Well logging involves lowering instrumentation into oil and gas wells to collect data about the surrounding rock formations. A well log records measurements from these instruments and provides information to geologists, geophysicists, drilling engineers, and reservoir engineers. Common logs measure properties like electron density, acoustic travel times, resistivity, and neutron absorption to characterize formations and identify potential reservoirs. Well logs are presented as tracks on a plot against depth to interpret features like formation tops, pore fluid content, and stress orientations deduced from borehole breakouts.

1.  What is well Logging?
A log is a record of a voyage , like a ship's log or a travelogue. A well-log is a record of the
voyage of a measuring instrument into a well bore. The instrument itself is something called a
log or a logging tool. The log is the paper or digital recording of the measurements made by the
logging tool, versus depth or time.
Well logging is a continuous record of measurement made in bore hole respond to variation
in some physical properties of rocks through which the bore hole is drilled.
Traditionally Logs are display on girded papers. Now a days the log may be taken as films,
images, and in digital format.
 Logging Units:
Each unit will contain the following components:
• logging cable.
• winch to raise and lower the cable in the well.
• self-contained 120-volt AC generator.
• set of surface control panels.
• set of downhole tools (sondes and cartridges).
• digital recording system.
 Uses of Logs:
A set of logs run on a well will usually mean different things to different
people. As,
1) The Geologist:
The Geologist may ask:
• What depths are the formation tops?
• Is the environment suitable for accumulation of Hydrocarbons?
• Is there evidence of Hydrocarbon in this well?
• What type of Hydrocarbon?
• Are Hydrocarbons present in commercial quantities?
• How good a well is tie?
• What are the reserves?
Well
Logging
Well
Logging

2. • Could the formation be commercial in an offset well?
2) The Geophysicist:
As a Geophysicist what do you look for?
• Are the tops where you predicted?
• Are the potential zones porous as you have assumed from seismic data?
• What does a synthetic seismic section show?
3) The Drilling Engineer:
• What is the hole volume for cementing?
• Are there any Key-Seats or severe Dog-legs in the well?
• Where can you get a good packer seat for testing?
• Where is the best place to set a Whipstock?
4) The Reservoir Engineer:
The Reservoir Engineer needs to know:
• How thick is the pay zone?
• How Homogeneous is the section?
• What is the volume of Hydrocarbon per cubic metre?
• Will the well pay-out?
• How long will it take?
5) The Production Engineer:
The Production Engineer is more concerned with:
• Where should the well be completed (in what zone(s))?
• What kind of production rate can be expected?
• Will there be any water production?
• How should the well be completed?
• Is the potential pay zone hydraulically isolated?
 What Logs Can Measure?
1) Electron density of the rock.
2) Acoustic travel-time of the rock.
3) Resistivity, at various distances from the borehole, of the rock.
4) Neutron absorption rate.
5) The self potential of the rock/borehole fluid interface.
6) The size of the borehole drilled in the rock.
7) The flow rate and density of fluids in the wellbore.
8) Other related or derived properties.
 Well Logging and the Well Log Plot:

3. Logging is an operation with a device consisting of a Bakelite cylinder with a couple of
metallic electrodes on its exterior. Connecting the device to the surface with a cable/wire, thus
providing us with the term wireline logging. Wireline refers to the armored cable by which the
measuring devices are lowered and retrieved from the well and, by a number of shielded
insulated wires in the interior of the cable, provide for the electrical power of the device and a
means for the transmission of data to the surface. More recently, the devices have been
encapsulated in a drill collar, and the transmission effected through the mud column. This
procedure is known as logging while drilling (LWD).
So an oil well is logged by lowering a set of sensors (a logging tool), attached to a telemetry
cable, down a borehole. The logging operation is accomplished by slowly pulling the tool
uphole while recording the sensor measurements at appropriate time and depth intervals.
To log the well, drilling must be stopped and the drill string must be removed from the
borehole. Typically, the winch lowers the wireline cable and sonde to the bottom of the
borehole. When the sonde is on the bottom, the winch slowly rises it up to the surface. When it
moves up through the borehole, the sonde takes formation measurements. This is called
"logging up. In some cases, measurements may be taken as the sonde is lowered into the
borehole. The electrical cable carries power to the sonde. Measurements are not stored in the
sonde. Instead, formation measurements are transmitted uphole through the cable to the logging
unit for processing.

4. Fig. 1. Well logging and Examples of four logging tools.
The well log plot is made by drawing a long and narrow strip chart. As a well log plot is
typically viewed, the longitudinal direction represents vertical well depth measured in scaled
feet or meters, and the latitudinal direction (across the paper) represents sensor response, scaled
to some appropriate coordinate system.
A sequence of data for a sensor is called a log trace, and is typically drawn using a curve that
runs along the vertical direction. Often, this vertical direction is called the logging direction.
During logging operations, one never knows how long the log will be, so the length of this axis
is considered to be indefinite. This is also sometimes called the "continuous rendering"
direction.
Well log plots are graphs which display data obtained during various data acquisition and
processing operations. These graphs are meaningless unless they are plotted on some form of
scaled graph paper from which the data values may be read and interpreted. Timing lines, depth
lines, and grid line objects are used to draw this graph paper. In a typical log (see fig.2), tracks
run vertically from the bottom to the top of the plot. A standard log has a left track (Track_1),
2.5 inches wide, extending from the left edge of the paper. Next, a 0.75 inch wide depth track is
used to record vertical depths, typically every 10, 20, 50, or 100 feet/meters, depending on log
scale and coordinate system. Next come two 2.5 inch wide tracks, sometimes used separately,
and sometimes used in combination. In this case, Track_2 and Track_3 are divided into ten
equal increments.
Summing up these track widths gives a standard log width of 8.25 inches. Since vertically
oriented data is to be plotted in these tracks, horizontal depth lines are used to delineate the
depth scale: thin depth lines occurring most frequently (every 2 borehole feet); less thin depth
line occurring less frequently (every 10 borehole feet); thickest depth lines occurring least
frequently (every 50 borehole feet).

5. Fig.2. Typical Three-Track Log Layout

6. Fig.3. An example of well log showing parameters recorded simultaneously namely : Gamma-
Ray / Spontaneous Potential / Normal Resistivities 8 - 16 - 32 - 64’’ and Single Point
Resistance.
 Borehole Environment:
When a hole is drilled into a formation, the rock and the fluids in it (rock-fluid system) are
altered in the vicinity of the borehole. A well’s borehole and the rock surrounding it are
contaminated by the drilling mud, which affects logging measurements.
Fig.4. A schematic illustration of a porous and permeable formation which is penetrated by a
borehole filled with drilling mud.

7.  Introduction:
The Caliper Log is a tool for measuring the diameter and shape of a borehole. It uses a tool
which has 2, 4, or more extendable arms. The arms can move in and out as the tool is
withdrawn from the borehole, and the movement is converted into an electrical signal by a
potentiometer.
In the two arm tool (Fig. 9.1), the borehole
diameter is measured. This is shown in track 1 of the
master log together with the bit size for reference.
Borehole diameters larger and smaller than the bit
size are possible. Many boreholes can attain an oval
shape after drilling. This is due to the effect of the
pressures
in the crust being different in different directions as
a result of tectonic forces. In oval holes, the two arm
caliper will lock into the long axis of the oval cross
section, giving larger values of borehole diameter
than expected. In this case tools with more arms are
required.
In the 4 arm (dual caliper) tool, the two opposite
pairs work together to give the borehole diameter in
two perpendicular directions. An example of a 4 arm
tool is the Borehole Geometry Tool (BGT). This has
4 arms that can be opened to 30 inches (40 inches as
a special modification), and give two independent
perpendicular caliper readings. The tool also
calculates and integrates the volume of the borehole
and includes sensors that measure the direction
(azimuth) and dip of the borehole, which is useful in
plotting the trajectory of the borehole.
Caliper LogsCaliper Logs

8. In the multi-arm tools, up to 30 arms are arranged around the tool allowing the detailed
shape of the borehole to be measured.
Some of the other tools have sensors attached to pads that are pressed against the borehole wall.
The pressing device is also a form of caliper, and so caliper information can sometimes also be
obtained from these tools.
 Log Presentation:
The caliper logs are plotted in track 1 with the drilling bit size for comparison, or as a
differential caliper reading, where the reading represents the caliper value minus the drill bit
diameter (Fig. 9.2).
The scale is generally given in inches, which is standard for measuring bit sizes.
The 4 arm (or dual caliper) tools are presented in a range of formats, an example of which is
given in Fig. 9.3. Note that data from the caliper pairs are shown together, and that they are
different indicating an oval hole. Also the tool rotates in the hole (the pad 1 azimuth P1AZI
changes). The hole azimuth is reasonably constant at HAZI180o, and the dip is almost vertical
(DEVI is about 0o).

9. The ticks represent borehole volume. This information is useful to estimate the amount of
drilling mud in the borehole, and to estimate the amount of cement required to case the hole.
There are engineering approximation formulas to calculate both of these from caliper data.
 Simple Caliper Interpretation:
Table 9.1 describes the main influences on caliper values. Note that when a hole is the same
diameter as the bit-size it is called on gauge.

10.  Uses of the Caliper Log:
The commoner uses of the caliper log are as follows:
- Contributory information for lithological assessment (see Table 9.1).
- Indicator of good permeability and porosity zones (reservoir rock) due to development of
mud cake in association with gamma ray log.
- Calculation of mudcake thickness, hmc = (dbit – dh)/2, where h stands for the hole, in
inches.
- Measurement of borehole volume, Vh = (dh 2/2)+1.2%, in litres per metre.
- Measurement of required cement volume, Vcement = 0.5 (dh 2 – d2 casing) + 1%, in liters
per meter.
- Indication of hole quality for the assessment of the likely quality of other logs whose data
quality is degraded by holes that are out of gauge. Other log data can often be corrected for
bad hole conditions using the caliper readings, but the larger the correction, the less reliable
the final data will be. Centralized tools are designed to be about 4 inches in diameter for a
standard 8.5 inch hole, and they are designed to work with 2.25 inches of drilling mud
between them and the formation. If the hole caves to 14 inches, which is not uncommon, the
distance to the formation becomes 5 inches and the tool responses are degraded. This can be
corrected for to some extent if the caliper value is known. Tools that work by being pressed
up against the side of the borehole wall have even greater problems because the irregularity
of the borehole wall makes it impossible to obtain reliable readings. In both cases the
recognition that a borehole has bad caving or thick mud cake can help us judge the
reliability of other tool’s readings.
- Selection of consolidated formations for wireline pressure tests, recovery of fluid samples,
for packer seating for well testing purposes, and for determining casing setting depths.

11.  Borehole breakout analysis from four-arm caliper logs:
Borehole breakouts are stress-induced enlargements of the wellbore cross-section. So
borehole breakouts are an important indicator of horizontal stress orientation, particularly in a
seismic regions and at intermediate depths (<5 km). Approximately 19% of the stress
orientation indicators in the World Stress Map (WSM) database have been determined from
borehole breakouts. Here we present the procedures for interpreting borehole breakouts from
four-arm caliper log data and for WSM quality ranking of stress orientations deduced from
borehole breakouts.
When a borehole is drilled the material removed from the subsurface is no longer supporting
the surrounding rock. As a result, the stresses become concentrated in the surrounding rock (i.e.
the wellbore wall). Borehole breakout occurs when the stresses around the borehole exceed that
required to cause compressive failure of the borehole wall. The enlargement of the wellbore is
caused by the development of intersecting conjugate shear planes, that cause pieces of the
borehole wall to spall off (Figure 9.4).

12. Figure 9.4: Results of a hollow cylinder lab test simulating borehole breakout .
Intersection of conjugate shear failure planes results in enlargement of the cross-sectional
shape of the wellbore. SH and Sh refer to the orientations of maximum and minimum horizontal
stress respectively. Around a vertical borehole stress concentration is greatest in the direction of
the minimum horizontal stress Sh. Hence, the long axes of borehole breakouts are oriented
approximately perpendicular to the maximum horizontal stress orientation SH.
However, unprocessed oriented four-arm caliper logs can also be used to interpret borehole
breakouts.
Figure 2:The four orthogonal caliper arms and its Geometry in the borehole and data used for
interpreting borehole breakouts.

13. To identify zones of breakout and the orientation of the enlargement we suggest:
1. Tool rotation must cease in the zone of enlargement.
2. There must be clear tool rotation into and out of the enlargement zone.
3. The smaller caliper reading is close to bit size. Top and bottom of the breakout should
be well marked.
4. Caliper difference has to exceed bit size by 10 %.
5. The enlargement orientation should not coincide with the high side of the borehole in
wells deviated by more than 5°.
6. The length of the enlargement zone must be greater than 1 m.
Breakout orientations can rotate in inclined boreholes and may not always directly yield the
horizontal stress orientations. Hence, the maximum horizontal stress orientation can only be
reliably estimated from breakouts in approximately vertical boreholes (less then 10° deviation
from vertical). All orientations measured from four-arm caliper tools need to be corrected for
the local magnetic declination at the time of measurement.

15. Figure 9.5: Four-arm caliper log plot displaying borehole breakouts. Caliper one (C1) locks into
breakout zone from 2895-2860 m (P1AZ ≈ 200°), the tool then rotates 90° and Caliper two (C2)
locks into another breakout from 2845-2835 m (P1AZ ≈ 290°). Both breakout zones are
oriented approximately 020° and suggest a SHmax direction of 110°. The borehole is deviated
4° (DEVI) towards 140° (HAZI).