Page 184 - Primer on Enhanced Oil Recovery
P. 184

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
   179   180   181   182   183   184   185   186   187   188   189