Geophysical wireline log: Caliper log

1. Presentation on Caliper log
Md. Zubayer Salehin
M.S. (Geophysics)
Department of Geology
University of Dhaka
Bangladesh

2.  What is well Logging
Well log 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

3. Caliper Log
•Caliper tools measure hole size and shape.
•The simple mechanical caliper measures a vertical profile
of hole diameter.
•The more sophisticated geometry tools records two
simultaneous calipers and give an accurate borehole shape
and orientation.

4. Caliper log
showing
hole
Diameter
and some
Typical
Responses

5. Schematic Caliper tool showing
the conversion of a mechanical
movement to an electrical signal
using a variable resistance
Function of the Tool:
•The mechanical caliper
measures variations in bore hole
diameter with depth.
• The measurements are made
by two articulated arms pushed
against the borehole wall.
•The arms are linked to the
cursor of a variable resistance.
Lateral movements of the arm
is translated into the
movements of the cursor along
the resistance, and hence
variation in electrical out put.
•The variations in output are
translated into diameter
variations after a simple
calibration.

6. •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.
•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.
•In the multi-arm tools, up to 30 arms are arranged around the tool
allowing the detailed shape of the borehole to be measured.

7. Various types of caliper
 ƒMechanical Caliper -arm averaging
 ƒUltrasonic Caliper -An Ultrasonic transducer scans around the
borehole walls, and the reflected travel time is converted to
the distance between the sonde and the wall.
 Acoustic caliper – calculated from acoustic transit time and
velocity

8. Mechanical 3-arm
averaging caliper
Mechanical 2 arm caliper

9. 3D image of the ultrasonic caliper.
(FromSchlumberger)

10. Caliper Log Presentation
Presentation of the Caliper Logs: 1. in
ordinary format; 2. in differential
format; BS = Bit size
•The caliper log simply is a
measure of continuous value
of hole diameter with depth.
•The curve is traditionally a
dashed line.
•The horizontal scale may be
in inches of diameter or in
differential caliper,
expressed as increase or
decrease in hole diameter
about a zero defined by the
bit size.
•The ordinary caliper log is
accompanied by a reference
line indicating bit size.

11. •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.
•The 4 arm (or dual caliper) tools are
presented in a range of formats, an example
of which is given in Figure.
•Data from the caliper pairs are shown
together, and that they are different
indicating an oval hole.
• 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.

12. Table: Factors influencing caliper responses
Hole Diameter Cause Possible Lithologies
On Gauge Well consolidated formations
Non-permeable formations.
Massive sandstones
Calcareous shales
Igneous rocks
Metamorphic rocks
Larger than Bit
Size
1. Formation soluble in
drilling mud.
2. Formations weak and
cave in.
1. Salt formations drilled with
fresh water.
2. Unconsolidated sands,
gravels, brittle shales.
Smaller than Bit
Size
1. Formations swell and flow
into borehole.
2. Development of mudcake
for porous and permeable
formations.
1. Swelling shales.
2. Porous, permeable
sandstones.

13. Simple 2 arm caliper
interpretation

14. Figure shows a schematic
hole with caliper
information.

18. Quality control using Caliper
 When caving is serious, the quality of log reading is
impaired.
 It should be pointed out that simple caliper attached to
the open hole tools (microlog, formation density) will
be pessimistic in terms of hole condition, because in
oval holes a simple caliper will naturally open to the
maximum diameter of the borehole.
 So, the log measurements recorded will be made across
the larger diameter, the hole condition is not as bad as
may first appear.

19. • For this reason, 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.

21. Four-arm caliper interpretation
 More information can be found by dual(4-arm) caliper
than simple caliper tool.
 A hole can be seen to be “on gauge” and round, or
“washed out” and oval, or enlarged by “key seat”.
When oval, the direction of enlargement can be given.
 However, much more can be interpreted from borehole
shape.

23.  Keyseats: asymmetrical oval hole, formed by wear
against the drill string at points where the borehole
inclination changes(doglegs).
 Washouts: develop from general drilling wear, especially
in shaly zones and dipping beds. On the geometry logs,
it has a considerable vertical extent and both calipers
are larger than the drill bit size with one caliper being
much larger than other.
 Both have general drilling phenomena, but breakouts
have a specific cause.
 Breakouts: consider to form as a result of the
interaction of stresses induced by drilling and the
existing stress regime of the country rock.

25. • breakouts indicates the present day stress-field orientation and
are independent of lithology, dip and existing fracture or joint.
•On global scale, breakout derived stress field orientation are
similar to those derived from earthquake studies and tend to
indicate intraplate tectonic stresses
•On local scale, it have an importance for
field development.

26. The common uses of the caliper log are as follows:
•In a logging context, the greatest application of caliper data is
in environmental corrections to other logs that are sensitive to
hole size such as Natural gamma ray , Density, Neutron logs.
•Contributory information for lithological assessment.
• Indicator of good permeability and porosity zones (reservoir
rock) due to development of mudcake in association with gamma
ray log.
•Calculation of mudcake thickness, hmc = (dbit– dh)/2, where h
stands for the hole, in inches.

27. The common uses of the caliper log are as follows:
•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.
•calipers can be used to center or eccenter logging-tool strings.
•Openhole caliper data are used to estimate the volumes of gravel
and cement needed for well-completion planning.