Page 351 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

318 Reservoir Engineering

coring, the time taken to obtain results tends to be longer than other methods
because of transportation and the specialized core analysis work that is needed.
After determining residual oil, the extracted cores are used to obtain needed
information on reservoir properties, in particular vertical heterogeneity, and
they also can be used in laboratory displacement tests. The reduced diameter
(-2 y2 in.) of pressure cores compared to those obtained by conventional coring
is a disadvantage with respect to making laboratory displacement studies. The
use of the sponge coring technique has increased because of the lower coring
costs, reduced analysis costs, and, since larger diameter cores are obtained, core
plugs can be obtained for subsequent conventional or specialized core testing.
The tracer test samples about a half million times more pore volume than
the pressure core barrel. Results with the tracer test will be conservatively low
because they are usually weighted towards the more permeable zones, where
residual oil will tend to be lower. On balance this is probably an advantage
because these zones will also tend to be swept more readily during a tertiary
process. The tracer test can be used in old wells, but it is important that the
well has not been fractured or stimulated severely.
Of the various log-inject-log procedures, the pulsed-neutronxapture method
is the most widely tested. It has the advantage that it can be used in cased holes.
Problems can arise with borehole rugosity effects and high values of residual
oil if displacements are incomplete during the log-inject-log procedure. Results
can be affected by flushing because of the small depth of investigation. The
method can give accurate results under favorable circumstances. Stabilization
of capture-cross-section values for the log-inject-log procedure can take unex-
pectedly long times and can present problems in interpretation. The nuclear
magnetic log has been rated highly as to accuracy, but cannot be used in cased
holes and it still has limited commercial availability. Log-inject-log resistivity
measurements can give good results under favorable circumstances but are not
applicable to cased holes.
Recommended Methods for Assessing Residual 011

In determining residual oil saturation, at least two reliable methods should
be compared. In most cases, a tracer test combined with injectivity profiles
should be run in all situations unless there are clear reasons, such as excessive
drift, why the tracer test would fail. The second method selected should provide
information on vertical distribution of residual oil. The situations of old and
new holes will be considered separately.
Existing Wells

Considerable cost can be saved if first measurements can be made on existing,
preferably watered-out, producing wells. These wells will usually be cased. Old
resistivity logs and core analysis data may be of value in estimating oil distribu-
tion and related heterogeneity. For more accurate determination of residual oil,
the tracer test should be run together with the pulsed neutron capture log.
However careful consideration must first be given to the past history of the well.
There is no reliable method for determining residual oil if the well has been
fractured or subjected to excessive stimulation. Inadvertent fracturing could have
occurred during stimulation. Various forms of acidizing can have serious effects
on near-borehole rock properties. The operator must also be sure that any
injected chemicals which could affect results are absent from the test region,
and that the well is clean and can be put on injection.