Page 347 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

314 Reservoir Engineering

(5-256)

where E’: is the measured value after the chlorinated oil flush.
The technique just outlined will suffer if Ew is small corresponding to low
chloride concentration (below 30,000 parts per million). If Xw is unknown, it
can be controlled by an initial water flush with water of known cross-section.
The most common practice has been to run a PNC log, inject fresh water,
and run a second PNC log. Excellent results have been reported for initial water
salinities in excess of 30,000 parts per million.

Carbon-Oxygen. The oil industry has long sought a logging method that directly
measures oil saturation. The carbon-oxygen (C/O) log is the most recent method
in this continuing effort. Since the method is sensitive to formation carbon and
since oil is largely carbon, the ideal result would yield directly formation oil
content [218,219,354-3571.
The C/O log utilizes apparatus similar to that of the PNC log; namely, a
pulsed neutron source and a gamma ray detector. The neutron bursts and the
detector device are timed to emphasize gamma rays produced by high-energy
neutrons scattering off of carbon and oxygen. The gamma rays are not simply
counted but are also analyzed for their energy. The gamma rays produced during
the neutron burst are primarily inelastic gamma rays, and it is in this time period
that carbon contributes gamma rays to the detector. When the neutron source
is off, capture gamma rays are detected and analyzed. Carbon does not con-
tribute to the capture-gamma-ray spectrum. The capture gamma-ray spectrum
is used to correct for interference in the carbon region of the inelastic spectrum
due to calcium, silicon, and oxygen [355].
The device has serious limitations. These include problems with counting
statistics, interfering gamma rays, carbonate rocks, and perturbations caused by
casing and borehole. Best results are obtained when the tool is stationary for
several minutes to overcome counting statistics, and when the formation porosity
and oil saturation are high. The measurement may be improved using log-flush-
log techniques, but only a few efforts have been documented at this time [344].

Nuclear Magnetlsm. This method has been thoroughly discussed in the litera-
[358-3601. The technique involves a polarization of the hydrogen magnetic
moments via a large coil (3 ft long and 3 ‘/s in. in diameter) carrying a large
direct current. The idea is to align the hydrogen magnetic moments along the
field created by the coil. This field ideally is at right angles to the earth’s
magnetic field. After a few seconds (up to about four), this coil-produced field
is reduced to zero as quickly as possible. The polarized hydrogen moment then
processes about the earth’s field at about 2,200 Hz. This induces a voltage into
the coil which is detected and processed to yield a measure of the total number
of hydrogen nuclei in the formation fluid. This number or concentration is
called the free fluid index (FFI). Since oil and water have about the same
concentration of hydrogen nuclei, FFI is a measure of porosity as given by the
fluid contributing to the signal.
For the detection of residual oil saturation, one relies on the fact that the
addition of paramagnetic ions to the formation water will cause the signal from
the water to be annulled. The measurement thus utilizes the log-flush technique.
A first measurement is made of FFI. The formation is then flushed with water