Well logging - Neutron density and sonic logs

1. Asst. Lecturer: Amir I. Abdelaziz
Helwan University
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3. Porosity
• Porosity: is the pore volume per unit
volume of formation; it is the fraction of the
total volume of a sample that is occupied by
pores or voids.
• The symbol for porosity is f. A dense,
uniform substance, such as a piece of glass,
has zero porosity; a sponge, on the other
hand, has a very high porosity.
• Porosities of subsurface formations can vary
widely.
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4. Porosity
• Dense carbonates (limestones and dolomites)
and evaporites (salt, anhydrite, gypsum,
sylvite, etc.) may show practically zero
porosity; well-consolidated sandstones may
have 10 to 15% porosity; unconsolidated
sands may have 30%, or more, porosity.
• Shales or clays may contain over 40% water-
filled porosity, but the individual pores are
usually so small the rock is impervious to the
flow of fluids.
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5. Porosity log types
3 Main Log Types
• Bulk density
• Sonic (acoustic)
• Compensated neutron
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These logs do not measures porosity directly. To accurately calculate
porosity, the analyst must know:
• Formation lithology
• Fluid in pores of sampled reservoir volume

6. Porosity log types
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Neutron tool
• Neutron source.
• High energy neutrons are slowed down by hydrogen
atoms in water (or oil) and detected by tool
• Porosity is function of rock type.
Density tool
• Gamma ray source.
• Electrons reflect gamma rays back to detector in tool.
• Electrons in formation proportional to density.
• Porosity is function of rock type and density.
Sonic tool
• Measures speed of sound in formation.
• Porosity slows sound.
• Porosity is function of rock type and measured speed
of sound.

7. Porosity from logging
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Gamma
ray
Resisitivity Porosity
Increasing
radioactivity
Increasing
resistivity
Increasing
porosity
Shale
Shale

8. Density logs
 Measures formation’s bulk density
 Used as a porosity measure Differentiates lithologies with Neutron
Log.
 Used with Sonic Logs to generate synthetic seismic traces to
match to seismic lines.
 Uses radioactive source to generate gamma rays.
 Gamma ray collides with electrons in formation, losing energy.
 Detector measures intensity of back-scattered gamma rays, which
is related to electron density of the formation.
 Electron density is a measure of bulk density. 8

9. Density log
 Bulk density, b, is dependent upon:
 Lithology
 Porosity
 Density and saturation of fluids in pores.
 Saturation is fraction of pore volume occupied by a
particular fluid (intensive)
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10. Density log
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The formation density log is a porosity log that measures
electron density of a formation.
Dense formations absorb many gamma rays, while low-
density formations absorb fewer. Thus, high-count rates at
the detectors indicate low-density formations, whereas low
count rates at the detectors indicate high-density
formations.
Therefore, scattered gamma rays reaching the detector is
an indication of formation Density.
Scale and units:
The most frequently used scales are a range of 2.0 to 3.0
gm/cc or 1.95 to 2.95 gm/cc across two tracks.
Formation (b)
Long spacing
detector
Short spacing
detector
Mud cake
(mc )
Source

11. Density log
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Formation (b)
Long spacing
detector
Short spacing
detector
Mud cake
(mc )
Source
• A radioactive source, applied to the borehole wall in a
shielded portion of the tool, emits medium-energy
gamma rays into the formations. These gamma rays
may be thought of as high-velocity particles that
collide with the electrons in the formation. At each
collision a gamma ray loses some, but not all, of its
energy to the electron, and then continues with
diminished energy.
• The scattered gamma rays reaching the detector, at a
fixed distance from the source, are counted as an
indication of formation density.
• Electron density is related to the true bulk density b,
which, in turn, depends on the density of the rock
matrix material, the formation porosity, and the
density of the fluids filling the pores.

12. Density log
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GRC
0 150
SPC
MV-160 40
ACAL
6 16
ILDC
0.2 200
SNC
0.2 200
MLLCF
0.2 200
RHOC
1.95 2.95
CNLLC
0.45 -0.15
DT
us/f150 50
001) BONANZA 1
10700
10800
10900
Bulk Density
Log
RHOC
1.95 2.95

13. Density log
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• For a clean formation of known matrix density, ma, having a porosity f, that
contains a fluid of average density f the formation bulk density b, will be:
bmabflb ff )1( 
bmabfl
bmab
density


f



DENSITY POROSITY
Clean formation

14. Density log
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DENSITY POROSITY
EXAMPLE
?
/31.2
/10.1
/71.2
density
ccgb
ccgfl
ccgma
f







15. Density log
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SOLUTION
bmabfl
bmab
density


f



25.0248.0
61.1
4.0
71.210.1
71.231.2







density
density
f
f

16. Density logs
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For liquid-filled sandstones,
limestones, and dolomites the tool
reading, a, is practically identical
to actual bulk density, b. For a few
substances, such as sylvite, rock salt,
gypsum, anhydrite; coal, and gas-
bearing formations, the corrections
shown in the picture are needed to
obtain bulk density values from the
density log readings.

17. Neutron logs
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 Measures porosity of formation.
 detect quantity of hydrogen
present.
 Measures lithology when used
with Density Log.

18. Neutron logs
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 The Neutron Log is primarily used to
evaluate formation porosity.
 It is used to detect gas in certain
situations.
 The Neutron Log can be summarized
as the continuous measurement of the
induced radiation produced by the
bombardment of that formation with a
neutron source contained in the
logging tool whose sources emit
fast neutrons that are eventually
slowed by collisions with hydrogen
atoms until they are captured.

19. Neutron logs
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20. Porosity from Neutron log
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Theoretical equation
 
  Nmashshsh
NhcxoNmfxoN
V1V
S1S
fff
fffff
fN = Recorded parameter
f Sxo fNmf = Mud filtrate portion
f (1 - Sxo) fNhc = Hydrocarbon portion
Vsh fNsh = Shale portion
(1 - f - Vsh) fNhc = Matrix portion where f = True
porosity of rock
fN = Porosity from neutron log measurement,
fraction
fNma = Porosity of matrix fraction
fNhc = Porosity of formation saturated with
hydrocarbon fluid, fraction
fNmf = Porosity saturated with mud filtrate, fraction
Vsh = Volume of shale, fraction
Sxo = Mud filtrate saturation in zone invaded
by mud filtrate, fraction

21. Neutron logs
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• Gas zones can often be
identified by comparing the
neutron log with another
porosity log or a core
analysis.
• A combination of the neutron
log with one or more other
porosity logs yields even
more accurate porosity
values and lithology
identification even an
evaluation of shale content.

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Porosity from Neutron log
Porosity log crossplots are one of
the basic tools used in formation
evaluation, since each of the
common porosity logs such as the
acoustic, density and neutron
responds uniquely to different
lithologies.
Crossplotting multiple porosity
responses also defines actual
porosity more accurately.

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24. Density-Neutron log
• Can be an Indicator for Gas ( Gas zone boundary).
• We can see in ( Density – Neutron) log an OVERLAP + SEPARATION
and the case may reversal in case of Oil zone.
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• Determine the different lithologic units in the following composite log.
Detect the OWC and The GOC.

33. Sonic (Velocity) log
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• Measures speed of sound in the formation in microseconds/ft.
• Each rock type (lithology) has a characteristic DT 40-70 us/ft.
• Fluids have a much slower DT of 180-230 us/ft, gas even slower.
• Links seismic depth in time to log depth in ft or m.
• Used to determine porosity and lithology.
Sonic (DT)
Acoustic energy emitted by a
transmitter, travels through the
formation/fluids, detected by
multiple detectors.
Log displays the interval transit
time (Dt) in msec/ft (actually
an inverse velocity)

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f f
f

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Subsurface temperature
• Temperature Increase with depth.
• This temperature-depth relationship is commonly a
linear function of the following form.
TD = Ts + α D
• TD : Temperature of the reservoir at any depth ;
Ts: Average surface temperature ; α : Temperature
gradient (degree/ft) and D : Depth, ft.

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Subsurface temperature
• Find the temperature at depth of 5000 ft when
the average surface temperature is 75 F and the
temperature gradient is 1.5 F/100 ft.
• Calculate the Geothermal gradient of a sandstone
layer at depth 2200 ft where, the surface
temperature=80 0F and the formation
temperature=122 0F.