Page 338 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Estimation of Waterflood Residual Oil Saturation 305

the laboratory, the amount of oil stripped from cores varied directly with the
overbalance pressure, filtration production rate, and core permeability; it varied
inversely with penetration rate and core diameter. The overbalance pressure is
usually the critical variable [195].

Overbalance Pressure. Unintentional displacement of residual oil may occur in
coring operations when large pressure gradients exist near the core bit. In this
region when fluid velocities are high, the resulting viscous forces may become
sufficient to overcome the capillary forces that hold the residual oil in place.
Results from an extensive laboratory coring program [195] showed a reduction
in pre-test residual oil saturation of almost 20 percent to about 60 percent as
the pressure gradients varied from about 350 to 1,700 psi/ft. In an evaluation
of the same data, other authors [123] contend that when analyzing larger
diameter cores (4-in. diameter) and considering radial flow, the estimated
penetration would result in only a 10% change in residual oil. In addition, they
contend that core samples used in retort analysis are usually taken from the
center of the core where mud solid penetration into the core would be minimal.
However, it is generally recognized that residual oil, which is immobile after
normal waterflood operations, can become mobile and be stripped from the
core, especially in the region adjacent to the core bit.
The ratio of viscous to capillary forces has been expressed as AP/La, where
AP/L is the pressure drop per unit length, and a is the interfacial tension. In
water-wet cores, at least, a critical value of Ap/Lo must be exceeded before
production of residual oil occurs [194]. In coring, the overbalance pressure must
be kept low in order to minimize flushing, Furthermore, the drilling mud should
not contain additives that cause significant reduction in interfacial tension that
could mobilize residual oil. The use of dispersants, emulsifiers, lubricants, lost
circulation materials, and oil should be avoided. If the overbalance pressure
causes the critical displacement ratio to be exceeded, then there will be some
displacement of residual oil. Linear displacement tests run in the laboratory show
the critical displacement pressure to vary from about 1 (psi/ft)/(dyne/cm) for
a 1,000 md sandstone to about 25 for a 100 md sandstone [194]. Thus, the
permissible overbalance pressure will have significant dependence on the
properties of the formation that is being cored (Figure 5-173).

Drilling Mud Properties. At bottomhole conditions, API filter loss for water-base
muds is often in the range of 5 to 10 cc for 30 minutes, which is sufficient to
drive most 3-in. to 4-in. diameter cores to the equivalent of the waterflood
residual oil saturation if the region being cored is not already at this condition
[123]. Higher mud water loss or smaller core diameters can lead to displacement
of some of this residual oil. However, only general agreement has been found
between API filter loss and the amount of oil stripped from cores. More
consistent agreement has been observed between the amount of mobilized
residual oil and spurt loss (the rapid fluid loss to the formation that occurs
before an effective mud filter cake has built up). Spurt loss has been shown to
correlate with solids content and particle size distribution which also influence
filtration rates and amount of oil-stripping [195]. In general, when taking cores
it is always preferable to use a bland water base drilling fluid which contains
no oil or surface active materials [319].
Shrlnkage and Bleedlng. In reservoirs which have been depleted to low pres-
sures and waterflooded to high water-oil ratios, changes in residual saturation
in bringing the core to the surface should be fairly minimized. However, in most