Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 171

Provided the capture cross section of the fresh and brine flushes are known, all
the unknown quantities may be normalized out and the residual oil saturation found
using:

S S
fS S

-
-

log brine log fresh

S o =- 1

( )

brine fresh

Note that it is not necessary to know either ʣ ma or ʣ o . The technique has many variations,
some of which use specially chlorinated oil that has a high capture cross section.

Departure Curves

Ideally, pulsed neutron logs should be usable for quantitative interpretation without
having to make any corrections to the values read from the log. However, in some
cases (e.g., when a base log is run with a fresh completion fluid and a subsequent
log is run with a salty completion fluid in the borehole, or if the base log is run
without a liner and a subsequent log with a liner), corrections will be required to the
raw log measurement of sigma before quantitative interpretation can be made. The
required corrections are a function of three variables: casing size, hole size, and
salinity of the borehole fluid.
Many sets of departure curves are published by the service companies for their
specific tools as functions of open-hole size and casing size. Considerable contro-
versy exists in the literature regarding the need for departure curves. One school of
thought holds that the diffusion of neutrons from the borehole to the formation
necessitates the use of departure curves. Others maintain that proper tool design and
the associated gating systems used to calculate ʣ eliminate the need for corrections
since they are supposedly free of diffusion effects and the need for departure curves.
Some pulsed neutron tool design call for a “dual burst” of neutrons. The decay of
the neutron population in the borehole is monitored by a first burst and a second
burst is used to monitor the decay in the formation proper.
Essentially pulsed neutron tool design is a delicate balancing act. On the one
hand technological advances need to be incorporated in succeeding generations of
any given service company's tool. When better gamma ray detectors become avail-
able allowing for greater sensitivity, higher count rates, and lower “dead” times then
they are incorporated. When additional detectors, above and beyond the basic two
conventionally used, then the door is opened for more sophisticated data analysis
and better estimates of the true formation ʣ, free from the disturbing effect of the
borehole ʣ. Where the log user is monitoring changes in ʣ over time periods longer
than tool development cycles sometimes the tool design changes may complicate
legitimate log comparisons between today's version of what formation ʣ is and
what it was 10, 15, or 20 years ago as logged by an older version of the tool which
was less technically equipped to unravel the effects of neutron diffusion, etc. As a
result multi-detector tools are now emerging on the market (Zett et al. 2012a , b ;
Bertoli et al. 2013 ) as well as tools equipped with neutron detectors rather than
gamma ray detectors which aim to directly measure the rate of decay of a pulsed
package of fast neutrons (Arbuzov et al. 2012 ).