Page 334 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2
P. 334

Estimation of  Waterflood Residual Oil Saturation   301


                       ESTIMATION OF WATERFLOOD  RESIDUAL OIL SATURATION
                    It  is  recognized  throughout  the  industry that  there  is  no  single generally
                  accepted method of measuring residual oil [319]. The available methods should
                  be considered on their merits and some combination of methods should always
                  be used as a cross-check. Each method provides somewhat different information
                  about the  amount and distribution of  residual oil. This section discusses the
                  various methods that are in current use, and recommendations are made as to
                  how  residual oil should be measured [320].
                                            Material Balance

                    Material balance was one of the first and is the most widely  used technique
                  employed in  estimating oil  reserves and  depletion.  Overall estimates of  the
                  amount of  in-place and recoverable oil are based mainly on material balance.
                  These  calculations are  also used  as  a  first  screening point  to  determine  if
                  sufficient oil remains  after waterflooding for  application of  tertiary recovery.
                  The quantity of oil remaining in the reservoir, having pore volume V,,  in stock
                  tank barrels, is given by  the difference between the initial oil-in-place,  N,  and
                  the amount of  oil produced, N,.  The overall residual saturation (Sor)MB is based
                  on the volume of  oil relative to the reservoir pore space [319].


                                                                               (5-239)


                  where Bor is  the oil formation volume factor after waterflooding.
                    Many  of  the  projects currently being evaluated as  tertiary prospects  were
                  initially developed twenty to thirty years ago or longer. At  that time, methods
                  for estimating hydrocarbon content were  not as  accurate as methods presently
                  available. In addition, adequate information relating to hydrocarbon volumes may
                  be limited to only a few wells  in a field and production data for the field may
                  not always be reliable. In any event, even where material balance might yield a
                  reasonable estimate of  the amount of residual oil-in-place, it does not indicate
                  the distribution of  oil within the reservoir.
                    The value of (SJMB will be dependent on both the microscopic displacement
                  efficiency in  swept zones and the vertical and horizontal sweep efficiency. It
                  has been  suggested that  a high ratio  of   to Sor, determined by  methods
                  applying to the swept zone, indicates that the reservoir still contains an unusually
                  high  amount of  oil in  the unswept region. The reservoir should then  receive
                  special consideration as  a candidate for obtaining additional oil recovery through
                  infill drilling [319].
                    In  application of  tertiary recovery, the  residual oil  remaining in  the  swept
                  zone  is  of  most  interest because  this is the  region  that  will  most  likely be
                  contacted  by  a  tertiary  process.  Thus,  the  material  balance,  even  in  more
                  sophisticated forms than the foregoing simple volumetric balance, should only
                  be  used  for  rough  screening  in  evaluating prospects  for  tertiary  recovery.
                  Furthermore, because of  the many  uncertainties in the measurements used to
                  obtain a material balance,  an  enhanced recovery prospect  should  not be  dis-
                  counted on the basis of material balance alone.
                   When  the volume of  oil produced is  known with  reasonable precision, the
                  accuracy of  this method depends mainly on the reliability of  the original oil-in-
                  place estimate.
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