Page 30 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL [INTERPRETATION OF WELL LOGS -

COUNTRY-NGRIH SEA LAT-S70 SIM N.
//3- COMPANY -AMNOCO LONG-020 45M E.
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TEMPERATURE CONDUCTIVITY HEAT FLOW LT THOLOGY AGE
HOEG-C.1 UW. 70 DEG.) (AW. AN)
. 200 6 400—CG M20 4d OO 4 2 3 4 §& 6 90 60 70 80 90 140

$00 ‘
. ; clay u TERTIARY
1000

1800
“27.6 GEG.C. san.

SHALE PALE OCE NE

2000
DEPTH IMETRES)
CHALK
ue CRET.
SHALE /HARC
L. CRET.
2500
TC
LURASS
3000
3800

YU. PERBIRN
HAL J TE ZANHTOR STE
4000
aL
SHALE SALTS .745S1.
L. PERMIAN
4500
Figure 3.1 Temperature gradient, thermal conductivity and heat flow compared in North Sea well 7/3-1. Lithology and age are
indicated. (From Evans, 1977).

Table 3.2 Some typical geothermal gradients in continuous temperature may be read either going into the
sedimentary basins. hole or coming out.
When more detail is needed, a special thermometer
Gradient
tool may be used to give continuous temperature read-
°CéAm °F/1OOft
ings. The tool uses a thermistor, or metal whose

Rhine Valley 66 3.6 resistance is sensitive to temperature changes, fitted into
Red Sea 45 2.5 the circuit of a Wheatstone bridge. Typically the tool will
Central Ruhr basin 40 2.2 give not just the absolute temperature but also, using two
Madagascar 38 2.1 separate thermistors, a temperature differential. Such is
Western Canada, Alberta* 31.8 1.8
the case, for instance, in the Temperature Survey of Atlas
Mississippi (typical)* 29 1.6
Wireline (Figure 3.4).
Louisiana (typical}* 24 1.3
The absolute accuracy of temperature logs is generally
Eastern Canada* 22 1.2
not good and is in the region of +2.5°C (15°F),

*typical oil regions although the resolution is good, around 0.025°C (0.05°F)
(Hill, 1990),
the borehole. Up until the mid 1980s, bottom hole
temperature (BHT) was most commonly measured by
3.3 True formation temperatures
strapping to the tool, usually three, pressure resistant,
(BHT corrections)
maximum thermometers. The three temperatures
(maxima) were noted on the log-head, and an average or The temperature measured in boteholes is not the
the most reliable temperature used as the bottom hole formation temperature, it is the temperature of the mud in
temperature (BHT). Although this habit is still generally the borehole. Borehole mud is cooler than the formations
continued, in every tool string (at least from being drilled (apart from the immediate surface strata).
Schlumberger), a small, special sonde is included which Thus, the invasion of mud filtrate into a formation will
measures temperature continuously along with mud cool it down immediately by convection. As a result of
resistivity and cable tension. With this sonde (AMS), a the infinite mass around the borehole, the true formation

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