1. Well Logging Course (1st Ed.)

2. 1. Rudimentary definitions
2. hydrocarbons presence determination
3. hydrocarbons quantity and recoverability
determination
4. The Borehole Environment

3. 1. Reading A Log
2. Examples of Curve Behavior And Log Display
3. Electrical Properties Of Rocks And Brines

5. Standard log presentation formats
Reading a log with ease
requires familiarity with
some of the standard log
formats.
The formats for
traditional logs and most
field logs are shown in
Figure.
It contains three tracks.
A narrow column
containing the depth is
found between track 1
and tracks 2 and 3.
The latter are contiguous
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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6. Different scale types
In the normal linear presentation,
the grid lines in all three tracks
having linear scales each with ten divisions
In the logarithmic scale
We have logarithmic presentation for tracks 2 and 3
Four decades are drawn to accommodate the electrical
measurements, which can have large dynamic ranges
scale begins and ends on a multiple of two rather than unity
In a hybrid scale
We have a logarithmic grid on track 2 and a linear in track 3
Electrical measurements that
may spill over from track 2 into track 3 will still be logarithmic
even though the indicated scale is linear
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
6

7. SP and GR log headings used for
clean formation determination
Figure shows the typical
log-heading presentation
for several of the basic logs.
The upper two
presentations show
two variations for SP,
 which is always in track 1.
 the SP decreases to the left
The bottom presentation
shows
 the caliper,
 a one-axis measurement of
the borehole diameter,
 the gamma ray,
 which are also generally
presented in track 1.
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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8. clean sections determination
The rule given for finding clean sections was that
the SP becomes less negative for increasing shale,
so that deflections of the SP trace to the right
will correspond to increasing shale content
The GR curve, as it is scaled
 in increasing activity
(in American Petroleum Institute (API) units) to the right,
will also produce curve deflections to the right
for increasing shale content.
Thus the two shale indicators can be expected to
follow one another as the shale content varies.
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
8

9. The induction log heading and
schematic of the formation
Although modern tools
have a larger selection of
curves with different
depths of investigation,
 the displays are similar
A traditional resistivity log
heading along with a
schematic indication of the
zones of investigation is
shown in the figure
 three zones corresponding
approximately to the
simultaneous electrical
measurements of different
depths of investigation
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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10. dual induction-SFL
The particular tool associated with this format (previous slide)
 is referred to as the dual induction-SFL and
 will normally show three resistivity traces (units of ohm-m)
The trace coded ILD (induction log deep)
 the deepest resistivity measurement and
 correspond to Rt when invasion is not severe
The curve marked ILM (induction log medium)
 is an auxiliary measurement of intermediate depth of penetration and
 is highly influenced by the depth of invasion
The third curve, in this case marked SFLU (spherically focused log),
 is a measurement of shallow depth of investigation and
 reads closest to the resistivity of the invaded zone Rxo.
By combining the three resistivity measurements,
it is possible, in many cases,
 to compensate for the effect of invasion on the ILD reading
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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11. Log headings
for three porosity devices
The top two correspond to
 two possible formats
 for simultaneous
density and neutron logs
 The porosity is expressed as
a decimal (v/v) or in porosity
units (p.u.), each of which
corresponds to 1% porosity
The bottom
is the sonic log format
 It is with the apparent transit
time Δt increasing to the
left.
In all three presentations,
 the format is such that
 increasing porosity produces
curve deflections to the left
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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12. matrix setting
in neutron and density logs
For the neutron and density logs,
another point to be aware of is the matrix setting
This setting corresponds to
a rock type assumed in a convenient pre-interpretation
that establishes the porosity
from the neutron and density device measurements
the matrix setting SS,
means that the rock type is taken to be sandstone
If the formations being logged are indeed sandstone,
• then the porosity values recorded on the logs
will correspond closely to the actual porosity of the formation
if the actual formation matrix is different, say limestone,
• then the porosity values will need to be shifted or corrected in
order to obtain the true porosity in this particular matrix
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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14. An SP log over
a clean section bounded by shales
shale sections
 The intervals of high SP
 above 8,500 ft and
below 8,580 ft
The value of
the typical flat response
 is called the shale base line
Sections of log
with greater SP deflection
 (with a more negative value
than the shale base line)
 are taken as clean, or
at least cleaner, zones
 One clean section is
 the zone between
8,510 and 8,550 ft
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Well Logging Course: Getting familiar with logs
14

15. A GR and caliper log over the same
section as previous slide
Note the similarity between
the GR trace and the SP trace
GR (solid)
 In the clean sections,
 the gammy ray reading is
on the order of
15 to 30 API units,
 while the shale sections
 may read as high as
75 API units
the caliper (broken)
 It follows much of the same
trend as GR because
 the shale sections can
“wash out,”
• increasing the borehole size
compared to the cleaner
sand sections that retain
their structural integrity
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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16. An induction log over a water zone
with a HC zone above it
The shallow, deep, and
medium depth resistivity
curves are indicated.
The zone below 5,300 ft
is possibly water,
Assuming the resistivity of
the formation water
is much less
(i.e., the water is much
more saline) than the
resistivity of the mud
Mud resistivity effect:
the shallow resistivity
curve,
which for the most part
stays around 2 ohm-m
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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17. An induction log over a water zone
with a HC zone above it (Cont.)
 At 5,275 ft,
a possible hydrocarbon zone
 ILD is much greater
than in the supposed water zone
 However, this increase in resistivity
may not be the result of hydrocarbon
presence.
 A decrease in porosity
could produce the same effect for a
formation saturated only with water
 The real clue here is that
 even though the Rxo reading
has also increased
(means the porosity has decreased),
there is less of a separation
between the Rxo and Rt curves
than in the water zone.
 This means that
the value of Rt is higher
than should be expected from the
porosity change alone.
By this plausible chain of reasoning,
we are led to expect that this zone
may contain hydrocarbons.
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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18. Sample neutron and density logs
 the density-porosity estimate
 (φd , or DPHI, on the log heading), in solid,
 the dotted neutron porosity,
 the compensation curve Δρ (or DRHO)
 (The auxiliary curve Δρ)
 indicates little borehole irregularity
 is the correction which was applied to the
density measurement in order to correct
 for mudcake and borehole irregularities
 It can generally be ignored if it hovers about
zero, as is the case at certain depths.
 Note, once again, the built-in assumption
that the matrix is sandstone.
 Density and neutron derived
porosity equality:
 the presence of liquid-filled sandstone is
confirmed.
 (for the 20 ft section below 700 ft)
 Density and neutron derived
porosity separation:
 caused by an error in the assumed matrix or
by the presence of clay or gas
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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19. A neutron and density log
exhibiting gas in the formation
presence of gas from
a comparison of the neutron
and density logs.
 With gas in the pores the
formation density is less than
with oil or water,
 so that the apparent density
porosity is higher.
 At the same time
the hydrogen content of gas is
less than oil or water
 so the neutron porosity is
lower.
Thus, in the simplest of cases,
gas is indicated in any zone in
which
 the neutron porosity
is less than
the density porosity.
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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20. The signature of shale on
a neutron and density combination log
Shale produces
the opposite effect
[rather than gas]
the neutron porosity
may far exceed
the density porosity
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Well Logging Course: Getting familiar with logs
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21. Neutron and density crossover
caused by changes in lithology
All of these generalities are
true only if the principal
matrix corresponds to the
matrix setting on the log.
The effect of having the wrong
matrix setting on the log
(or having the matrix change
as a function of depth)
is shown in Fig figure.
 Several sections show
negative density porosity.
 These are probably due to
anhydrite streaks,
• which, because of their
much higher density,
are misinterpreted
as a negative porosity.
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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22. An example of
an LWD log in a horizontal well
 In track 1 is
 the familiar GR along
 with three curves indicating
 the time delay between drilling
and the three types of
measurements made;
 depth track
 the tool rotation rate is there
 Track 2 contains
 two types of resistivity
measurements,
 each with multiple
depths of investigation
that overlay in this example.
 The third track contains
 the LWD versions of
 the neutron measurement (TNPH),
 the density measurement (ROBB),
 and the density correction (DRHB).
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
22

23. A basic set of logs for
performing a wellsite interpretation
clean and possibly
permeable zones
identification
an inspection of
the SP and GR
four clean, permeable
zones
labeled A through D
resistivity readings are
contained in the second
track.
What is the fluid in each
zone?
the lowest resistivity
values =water
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Well Logging Course: Getting familiar with logs
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26. electrical property measurements
An important component of the well logging suite
is
the measurement of
electrical properties of the formation.
These measurements deal with
• the resistivity of the formation or
• the measurement of spontaneously generated voltages.
o These voltages are the result of
an interaction between
the borehole fluid and
the formation with its contained fluids.
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Well Logging Course: Getting familiar with logs
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27. spontaneous potential
Historically, the first logging measurements were
electrical in nature.
The first log was a recording of
the resistivity of formations as a function of depth and
was drawn painstakingly by hand.
Unexpectedly, in the course of attempting
to make other formation resistivity measurements,
“noise” was repeatedly noted and
was finally attributed to a spontaneous potential.
• It seemed most notable in front of permeable formations.
Both of these measurements
are still performed on a routine basis today.
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Well Logging Course: Getting familiar with logs
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28. Resistivity
Resistivity is
a general property of materials,
 as opposed to resistance,
 which is associated with
the geometric form of the material
the dimensions of resistivity are
ohms-m2/m, or ohm-m
 The units of its reciprocal, conductivity,
are Siemens per meter.
 In well-logging,
milli Siemens per meter (mS/m)
a material of resistivity 1 ohm-m
with dimensions of 1 m on each side
 will have a total resistance, face-to-face,
of 1 ohm.
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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29. Resistivity measurement
Thus a system to measure resistivity
would consist of a sample of the material
to be measured contained in a simple fixed geometry.
If the resistance of the sample is measured, the
resistivity can be obtained from the relation:
which becomes, using Ohm’s law:
This constant k,
referred to as the system constant,
converts the measurement of a voltage drop V,
for a given current I , into the resistivity of the material.
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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30. A schematic diagram of a mud cup
for determination of its resistivity
A current, I ,
is passed through the sample of
drilling fluid and the corresponding
voltage, V, is measured.
the system constant can be
calculated to be 0.012 m.
The resistivity, ρ, in ohm-m, is then
obtained from the measured
resistance R by:
a sample of salt water with a
resistivity of 2 ohm-m in the
chamber would yield a total
resistance of 166 ohms
Fall 13 H. AlamiNia
Well Logging Course: Getting familiar with logs
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32. Resistivity values
There are two general
types of conduction:
electrolytic and
the mechanism is
dependent upon the
presence of dissolved
salts in a liquid
• such as water
electronic
Examples of electronic
conduction are provided
by metals, which are not
covered here
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Well Logging Course: Getting familiar with logs
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33. Resistivity in different materials
Notice the range of resistivity variation for salt water,
which depends on the concentration of NaCl.
Typical rock materials are in essence insulators.
The fact that reservoir rocks have any detectable
conductivity is usually the result of
the presence of electrolytic conductors in the pore space.
The conductivity of clay minerals is also
greatly increased by the presence of an electrolyte.
In some cases, the resistivity of a rock may result from
the presence of metal, graphite, or metal sulfides.
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Well Logging Course: Getting familiar with logs
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34. sedimentary rocks resistivity
the resistivity of formations of interest may range
from 0.5 to 103 ohm-m, nearly four orders of magnitude.
The conductivity of sedimentary rocks
is primarily of electrolytic origin.
It is the result of the presence of water or
a combination of water and hydrocarbons
in the pore space as a continuous phase
will depend on
the resistivity of the water in the pores and
the quantity of water present.
To a lesser extent, it will depend on
lithology of the rock matrix, its clay content, and its texture (grain
size and the distribution of pores, clay, and conductive minerals).
will depend strongly on temperature
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Well Logging Course: Getting familiar with logs
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35. Determination of the resistivity of an
NaCl solution f(NaCl concentration, T)
the resistivity
of saltwater
(NaCl)
solutions is a
function of
the
electrolyte
concentration
and
temperature
G/G
is grains per
gallon
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Well Logging Course: Getting familiar with logs
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36. 1. Ellis, Darwin V., and Julian M. Singer, eds. Well
logging for earth scientists. Springer, 2007.
Chapter 2 and 3