Post on 02-Jun-2018
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Modern logs have more
measurements but the principle is
the same
1. Are the reservoir have
good properties?....
2. Where is
hydrocarbon?.
What these curve mean?
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We have a problem.
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Petrophysically, to quatify:
1. (Porosity)
2. Sw (Water Saturation)
3. k (Permeability)
4. NetPay (Pay Thickness)
5. h (Height above free water)/contact
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The simple mechanical caliper
measures a vertical profile of
hole diameter
1. Caliper Logs
CALIPER LOGS
- Applications:
Measure borehole diameter (borehole geometry if multi-arm caliper
tools with 2 or 3 hole diameters measurements 90 or 60 relative to
each other).
Important measurement for drillers: hole geometry, hole/cementvolume.
Hole diameters are an import input parameter for the environmental
correction of petrophysical logs.
- Basic Quality Control:
Perform casing check - should read nominal casing ID.
CALI, C1, C2
Washout: Shale
zone?
Mudcake:
Permeable zone?
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3. Resistivity and Conductivity Logs
The Resistivity log is a
measurement of a
formations resistivity,
that is its resistance to
the passage of anelectric current.
Schematic illustration of the behaviour of
resistivity logs
Minimum bed resolution under best
conditions (modified from Hartmann, 1975)
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Resistivity Logs
Resistivity was the first log, until the
1950s it was the only log.
In many basins it is still the most common
log measurement.
This familiarity obscures the fact that it is
one of the most difficult measurements to
interpret.
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Resistivity Logs (cont.)
Resistivity tools read deeper than any
other logs.
Several metres for the deepest reading
devices.
The current can be focussed so it
always follows the same path.
Several different depths of investigation.
BUT
Resistivity is a directional property.
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Why is resistivity difficult to interpret?
In anisotropic media (e.g. bedded formation)
the resistivity depends on the current path.
In the well shown opposite the BLUE CURRENT
flows more or less parallel to bedding whereas
the RED CURRENT flows perpendicular.
The RED current, corresponding to a Laterolog
tool can flow in the low resistivity beds.
The BLUE corresponding to an Induction
is forced to flow through all the beds.
What would happen if the well was drilled parallel
to the beds?
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Formation Resistivity
Resistivity varies more than any other property.
0.1 Ohmm for a high porosity sand saturated with saline
formation water.
>1000 000 Ohmm for most evaporites. Electrical Properties can be measured more accurately
than anything else.
Unfortunately
Resistivity is a directional measurement. Different tools can give different readings, different readingsobtained in horizontal and vertical wells.
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How to Read Resistivity Logs
4-decade logarithmicX10 linear
When resistivity exceeds 10 Ohmm
scale switches to 0-100 Ohmm
Because resist ivity
can vary so much
a simple linear scal
is of ten not adequa
X10 Linear scalesare commonly found
in N. America and
E. Europe.
Logarithmic are
used in N. Sea andby W. European
operators.
1 10 100 1000
300 Ohmm
8 Ohmm
16 Ohmm
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Unfocussed Resistivity Tool
Pass Current -I- Between Electrodes A and B.
Measure Voltage -V- Between M and N.
Resistivity is Given by:
R = KV/ I (Ohms Law, K constant for the tool)
Problems:
1. The Current path depends on the relative
resistivity of the mud and the formation.
If Rmud is low most current will flow in
the borehole, if it is high most current
flows in the formation.2. What happens at a bed boundary?
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Focussed Devices.
The problem with Unfocussed devices is the
current path depends on the formation resistivity.
Focussed devices work by forcing the current to
follow the same path no matter what theformation resistivity is.
Two Basic Types: Laterolog and Induction.
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Laterolog
Bucking Current
Measure Current
Current Lines Cannot Cross.
This is exploited in laterologs to forcethe measure current to follow the same
path no matter what the relative
magnitudes of mud and formation
resistivity.
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Dual Laterolog
Bucking CurrentMeasure Current
The Heart of the tool is an array of
electrodes.Current passes from/to the
electrodes labeled Ax and voltages
are measured between the
electrodes labeled Mx. The Bucking
Currents are continuously adjusted
to force the measure currents to flowthe same path through the formation.
By operating at two different
frequencies two different current
paths are created: DEEP and
SHALLOW.
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Induction
1. RF field Produced by Transmitter Coil
2. RF Field Induces an AC Current that f low
around the tool axis.
3. AC current flowing around the tool induces
an RF field that causes a current to flow in the
Receiver Coil .
Insulated Housing
Tx
Rx
A series of Auxiliary Coils focus the current
into a do-nut shape.
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Induction and Laterolog Compared
Laterolog
Low frequency (35Hz)
Needs a conductive path to
the formation.
Conductive Mud.
Good vertical Resolution
Can read up to 100k Ohmm.
6m depth of investigation.
Induction
High Frequency(20kHz)
Works in OBM and air filled
holes
Vertical Resolution poor.
But Phaser and Array
tools are comparable to
laterolog.
Only reads to 100 Ohmm
4-8 m depth of investigation
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Gamma Ray log shows
natural radioactivity.
Spectral gamma ray log
gives abundances of
naturally radioactiveelements, thorium (th),
uranium (U) in ppm and
potassium (k) in %.
a) Average volume from which
radiation are detected
b) Depth of investigation shown to
be dependent on formation
density (Modified from
Hallenberg, 1992)
4. Gamma Ray and Spectral Gamma Ray Logs
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Sonic Log: Applications
Petrophysics and Geology. Porosity Tool.
Lithology (with other tools).
Mineralogy (Coals, evaporites). Over-pressure Detection.
Rock Mechanics.
Seismic.
Well-tie.
Attribute modelling.
5. Sonic or Acoustic Logs
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Measure the time it
takes for sound pulse
to travel between a
transmitter and
receiver, mounted a set
distance away along
the logging tool.
The Principle uses of the sonic log (Conventional, compressional wave
tools)
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Sonic Logs Does not use a radioactive source.
Can work in very poor holes. LWD tools available since mid-1990s
Modern tools can work behind casing.
But need good a good cement bond!
BUT Poor vertical resolution.
Very shallow depth of investigation.
Generally unpredictable response to porosity
actually responds to rock fabric.
The sonic is often the only porosity tool
run above the reservoir interval. It is the
link between geophysics and petrophysics.
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Density Logs
Good vertical resolution.
Predictable variation of porosity with density.
Small environmental effects.
(hole size, mud weight) Make it the preferred porosity tool.
BUT
It does not work in rugose holes.
Shallow Depth of Investigation.
It is less accurate at high density = low porosity
6. Density and Photoelectric Factor Logs
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The logging technique
of the density tool is to
subject the formation to
a bombardment of
medium-high energy
(0.2-2.0 MeV)
collimated (focused)
gamma rays and to
measure their
attenuation between
the tool source and
detectors.
The Principle uses of the density
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Density Logs (Summary)
Predictable response to porosity.
Good Vertical Resolution.
Can give information on lithology.
In-built QC (Delta-Rho)
BUT Shallow depth of investigation
Uses a Radioactive Source.
Needs a smooth borehole wall.
Only Open Hole.
AND
Accuracy Drops with Density (Porosity)
Volume of Investigation changes with Density
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The photoelectric factor (PEF) log
(the Litho-Density log of
Schlumberger) is a continuous
record of the effective photoelectric
absorption cross section index or
Pe of formation.
The Principle uses of the photoelectric factor
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7. Neutron Logs
The photoelectric factor (PEF) log (the Litho-Density
log of Schlumberger) is a continuous record of the
effective photoelectric absorption cross section index
or Pe of formation.
The Principle uses of the neutron log
Depth of investigation of the neutron
tool (modified from Serra, 1979)
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Neutron Logs Works in Cased Hole.
Reasonable Depth ofInvestigation.
Most accurate at lowporosity.
Complements Density log
ALSO
Responds to lithology,hydrocarbon and porosity.
Tool can be tuned to beparticularly sensitive or
insensitive to these.
Difficult Measurement toProperly Understand and
Appreciate. Many Different nuclear
reactions involved.
May have Poor verticalresolution(*)
Uses a RadioactiveSource(*).
Needs a good hole.
May Suffer Significantenvironmental effects(*)
temperature, pressure,salinity etc.
Many different tool typeswith significantly differentresponses.
* - latest tools like the Schlumberger APS are far better in this respect.
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Neutron Tool Principles of Measurement
1. Neutrons emitted from a chemical source.2. Neutrons are slowed down by collisions with
atomic nuclei (Hydrogen is especially
effective).
3. Slow neutrons form a cloud around the tool
4. Some are detected, some are not, some get
absorbed by other atoms (5).
12
2
3
4
5
High porosity
Low porosity
Thermal Neutron Tool
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Thermal Neutron Tool
Fe
1. High Energy Neutron Emission
2. Scattering by Medium Nucleus
3. Scattering by Light Nucleus
4. Scattering by Hydrogen
5a. Capture by Iron
5b. Diffusion and possible detection.
1
2
3
4
5a
5b
0.17c
0.000007c