Page 334 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2
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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.