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
4.
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 5
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 7
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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 9
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 10
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 11
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 12
13.
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 15
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 16
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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 17
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 18
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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 19
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 20
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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 21
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).
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 23
24.
25.
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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 26
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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 27
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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 28
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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 29
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 30
31.
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 32
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.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 33
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
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 34
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
concentratio
n and
temperature
G/G
is grains per
gallon
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 35
36. 1. Ellis, Darwin V., and Julian M. Singer, eds. Well
logging for earth scientists. Springer, 2007.
Chapter 2 and 3