Page 36 - The Geological Interpretation of Well Logs

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CALIPER LOGS

Caliper tools measure hole size and shape. The simple simple calibration. Frequently logging tools are automat-
mechanical caliper measures a vertical profile of hole ically equipped with a caliper, such as the micrologs
diameter (Figure 4.1). The more sophisticated borehole (Chapter 6) and the density-neutron tools (Chapters 9, 10)
geometry tools record two simultaneous calipers and give where the caliper arm is used to apply the measuring
an accurate borehole shape and orientation. head of the tool to the borehole wall. Sophisticated, dual
caliper tools, such as the Borehole Geometry Too] of
Schlumberger, also exist specifically for measuring hole
4.1 Mechanical calipers — the tools
size and volume. However, today, such information is
The mechanical caliper measures variations in borehole generally taken from dipmeter tools, which acquire
diameter with depth. The measurements are made by geometry data in order to derive dip (Chapter 12). These
two articulated arms pushed against the borehole wall. tools have four pads fixed at right angles, opposite
The arms are linked to the cursor of a variable resistance pairs being linked but independent of the perpendicular
(Figure 4.2}. Lateral movement of the arms is translated set. This, in terms of geometry, gives two independent
into movements of the cursor along the resistance, and calipers at 90°. The tool also contains gyroscopic orienta-
hence variations in electrical output. The variations in tion equipment so that the azimuth (bearing) of the two
output are translated into diameter variations after a calipers is permanently defined.
HOLE DIAMETER
# 4 Scale: inches —————»- 46
a
6
9
41
13
7
td
Lepet
ft
pt
bit ye waliger
SHALE rs size ‘caved hole’

nen LIMESTONE ptt «— ‘on gauge’

PERMEABLE mud cake thickness
»* SANDSTONE = caliper/2

IMPERMEABLE
* SANDSTONE

“tight spot’
SHALE | ‘pad hole’ or

1)

Figure 4.1 The caliper log showing hole diameter: some typical responses. *Limestone, dolomite, etc. equally applicable.

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