Page 341 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

908 Reservoir Engineering

cores in deoxygenated aqueous solution. This may be formation brine, synthetic
brine or mud filtrate [319,333].
Pressure Cores. Special handling is needed for cores obtained using the pressure
core barrel. This is normally carried out by the trained crew which assembled
the barrel prior to testing. After retrieving the core barrel, drilling fluid is
displaced at pressure by gelled kerosene. The complete barrel is then chilled in
dry ice for several hours in order to freeze the water in the core sample. The
pressure in the core barrel can then be released and an inner metal sleeve
containing the core is removed. The core is cut into convenient lengths, of about
three to four feet, and kept frozen by means of dry ice during transportation [319].

Measurement of Resldual Oil in Recovered Cores. Various techniques are
available for determining the oil content of cores. Examples that involve removal
of the oil are vacuum distillation, a combination of distillation and solvent
extraction (Dean Stark), and high temperature retorting. The Dean Stark method
with toluene as solvent is normally used when displacement tests are to
be carried out on the extracted cores. In this method., the oil saturation is
determined by difference from the amount of water removed from the core
[ 191,3?i3,334].
In general, cores obtained with the pressure core barrel under conditions of
minimal flushing are needed in order to obtain residual saturations that can
be treated with reasonable confidence. Special analytical methods have evolved
for treatment of pressure cores. The frozen cores are removed from the metal-
containing sleeve and dressed while still frozen. The pressure cores are then
allowed to thaw in an inert atmosphere, and volumes of evolved gases are
recorded. Next, the free water is distilled from the core. Any remaining oil is
removed by a tolueneC0, leaching. The amount of oil in the core is determined
by adding the volume obtained by distillation to the volume removed during
extraction and then making a correction for evolved gas. As a check on extent
of penetration of mud filtrate into the core, a tracer can be added to the drilliig
mud which permits the radial depth of invasion to be estimated. However,
filtrate invasion does not necessarily imply flushing of residual oil [S19].

Residual 011 from Laboratory Core Floods. Most cores are subjected to
cleaning before measurement of permeability and porosity. However, when the
preservation of wetting properties is of main interest, displacement tests are run
on the cores prior to cleaning.
Another approach to the problem of reservoir wettability is the restored state
method. Cleaned cores are saturated with reservoir brine or brine of similar
composition. The brine is displaced by reservoir crude to an equivalent connate
water saturation. Recontacting the reservoir rock with the reservoir crude is
believed to result in adsorption of those components from the crude oil which
determined the in-situ wettability and hence restore the system to its original
wetting condition.
Relatively little is known about the causes of reservoir wettability and its
sensitivity to the numerous variables that may cause the wettability of recovered
cores to be changed. It has been shown that wettability can have significant effect
on residual oil [121,1!25]. This is the main reason why values of residual oil
saturation determined by laboratory core flooding tests are treated with caution.
Residual saturations determined by laboratory flooding tests are often used in
estimating the amount of oil that will be recovered by waterflooding. However,
when residual oil saturation is to be determined for evaluation of a tertiary