Page 40 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

Quality control using the caliper 4.4 Four-arm caliper interpretation
With the simple, two arm caliper, one extremely
Breakouts
important use is in the quality control of logs in general.
A great deal more information can be gained from dual
When caving is serious, the quality of log readings is
caliper tools than from the simple caliper tool. As
impaired. In some tools, such as the micrologs, a caliper
indicated above, dual caliper information is generally
is registered simply because the tool] sensors are pad
taken from the four-arm dipmeter tool.
mounted. Lack of pad contact with the formation, a prob-
Using just a single caliper, borehole shape cannot be
Jem in rugose holes with pad-mounted tools, is quickly
interpreted. Data from a four arm caliper however,
seen by using the caliper. In other tools, such as the for-
enables the shape of a hole to be much better defined. A
mation density, the caliper reading is used for an
hole can be seen to be ‘on gauge’ and round (Figure 4.9a)
automatic hole size and mud-cake thickness correction
or oval and ‘washed out’ (Figure 4.9¢) or enlarged by a
or compensation (Chapter 9). Caving will demand
‘key seat’ (Figure 4.95). When oval, the direction of
inordinately large corrections to many logs and the log
enlargement can be given. However, much more can be
values will be of little use. It is therefore essential to look
interpreted from borehole shape.
at the caliper before using any logs (Figure 4.7).
However, it should be pointed out that the simple
caliper attached to the open hole tools such as the
micrologs and the formation density, will generally be — Caliper increase
woe . . Bit si
pessimistic in terms of hole condition, because in oval
ize
holes a simple caliper will naturally open to the maxi- L q
mum diameter of the borehole (Figure 4.8). And i | A
although the log measurements recorded will be made & §-(.)—¢ InGauge | f -=a— depth
across the larger diameter, the hole condition itself is not \ L }
ha
as bad as may first appear.

b :
= DIPMETER FORMATION } .
5 CALIPERS DENSITY 5 :
CALIPER F
faa
|e 18"| [8° 187

1960m 4 b |
= DIPMETER . A
2] 25
f c Ont Washout L é
2 po >
N
1980m} —fesa2%] BS12%
: Nt \ \ y ‘
‘ ao <
! - —
\ 7 ? caliper (‘oat)}

ee Caliper a... ae be
2. woo lee L
; FORMATION SS
oS ( DENSITY i j
' ad <-”%& Breakout ‘
2000m 4. _ 2 . As)
: . r / j
! \\

1
4
S Ow al A
z Cal 1-3——
a
Cal 2-4-----
3
2020m 4 ce
mud cake mud cake
Figure 4.9 Diagrammatic representation of types of borehole
shape and profile as identified on the two-arm caliper. a.
Round, in-gauge hole. 5. Key seat hole enlargement at a
Figure 4.8 Comparison between the simple caliper of the dogleg. c. Washout hole enlargement due to general drilling
formation density tool and the two-arm caliper of the wear. d. Breakout, showing characteristic oval hote with
dipmeter too} in an oval hole. The simple caliper normally abrupt vertical limits (re-drawn, modified from Plumb and
extends to the long axis in an oval hole. Hickman, 1985).
30