than the formation fluids. There is always vertical gravitational drift of steam
and this effect needs to be taken into the account.
However, the main effects generally are the oil viscosity reduction and the oil
distillation. All other processes have rather limited effects and in many cases can be
considered as secondary at the planning stages.
The main goal during continues steam injection is to raise the temperature in the
formation, especially in the vicinity of production wells, reduce viscosity, increase
pressure and enhance oil flow. It is evident that all this leads to increase in oil
production.
Basic processes during cyclic steam injection (huff and puff) are the same as
during continuous steam injection. One needs to remember that in this case we use
the production wells for the steam injection, which should be able to withstand
injection pressure. The difference in the processes is then in the phenomena devel-
opment direction. Injection of the steam is followed by soaking. The soaking is an
essentially waiting time so that the heat can penetrate the formation area around the
well. During soaking the direction of heat and stem/water flow is opposite to the oil
production stage flow direction. During initial steam injection stem on the first
place penetrates into the big openings which have big penetrability. During soaking
period there is slow re-distribution of steam and hot water into smaller and smaller
capillaries. As the result small capillaries will release oil into the big opening with
high penetrability.
It is possible to say that during cyclic steam injection main effect in increase of
oil recovery is provided by the rise of penetrability of oil in the areas around the
well. Moreover, the well drainage zone is increasing, further growing oil recovery
from the formation.
Development in ability to produce horizontal wells had allowed in 1980 in
Canada to develop technique named Steam Assisted Gravity Drainage (SAGD).
Over time this method became the industry standard in production of very viscous
(bitumen type) oil. In implementation of this method two paired horizontal wells
are drilled. Steam injection well is located vertically above production well. Again,
injected steam heats heavy oil and reduces oil viscosity. Mobile oil under gravita-
tion force sinks into the production well and is pumped to the surface.

8.1.2 Application criteria
One of the potential major difficulties for steam injection method is access to high
quality water, e.g. water with low salinity (mineral content) and low organic impu-
rities content. The impurity content should be below 5 µg/l. Moreover, presence of
Mg and Ca cations should be particularly avoided. Otherwise production of high
value steam (steam quality above 80% and thermal capacity 5 MJ/kg) is impossible.
Due to this limitation, water needs to be chemically purified with expensive chemi-
cal reagents. Cost of the chemicals can be as high as one third of steam production
coast.