Page 375 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2
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Enhanced Oil Recovery Methods   341






                                  I  I  ’   111’1  I  I  ‘  ‘
                  Ill  I                                 11111  ‘I  I   I
                           -NOT    CRITICAL  IF  UNIFORM -               HP  GAS


                                 HIGH  ENOUGH  FOR
                               GOOD  INJECTION  RATES                    co2

                                                                         POLY MER



                                                                         SUR FACTA NT

                                                                         ALKALINE



                                                                         IN-SITU


                                                                         STEAM

                         Figure 5-178.  Permeability ranges for  EOR methods [386].




                   For completeness, we  have  included  the  two  “last resort” methods (special
                 steamflooding techniques with shafts, fractures, drainholes, etc., and mining plus
                 extraction)  are listed in Figure 5-176. These methods are not included in Figures
                 5-177 and 5-1 78 because these unconventional techniques are not considered in
                 most reservoir studies.
                   Figure 5-177 shows that those enhanced recovery processes that work well with
                 light  oils  have  rather  specific  depth  requirements.  As  discussed,  each  gas
                 injection method has a minimum miscibility pressure for any given oil, and the
                 reservoir must be  deep enough to accommodate the required pressure.
                   Figure 5-178 shows that the three methods that rely on gas injection are the
                 only ones that are even technically feasible at extremely low permeabilities. The
                 three  methods that  use  backup waterflooding need  a permeability of  greater
                 than  10 md  in order to inject the chemicals or emulsions and to produce the
                 released oil from the rock. Although most authors show a minimum permeability
                 requirement of  20 md  for polymers, we  indicate a possible range down as low  as
                 3 md  for low molecular weight polymers, especially in some carbonate reservoirs.
                   The screening guides in the figures can perhaps be summarized by stating a fact
                 well-known  to  petroleum engineers: oil recovery is  easiest with  light oil in very
                 permeable reservoirs and at shallow or intermediate depths. Unfortunately, nature
                 has not been kind in the distribution of hydrocarbons, and it is necessary to select
                 the recovery method that best matches the oil and reservoir characteristics.
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