1. Bacha Khan University Charsadda
Caliper Log
Tahir Nagra
Department of Geology
Bacha Khan University Charsadda

2. Caliper log Uses Hole size & shape
Direction (Azimuth/angle) and Dip of the Hole
Use to calculate mud cake thickness
Provide Vertical Profiling of Hole Diameter.
Through this log we can determine the caving size & wash outs
Identification of porous & permeable zone
The measurement of hole volume in order to obtain an estimation of cement volume.
The correction of several log types for the effect of the borehole and of mud cake in order to obtain a more precise interpretation.

3. Units and Log Representation
Caliper measure in Inches.
Always displayed in 1st track with Gamma Ray (GR) and SP logs.
Curve of caliper log is dashed line.
The horizontal scale is in inches on track.

4. Caliper Tools Caliper tools consist of two, four, 32 or upto 64 arms.
Methods of Recording.
Spring-driven arms that contact the wall of the borehole.
The instrument is lowered to the total depth, and the arms are released either mechanically or electrically.
The spring tension against the arms centers the tool in the well.
The arms move in and out with the change in wellbore diameter.
The arm motion is transmitted to a rheostat so that change in the resistance of an electric circuit is proportional to the hole diameter.
The borehole diameter is recorded at the surface by measuring the potential across this resistance using potentiometer.

5. Continues……. Two Arms Tools
In two arms tools, pads prefer to open in the maximum direction of the hole when hole is oval shape and minimum diameter is skipped.
For precise measurement four arms tools are used.
Four Arms Tools
Four arms caliper also called dual caliper.
Arms open in two different perpendicular directions upto 30 or 40 inch diameter.
It measures size, shape, volume and orientation (azimuth) of the hole
Now mostly dipmeter tools are used for measuring these parameters instead of dual calipers.
Borehole Geometry Tool (BGT) is one example of four arms caliper tool by Schlumberger.

6. Two, Four and Multi (64) Arms Caliper Tools

7. Factors Effecting the Borehole Size
Borehole size might be effected by the lithology of the rocks such as
Because of soluble salt.
Gravel, shales, unconsolidated material etc.
Sloughing shale
Texture and structure of the rocks
There might be decrease or increase in hole size because of following factors.
Decrease in hole size
Mud cake
Tight spots
Increase in hole size
wash out or caving

8. Continues……….. In or On Gauge
If the hole size is same as the bit size diameter it is said in or on gauge.
Under Gauge
If the hole size is less than that of bit size called under gauge.
Over Gauge
In this situation hole become lager than the bit size
Bad-Hole or Tight Spot
The boreholes with smaller diameter than bit size. It is because of forming of mud cake against the wall of borehole or tight spots. Tight spots form when tool stick or bit gets stuck while being pulled out of the hole.
The cause of tight spots is abundant smectite in the clay mineral mixture. Smectite is swelling clay which takes water from the drilling mud so it expands and breaks from the formation and sloughs or collapses into the hole.
On caliper log there is smooth curve for mud cake upto a significant interval while in case of tight spot the curve is not smooth.

9. What is Sloughing Shales?
Shales make up over 75% of drilled formations and cause over 90% of wellbore stability problems.
When the hole is being drilled the shales are the most unstable formations in the wellbore.
Shales are impermeable, they do not develop a filter cake on the surface of the formation like a porous sandstone or a porous limestone or dolomite.
The filter cake helps stabilize the formation and prevents it from sloughing (flowing).
Sloughing shale, also called "heaving shale", is soft shale, which when penetrated by the borehole of a oil or gas well, expands, runs, falls, swells, or squeezes into a borehole.
The expansion of sloughing shale causes it to falls from the side of the borehole into the bottom of the borehole.
Sloughing shale can also result in the caving and bridging of the borehole.
Sloughing shale typically contains significant amounts of "sensitive clays", i.e. montmorillonite, that absorb water from the drilling mud and expand (swell).

10.
CAL
(inch)
Bit Size
Caving or Wash out
On Gauge
Mud Cake
Bad Hole or Tight Spot
Gauge On
Brittle Shale
Impermeable
Limestone
Permeable
Sandstone
Shale
Sloughing
Bit
Size
Caving
Mud
Cake

11. 𝑽 𝒄𝒆𝒎𝒆𝒏𝒕 = 𝟏 𝟐 𝒅 𝒉 𝟐 − 𝒅 𝒄𝒂𝒔𝒊𝒏𝒈 𝟐 + 1% (in 1/m)
Quantitative Uses
Caliper log can be used to determine the mud cake thickness
𝒉= 𝒅 𝒃𝒊𝒕 − 𝒅 𝒉 𝟐
Where h = mud cake thickness in inches
𝒅 𝒃𝒊𝒕 = diameter of bit (bit size) in inches
𝒅 𝒉 = caliper reading from log in inches
Hole volume can be measured.
𝑽 𝒉 = 𝒅 𝒉 𝟐 𝟐 +𝟏.𝟐 % 𝐢𝐧 𝟏 𝐦
Volume of cement
𝑽 𝒄𝒆𝒎𝒆𝒏𝒕 = 𝟏 𝟐 𝒅 𝒉 𝟐 − 𝒅 𝒄𝒂𝒔𝒊𝒏𝒈 𝟐 + 1% (in 1/m)
𝒅 𝒄𝒂𝒔𝒊𝒏𝒈 = diameter of casing in inches