Page 221 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

190 Reservoir Engineering

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from saturations determined from earlier runs. These values (ASw) show the
change in the position of the water table (hydrocarbon-water contact) versus time.
Another application is monitoring residual oil saturation in waterflood
projects. The procedure outlined by Bateman [217] involves first running a base
log (prior to injection). Next, salt water is injected and another log is run. Then
fresh water is injected and another log run. If Ebrine and Zhsh are known,
Bateman [217] suggests using:

(5-120)

to find residual oil saturation (So,) Additional details of estimating So= are
given later.
The main problem with using these logs is the presence of shale. Shale
normally appears wet, and shale will make a reservoir look like it has higher
Sw. Openhole logs and an NGS log are needed to confirm this interpretation
although shaly sand corrections can easily be made [217].

Gamma Spectroscopy Tools (GST). Also known as the carbon-oxygen log, this
device has recently been incorporated into pulsed neutron tools to aid in
differentiating oil and gas from water. GST tools operate with the same neutron
generator as the pulsed neutron devices, but gamma rays returning from the
formation are measured.
Two types of gamma rays are produced when neutrons are fired into
a formation:
1. Those that result from neutron capture by chlorine and boron.
2. Those that result from inelastic collisions with atoms.
The detector on this tool has energy windows set to receive certain returning
gamma rays [218]. The detectors are protected from the fast neutron source by
an iron shield, and from returning thermal neutrons by a boron shield.
The energy of the returning gamma rays depends on the atom involved in
the collisions. The atoms of interest include carbon, oxygen, silicon, and calcium.
Carbon-oxygen ratio is a carbon indicator and when combined with porosity,
gives an estimate of water saturation if matrix lithology is known. Figure 5-114
is used for this determination. Silicon-calcium ratio is an indicator of matrix
and is used to distinguish oil-bearing rock from calcareous rocks (such as limy sands
and limestones) [218,219]. Figure 5115 is an example of a carboncarygen log.
If capture gamma rays are also detected with separate energy windows,
chlorine and hydrogen content can be determined and related to formation water
salinity. Figures 5-116 and 5-117 are used for this purpose. All that is required
to estimate salinity of formation waters is knowledge of borehole fluid salinity,
Cl/H ratio, and response mode of the tool. These devices should not be confused
with the natural gamma spectroscopy log which only measures naturally-occurring
gamma rays.
The readings on the GST log are not affected by shale although carbonaceous
shales can cause trouble because of the sensitivity to .carbon. Usually, however,
these effects can be calibrated for or taken into account when this log is
interpreted. Much of the interpretation of this type of log is based in regional
experience; the analyst should have a good idea of the types of rocks present