Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 173

Fig. 11.33 Generic neutron
logging tool. Courtesy Baker
Hughes
In the figure note that fast neutrons are shown to be inelastically scattered by the
nuclei of four elements, Mg, Al, C, and Si. The neutrons thus scattered then are seen
to travel outwards to the second annular zone, further from the tool, where, once
they are slowed down to thermal energies become captured respectively by atoms of
Ca, Si, Fe, and S. The detectors in the tool gather the incoming gamma ray and
perform spectroscopic analysis in order to “finger print” the elements present.
Carbon/Oxygen Logging
Initially inelastic neutron scattering was widely used to determine the ratio of
carbon to oxygen in the formation surrounding a cased borehole. The underlying
principle of the method was the assumption that carbon atoms were to be found in
hydrocarbon molecules (e.g., gas and/or oil, C n H 2 n +2 ) and oxygen atoms found in
water (H 2 O). Thus, depending on the porosity, the C/O ratio would be an indicator
of formation water saturation, S w .
Figure 11.34 shows a typical “fan chart” relating the measured C/O ratio to S w.
Note that there are two “fans” with one labeled “Sandstone” and the other
“Limestone”. The reason for this is the ambiguity of any given value for the C/O
ratio. If the formation matrix is free of any carbon then, for example, a C/O ratio of
0.14 coupled with a formation porosity of 27.5 % would indicate a water saturation
of zero. However the same log reading and porosity would indicate 100 % water if
the matrix were limestone. This characteristic of C/O logs need not be fatal provided
the logging is performed where the matrix elemental composition is known and/or
the device is used to solely to monitor oil/water or gas/oil contact changes over time
by performing repeat logs over the productive life of the reservoir.