Page 348 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Estimation of Waterflood Residual Oil Saturation 815

containing paramagnetic ions so that FFI, is sensitive only to residual oil. The
result is that Sbr can be estimated from:
FFI'
s, =- (5-257)
FFI
This technique requires signal averaging over many repetitions of the basic
measurement in order to compensate for the very poor signal to noise ratio.
The method cannot be used in cased holes and requires a fairly large (>7 in.)
open hole. It has a very shallow penetration and highly viscous oils will not
contribute to FFI. In this latter case, a separate measure of porosity will be
needed. The service is still not widely available but in principle it is the most
promising of all logging methods for the determination of So= [361,362].
Dlelectric Constant. The dielectric constants of rock and oil are distinctly
different from that of water. The dielectric constant of bulk water is about 80
while those of oil and rock are 4 or less. In practice, however, due to polarization
effects on heterogeneous media, this difference in dielectric properties is masked
unless very high frequencies are employed in the measurement. One study shows
that results of the expected order are obtained with use of very high frequencies
and, furthermore, the measurements are unaffected by water salinity [363].
Several devices designed in accordance with these ideas have been designed and
successfully field tested [364-3741.
The use of this measurement for Sm determination could also benefit from
the log-inject-log procedure. For example, after logging initially, one could
displace the water with a fluid having about the same dielectric constant as oil.
A further flush with a fluid removing all oil and of known dielectric constant
could be made.
To interpret the measurements, one can use an equation similar to Equa-
tion 5-254:

At, = Atm(l - 6) + Atw@Sw + AtJ1 - $) S, (5-258)
where the Att refers to travel time of the electromagnetic energy between the
two receivers of the device. Att is the measured parameter while the other At
symbols refer to formation and fluid travel times. After the first flush, Sor is
found from:

(5-259)

After a second flush, @ could be determined from an equation similar to
Equation 5-256. Equation 5-259 can be used to obtain the amount of oil per
unit volume. If Sor is required, a porosity derived from a neutron-density log
could be used.

Volume of Reservoir Sampled

It is important to know how large a volume of the reservoir is investigated
by a given logging method. This is determined by the vertical resolution and
depth of investigation, which have been estimated as follows: