Jökull - 01.12.1961, Page 43

5. TEMPERATURE MEASUREMENTS IN
BOREHOLES.
Borehole temperatures are measured primarily
for the purpose of inferring formation tempera-
tures. At stationary conditions in a non-flowing
borehole the measured data are simply equal
to the formation temperature. On the other
hand, complications arise from any flow of
fluids. Thus the drilling fluid induces instati-
onary conditions by cooling or heating the form-
ations around the hole. All measurements carried
out during or shortly after drilling are therefore
seriously affectecl. This effect will be cliscussed
below.
(a) Experimental technique.
Three types of temperature measuring devic-
es have been applied, viz. mercury maximum
thermometers, electric resistance thermometers
and thermometers of the vapour pressure type.
(i) Tlie maximum thermometer is a simple
and inexpensive device. It has several disacl-
vantages, however. First, in practice only mono-
tonously increasing temperature-depth relations
can be recorded satisfactorily. The use of high-
inertia thermometers for decreasing tempera-
ture is possible but is quite time consuming.
Second, no telerecording is possible. Third, al-
though the accuracy may be good the mercury
column may tend to drop under hoisting out
of the hole. Moreover, the device is slightly sensi-
tive to pressure. These disadvantages are quite
serious and the maximum thermometer is there-
fore not a satisfactory instrument.
(ii) The semiconductor resistance thermo-
meters, the thermistors, are instruments of a
high accuracy which can be used for telere-
cording. In routine work an accuracy of 0.1
°C is easily attainable. The accuracy can
be increased consiclerably by a careful repeated
calibration. Moreover, the thermistor responds
relatively rapidly to temperature changes.
The thermistor is an ideal instrument for
low-temperature boreholes. Experience indicates
that the telerecorcling system is reliable up to a
temperature of about 150° C. On the other
hand, the insulation of the cable and the tlier-
mistor-connection presents difficulties at higher
temperatures. It is hoped, however, that the
insulation difficulties may be resolved.
(iii) The temperature recorders basecl on the
vapour pressure system are yet the only reliable
instruments available for temperatures aliove
150° C. Present instruments are applicable up to
260° C. They are quite rugged devices but the
accuracy is only about one °C. Another
drawback is a relatively slow response. Several
minutes are required for equilibrium. In most
high-temperature work, however, this accuracy
and response tirne do not impose serious limit-
ations.
(b) The influence of the drilling fluid.
The transient temperature conditions induc-
ed by the drilling result mainly from the cool-
ing effect of the drilling fluid. This effect is
particularly great in modern rotary drilling
where circulations up to more than 100 metric
tons per liour are applied. The fluid is kept
at a relatively low temperature which in drill-
ing for natural steam may be as niuch as 150
°C below the formation temperature.
At the moment drilling is discontinued,
and the circulation comes to rest, the hore-
hole temperature will be equal to the tem-
perature of the fluid during drilling. The
recovery to the formation value is very slow
due to the thermal properties of the rock.
The relaxation time for a borehole of 1,000
meters, which may be completed in three to
four weeks is more than two months.
On the other hand, the formation tempera-
ture is of fundamental importance and esti-
mates of this quantity may be required dur-
ing drilling. In natural-heat work the de-
cision whether the drilling of a well should
be continued or not will in general depend
to a large degree on the temperature of the
formations penetrated. The question arises
whether thc formation temperature can be
estimated on the basis of an extrapolation of
transient ltorehole data measured durlng breaks
in the clrilling operation.
The extrapolation of the borehole data in-
volves two steps. Firstly, an estimation of the
temperature field in the formation at the dril-
ling is discontinued. This represents the initial
condition for the second step. Secondly, a com-
putation of master curves for the temperature
rise at the various formation temperatures pos-
sible. A matching of the computed curves and
a sufficient amount of observed data shoulcl
41