Pulsed oxygen carbon\oxygen logging , How it uses for saturation logging with case study by schlumberger on preudo bay Alska.

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PULSED NEUTRON C/O
LOGGING
ABDUL BASITH C
M TECH PE

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PULSED NEUTRON LOGGING
• Gamma ray spectrometry measures gamma rays (counts) in time
and gamma ray energies. Some elements emit gamma rays
naturally; others can be bombarded with neutrons to induce
gamma ray emissions.
• Each element produces characteristic gamma rays of specific
energies. Moreover, the number of characteristic gamma rays
produced is proportional to the abundance of the element.
• Naturally occurring and induced gamma rays may be counted
and sorted according to energy.
• This produces a gamma ray spectrum that can be processed, or
decoded, to identify the elements and their concentrations.
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• RST A is 4 times faster than GST.
• RST B, is the only tool that can log
in flowing well, Speed is same as
GST.
• In RST B , 2 detectors are set in an
offset to axis, its helps them to
increase the sensitivity of the tool.
• Near detector-Borehole
sensitivity.
• Far detector – Formation
sensitivity.

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 Role of RST Tool
• Reservoir management.
• Saturation measurement over time to understand depletion.
• Water influx and Injection water breakthrough.
Why RST
• Can use well with tubing.
• The tool is very slim.
• It can be used under operating conditions.
• It can work in more saline conditions.
• We use the resistivity tool in the newly drilled well to understand
saturation, When we use GST(gamma ray spectroscopy) or RST .
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• When salinity of formation is less than 35000 ppm, we can use
C/O.
• C/O logging helps to understand relative concentration of carbon
and oxygen by the gamma ray emitted by inelastic neutron
scattering.
• A high C/O ratio indicates oil-bearing formation, Low C/O ratio
indicates water bearing formation or gas.
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Mode of operation of
c/o(inelastic capture mode)
• Gate A-In elastic spectra during
the burst.
• B,C records capture spectra after
burst.
• Net elastic spectrum= Gate B-
Gate A
• Interpretation- by comparing
data base.
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• Middle east well
• Both near and far detectors shows a
sharp increase in at 848ft –An oil
water interface in bore hole.
• The bore hole oil holdup during
flowing indicates that most of the oil
produced from 728-750 ft.
• Water saturation curve separates
770-850ft indicates oil from well
bore invaded formation during shut
in
• When well started flowing formation
water flushed out the oil from
formation
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Prudhoe bay
• Reducing water cut. The left track shows the
gamma ray and borehole oil holdup logs
measured with the well producing, The
borehole oil holdup curve shows no oil
production below 1170 ft.
• The right track is an ELAN Elemental Log
Analysis output based on openhole and RST
fluid data. It indicates oil depletion below
1170 ft. After the lower perforated interval
was plugged, production increased from 220
BOPD oil with 94% water cut to 300 BOPD oil
with negligible water cut.
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• One potential bypassed zone, A, was identified from X280 to
X291 ft. A second zone B, based on he open hole logs and a C/0
log from another service company, was proposed from X220 to
X231ft. The RST log shows Zone B to contain more gas and water
than Zone A.
• After assessing the open hole logs and the three C/0 logs, ARCO
decided to perforate Zone B. The initial production was 1000
BOPD with a 75% water cut. Production declined to 200 BOPD
with more than 95% water cut in a matter of weeks.
• The decline prompted ARCO to perforate Zone A, commingling
production from earlier perforations. Production increased to an
average of 600 BOPD and the water cut decreased to 90%.
• Subsequent production logs confirm that Zone A is producing oil
and gas and Zone B is producing all of the water with some oil.

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Q & A