Page 38 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

4.3 Simple, two-arm, caliper mud-cake build-up is indicated (Figure 4.6a). This is
interpretation an extremely useful indicator of permeability: only
permeable beds allow mud cake to form. The limits of
Increase in borehole diameter mud-cake indicate clearly the limits of the potential
The simple caliper log records the mechanical response reservoir, Mud cake thickness can be estimated from the
of formations to drilling. A hole that has the same size caliper by dividing the decrease in hole size by two (the
(diameter) as the bit which drilled it, is called on gauge caliper giving the hole diameter), i.e.
(Figure 4.1}. On gauge holes are the target for all drilling
and essentially indicate good drilling technique. Holes bit size (diam) - caliper reading (diam)

with a much larger diameter than the bit size are ‘caved’
2
or ‘washed out’. That is, during deepening of the hole, the
= mud cake thickness
borehole walls cave in, are broken by the turning drill
pipe, or are eroded away by the circulating borehole mud.
It should be remembered that this thickness may vary
This is typical of shales, especially when geologically
between tools. The caliper of a density tool is applied
young and unconsolidated, so that caving can have a
harder to the formation than the caliper of a micro-log:
general lithological significance (Figure 4.1).
the former probably causes a groove in the mud cake and
However, caving is also typical of certain specific
therefore gives a thinner, log derived mud-cake thickness.
lithologies such as coals or even organic shales. In
Boreholes with a smaller diameter than the bit size but
some fields, even with varied drilling fluids and drilling
rugose, are probably sloughed (Figure 4.65). The zones
techniques, it is found that certain stratigraphic levels
of small holes will be the ‘tight spots’ encountered during
habitually cave - generally for mechanical (textural)
drilling, trips or logging. That is, it will be at these points
reasons. The example (Figure 4.5) shows a section of
that tools stick or the bit gets stuck while being pulled out
Carboniferous shale from the UK East Midlands in which
of the hole. A frequent cause of tight spots is abundant
moderate caving occurs in the same organic rich interval,
smectite in the clay mineral mixture. Smectite is a
over a wide area. The shale is either very laminated or
swelling clay which takes water from the drilling mud,
locally fractured.
expands, breaks from the formation and sloughs or col-
Decrease in borehole diameter
lapses into the hole. The Gulf Coast ‘gumbo’, which
Calipers may show a hole diameter smaller than the
often causes hole problems, is smectite rich.
bit size (diameter). If the log has a smooth profile, a
Well 1 Well 2 Well 3
GR GR GR
50 100 150 100 150 50 100 150

8. 7 10" iva 8.5" vo-CALIPER , g- 8.5" io" CALIPER
15" 15°
J 4 1 1 1 1 i
Basen,

10m

20m

b 1 caliper GR
30m

organic rich shale

Figure 4.5 Consistent caving, indicated on the caliper log, over the same, organic rich, stratigraphic level in three different wells.
Upper Carboniferous, East Midlands, U.K.