Page 342 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Estimation of Waterflood Residual Oil Saturation 309

recovery prospect, laboratory flooding tests do not seem to be in favor. In a
recent monograph on residual oil determination, results of such tests received
a rating of only poor to fair [319]. Nevertheless, there seems to be consensus
view in the industry that the problem of wettability as it relates to residual oil
has been satisfactorily resolved. However, little attention appears to have been
given to restoring cores to their in-situ residual oil saturation at wetting
conditions which are representative of the reservoir even though this may be
critical to proper laboratory testing of a tertiary process [335,336].
Even though laboratory flooding of reservoir cores may not be a generally
acceptable method of determining residual oil, it is considered vital that tertiary
processes be tested in the laboratory using these cores.

Tracer Tests for Determining Residual Oil

How Tracer Tests Work

The tracer test was conceived by applying principles of chromatographic
separation to f hid movement in the reservoir. The outstanding advantage of
the tracer test is its ability to investigate a relatively large volume of the
formation. It was first suggested that the method could be applied to flow
between two wells [337]. The method depends on the effect which the relative
solubility of a tracer between oil and water has on the rate at which a pulse of
low concentration tracer passes through the formation.
The condition where the reservoir has been flooded out and the oil is
immobile is considered later, but the theory can also be applied where both oil
and water are flowing, or where the water phase is immobile. If a formation
containing residual oil is flooded with a bank of water containing a tracer which
is mutually soluble in oil and water, part of the tracer will pass into the oil
phase. If there is local equilibrium, the concentration of the tracer in the oil,
C,, is related to the concentration in the aqueous phase, C,, by the distribution
coefficient, K,.

Cm
K, = - (5-249)
c,

As a result of partitioning, part of the tracer temporarily resides in the immobile
residual oil, and the overall velocity of the tracer is less than that of the flowing
aqueous phase. The concentration of tracer in the oil together with the oil
volume determines the fraction of tracer resident in the oil phase. Since oil
volume is directly proportional to oil saturation, the rate at which a pulse of
tracer concentration passes through the formation depends on the oil saturation
in regions swept by the tracer.
Equilibration of the tracer between the residual oil and water following the
tracer bank coupled with dispersion effects determines the shape of a peaked
concentration distribution of tracer. Ahead of the peak there is net movement
of tracer into the residual oil. Behind the peak there is net movement of tracer
molecules from the residual oil back into the water.
From consideration of the way the tracer divides itself between the oil and
water phases and the effect of the magnitude of the residual oil saturation on
residence time of the tracer, the velocity of the tracer, vt, is related to the velocity
of the associated water, vw, by