4. Definition:The continuous recording of a
geophysical parameter along a borehole
produces a geophysical well log .
Main objective of well-logging is formation
evaluation.
Well-logging is done in most oil
wells, mining exploration wells, and in many
water wells.
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6. Geologically, bulk density is a function of the
density of the minerals forming a rock (i.e.
matrix) and the enclosed volume of free
fluids (porosity).
Density is one of the most important pieces
of data in formation evaluation.
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7. In the majority of the wells drilled, density is
the primary indicator of porosity.
In combination with other measurements, it
may also be used to indicate lithology and
formation fluid type.
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8. A radioactive source applied to the borehole
wall emits gamma rays into the formation so
these gamma rays may be considered as high
velocity particles which collide with the
electrons in the formation.
At each collision the gamma ray loses some
of its energy to the electron, and then
continues with diminished energy.
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10. This type of interaction is known as Compton
scattering. The scattered gamma rays
reaching the detector, at the fixed station
from the source, are counted as an indication
of formation density.
The denser the formation, the more electrons
are presented, and more energy is lost due to
collisions
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12. The density log is generally plotted on a
linear scale of bulk density.
The log is run across track 2 and 3.
Most often its scale is between 1.95 and
2.95 g/cm3.
The main log is accompanied by a curve that
shows the borehole and mud-cake
corrections that have been applied.
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13. A record of cable tension may also be
included, as the density tool tends to stick in
poor holes.
A correction curve, is sometimes displayed in
track 3 and less frequently in track 2.
The gamma ray and caliper curves usually
appear in track 1.
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15. The standard density tool has a collimated
gamma ray source (usually radio cesium
which emits gamma rays, radio cobalt is also
used).
It has two detectors (near and far) which allow
compensation for bore hole effects when
their readings are combined and compared in
calculated ratios.
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16. The near detector response is essentially due
to borehole influence which, when removed
from the far detector response enhance the
formation effects.
The most recent density tools use more
efficient scintillation detectors which separate
high and low energy gamma levels.
Source and detectors are mounted on a
plough shaped pad.
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18. The tool is run typically as a density-neutron
combination along with a gamma ray tool and a
caliper.
Its vertical resolution is 33 inches.
Depth of investigation is 1.5 inches.
The tool can be run in
Open hole
Cased hole.
Borehole fluid of gas or air, water or water based
mud, oil or oil based mud.
The logging speed of the tool is 60 feet/minute.
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19. POROSITY CALCULATION:
To calculate porosity from the log derived bulk
density it is necessary to know the density of all the
individual materials involved.
By knowing the grain (matrix) density and the fluid
density, the equation can be solved that gives
From the summation of fluids and matrix
components.
ρb = Ф x ρf + (1 – Ф) x ρma
Where ρma = matrix (or grain) density
ρf = fluid density
ρb = bulk density(as measured by the
tool hence include porosity and density of grains).
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20. When solved for porosity this equation
become:
PorosityФ = (ρma - ρb)/(ρma - ρf)
Erroneous porosities may be calculated when
the fluid density changes. This is the case
when a rock is saturated with gaseous
hydrocarbons. In the presence of gas the fluid
density drops dramatically. The density log
gives too high porosity.
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21. When oil is present the porosity given by the
density log is essentially correct because the
density of oil is quite close to that of water.
Gas is more mobile and frequently occurs
because of large density difference with
water.
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22. Figure showing the effect of gas on density log. In this example gas zone reads about 35%
porosity, it should be 27% 22
23. LITHOLOGY IDENTIFICATION
The densities of the common lithologies are
rarely diagnostic since there is too much
overlap. Overall, oilfield densities generally
measure between 2.0 g/cm3 and 3.0 g/cm3.
The density log is itself a poor indicator of
lithology, combined with the neutron log it
becomes best qualitative indicator of litholgy.
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25. The compaction of shales with burial is a well
known phenomenon and it can be followed
on the density log.
Shale compaction involves a series of textural
and compositional changes, resulting in a
progressive increase in density.
For example shallow, un-compacted clays
have densities around 2.0g/cm3, while at
depth, this figure commonly rises to
2.6g/cm3.
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26. Figure showing shale compaction with depth seen on a bulk density log plotted at a
compressed vertical scale 26
27. shale density is often indicative of age.
In general, older shales are denser.
Paleozoic clays are rare, as are Tertiary
shales.
The increase in shale density during
compaction, although essentially due to a
decrease in porosity is accompanied by
irreversible diagenetic changes.
In the subsurface, a change in compaction
trends will indicate a change in age, in other
words an unconformity.
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28. Figure showing tertiary shale uncomformably overlying cretaceous
shale. The abrupt change in density marks the unconformity.
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29. Local variations in shale density are more
likely due to changes in shale composition.
The increase in density is even more marked
when iron carbonate is involved. When
organic matter is present, the reverse occurs
and the density diminishes,
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30. Organic matter having a very low density.
An increase in carbonate content is generally
accompanied by an increase in shale density.
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31. Figure showing thin carbonate/siderite
cemented horizons in shale. The
intervals may be thin continuous bands
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32. Bulk density variations in sandstone generally
indicate porosity changes.
This is not true when there are changes in
grain density.
Overall grain density will change depending
on the non-quartz constituents.
Sands are commonly mixed with feldspars
(density 2.52 g/cm3), micas (2.65-3.1
g/cm3).
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33. Heavy minerals may also be a constituent
(2.7-5.0 g/cm3).
Changes in grain density in sands are gradual
and of a moderate order.
Abrupt changes, especially in homogenous
beds, often indicate diagenetic or secondary
changes.
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34. Figure showing the effect of muscovite on the bulk
density log in micaceous sands. The increase in
density below15m is due to mica content
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36. Density becomes a criterion for lithological
identification when it is either abnormally
high or abnormally low.
Coals, for example, are identified by very low
densities, between 1.2 g/cm3 and 1.8 g/cm3
Pyrite has a very high density between 4.8
g/cm3 and 5.17 g/cm3.
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37. Figure showing coal, with low density and pyrite
with high density, on the bulk density log
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38. Chemical deposits, because of their
purity, may be identified by their densities.
Most evaporates tend to give intervals of
constant density with little variation. When
this occurs, along with densities near the
pure mineral values, evaporates are probable.
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40. The presence of organic matter in shales
lowers their density.
The normal average matrix density of a
mixture of clay minerals is about 2.7g/cm3,
while organic matter has densities between
0.50 -1.80g/cm3.
The presence of organic matter therefore has
a marked effect on the overall shale bulk
density.
.
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41. This organic matter effect on the density log
can be quantified, so that the log can be used
to evaluate source rocks.
Difficulties arises when organic matter is
mixed with a high density mineral such as
pyrite (4.8-5.17g/cm3), Since the density of
the pyrite masks the effect of the low density
organic matter.
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