Page 184 - Primer on Enhanced Oil Recovery
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174 Primer on Enhanced Oil Recovery
process is quite straightforward many clays start to swell. Swelling by itself can
make some pores impassable for liquids. Swelling in part also leads to delamina-
tion. Delaminated clay (so named fines from clay) becomes mobile and will block
more pores. Overall effect will lead to development of very low or even zero per-
meability. Oil containing formations with significant clay content might be
completely incompatible with low salinity water injection. Clay nature and clay
content should be carefully analyzed before additional water injection into the
reservoir.
At this point it is worth remembering that water interaction with the rock on sig-
nificant part (some say on three quarters) defined by the interaction with ions on
the surface (hydrogen bonds and dipole interactions with surface adsorbed ions).
Rock wettability is determined to the great extent by the rock surface ions and their
concentration. Low salinity water will change ion concentration and the surface ion
speciation. It has been shown by many studies that most likely microscopic out-
come of this change will be rock becoming more water wet. It is possibly immedi-
ately to guess that this leads to some oil detachment from the rock (rock now
preferably coated with water) and will lower oil saturation after the sweep. It is
reported that it is possibly to extract additional OOIP by the low salinity water
injection. The reported extraction increase varies between 5% and 20% of OOIP in
carbonate reservoirs.
The interest to low salinity water EOR for light to medium gravity oil is based
on generally good results at low investment levels. This is assuming that the surface
water is available at the required quantity and can be prepared for the injection. But
even sea water has lower salinity compared to connate water. Addition of some
chemicals to sea water makes the process even more efficient. The amount of
injected water should be roughly just above 100% of pore volume. In many cases
consecutive alteration during injection of sea and fresh water were reported to pro-
duce good outcomes.
14.2 Electrical thermal EOR
Electrical thermal Enhanced Oil Recovery is mostly based on supplying energy to
the formation with the main goal to rise the temperature and reduce oil viscosity. It
comes probably without saying than other than just pure thermal effect processes
do take place, but they are difficult to evaluate and they are at the very beginning
of the research activities.
The simplest electrical thermal EOR is direct reservoir heating with low fre-
quency (50 or 60 Hz, country electrical grid standard dependant) electrical current
when heat is produced by ubiquitous Ohmic heating. The process is very simple
and straightforward. Connate water with high salt content conducts electrical cur-
rent well and the water acts as a big heat emitting resistor. The only things needed
are two electrodes to establish the current flow and big transformer to match the
power. Two production wells can do the electrodes job. However current