Page 31 - The Geological Interpretation of Well Logs

- TEMPERATURE LOGGING -

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Figure 3.2 Present-day North Sea geothermal gradients. temperature will be that at the eventual equilibrium: the
(After Comelius, 1975; Carstens and Finstad, 1981; Harper, mud and filtrate wil] be heated up to formation tempera-
1971). Taken from Cornford (1984). ture. This process can begin only when mud circulation
stops. Moreover, the process occurs through conduction
and is very slow. Examination of several temperature
SALT DOME
logging runs made at increasing time intervals after

drilling shows that equilibnhum may be established only

after months (Figure 3.5). Temperatures taken in bore-
clays (Tertiary) holes during drilling (as is usually the case) are therefore
consistently well below the rea! formation temperature.
To, correct BHT values, numerous methods have been
salt (Permian) devised (cf. Hermanrud and Shen, 1989). The modem
tendency is to mode] the thermal recovery of a well from

the available data but a generally accepted way of doing
clastics this does not exist and the results from presently available
(Carboniferous) methods show considerable variation (Hermanrud and
Shen, 1989). One of the ojder and still frequently applied

methods uses the Horner pijot (Fert) and Timko, 1972).
{SOTHERMS: interval 20°C
This method appears to be about as accurate as the
modelling methods.
Figure 3.3 The theoretical distribution of isotherms around
a salt dome indicated on a geological section: change in The Horner plot method relies on the concept of a
gradient is shown by isotherm spacing. (Redrawn from straight-line relationship between BHT and the log of
Evans, 1977). Ar/Ar + 14, where Af = time in hours since circulation
2}