Page 346 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Estimation of Waterflood Residual Oil Saturation 313

Relations other than Equation 5-253 are required for carbonates and shaly sands
[346,347].
Resistivity measurements cannot be made in cased holes. This may explain
the limited documentation of such methods for residual oil determination in
the literature.
Pulsed-Neutron-Capture. The device used for this measurement periodically
emits brief bursts of high energy neutrons. Between bursts these neutrons are
rapidly reduced in energy and then more slowly absorbed by formation nuclei.
It is the rate of this relatively slow absorption that is measured.
Upon neutron absorption most formation nuclei are left in high energy states
that decay to ground states through the emission of a characteristic set of gamma
rays. That is, the gamma rays emitted have various energies and numbers at
these energies that are unique to the capturing nucleus. However, carbon does
not participate in this process.
The gamma rays emitted above a fixed energy (the fixed energy used in
commercial tools varies from 50 kev to 200 kev) are detected and counted as a
function of time. The analysis of this counting rate yields the apparent thermal
decay time or equivalently the apparent capture cross-section of the formation.
The apparent cross-section can be corrected to true formation cross-section
through computation. Certain modes of operation [348] can yield results requir-
ing little or no correction.
The formation capture cross-section z, is related to the cross-sections of the
constituents of the fluid-saturated rock (see Equation 5-1 18) by the formula
[348,349]:
(5-254)

where E,, is the capture cross-section of the rock (including clay or shale), Q is
the porosity, Ew is the cross-section for the water, &, is the cross-section of the
hydrocarbons, and S, is the water saturation. Thus (1 - Sw)g is the oil volume
per unit volume of formation.
Of course, the desire is to determine the residual oil saturation (1 -Sw) or
the residual oil per unit volume, Q(1 - Sw). Because many of the quantities on
the right hand side of Equation 5-254 may be unknown, one employs the so-
called log-inject-log technique [350-3531.
If the porosity is known in addition to zw, then the simplest form of the log-
inject-log technique can be used. Here two successive logs are run. In the first,
the normal formation water of cross-section Ew occupies the pores. The section
of interest is then flushed (at low injection rate) with water of cross-section
EL as different from Ew as possible. Et and C: for these two conditions are
measured. Then from Equation 5-254 it follows that Sm = 1 - S, is

(5-255)

To obtain accurate values of Zt and XL one should use a very low logging speed,
or stationary measurements, or repeat passes that can be averaged.
If porosity is not known, the water flush can be followed by a flush with
chlorinated oil [352]. Here the chlorination is adjusted so that the oil has cross-
section E:. Then from the viewpoint of the PNC response, it is just as if we
had Sw = 100%. Then Equation 5-254 yields