Page 46 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

changes in potential: it gives no absolute values. The directly into the soil. Offshore, however, no such
surface electrode of the SP must be an effective earth possibility exists. Without an effective earth, the SP will
(Wallace, 1968). not be recorded. The SP from many offshore wells, espe-
cially from floating rigs, is useless and mostly ignored.
Log presentation: units and scales
This is a pity. The SP is a cheap and useful log.
SP currents are measured in millivolts (1 10° volts) and
The search for an effective earth offshore is difficult.
the scale is in + or — millivolts, negative deflections to the
The one most commonly used for floating rigs is the
left, positive to the right (Figure 5.5). The log is usually run
riser, but this is usually in electrical connection to the rig
in track | with a gamma ray or caliper log (Figure 5.5).
and any rig is electrically noisy. Using the rig legs is not
Unwanted logging effects helpful either since these are given a potential themselves to
As indicated above, for the SP tool to work effectively, it stop rusting and as waves pass, this potential changes and
must be connected to a surface earth. For onland wells causes a wavy SP — which is only indicative of sea state and
this causes no problem and an iron probe can be pushed not formation characteristics! (Wallace, 1968) (Figure 5.7).
SPONTANEOUS POTENTIAL
- >
millivolts

1900m

° \
q
Well data
R m 0.4424 40 5°C

R 0.682 25°C
mf
Rmc 2:18 2 25°C

Borehole temp. 80°C

Ry * 0.27 2 25°C
oC
1950m an

rm Lithology
a| /|$ ‘|
” & ‘| clean
xt ” . sand

\y o
c£
wh
ae i

L4 ‘en Produced hydrocarbons

aS
silt tt GAS

<
OIL
@
2000m

vopermeable
shaly
thin sand

!
4
Figure 5.8 Example of the shale baseline and the SSP defined on an SP tog. The shale baseline is the maximum positive deflection
(in this example) and occurs opposite shales. The SSP is a maximum negative deflection and occurs opposite clean, porous and
permeable water-bearing sandstones.
36