If a temperature record is obtained immediately after circulation, a flat 

 curve resembling curve 2 of Figure 16-4 will result. The same type of curve 

 will be obtained after several days have passed and equilibrium has been reached. 

 The most pronounced anomalies are obtained 24 to 36 hours after circulation has 

 ceased. 



Although from the foregoing discussion one can see that it is possible to 

 distinguish sands from shales, the temperature log will not yet replace the 

 electric log. Factors such as chemical reaction, changes in hole size, the move- 

 ment of fluids between sands, and the movement of hydrocarbons can alter the 

 temperature in a well. 



Thus far only open-hole-temperature measurements have been considered, 

 but the presence of a string of casing does not disturb the thermic state of a 

 bore hole. Therefore, all of this previous discussion applies to cased holes when 

 they have been cemented, provided the temperature log is made four or five 

 days after cementing. 



Cement generates considerable heat as it sets up, and this factor has 

 resulted in the principal application of temperature logs — the determination 

 of a cement top behind casing by means of thermal measurements. The mag- 

 nitude of this temperature increase varies with the time elapsed since cementing 

 and the quantity of cement used. 



Most of the heat is generated a few hours after the cement job has been 

 completed. After this interval it is quite possible that the formation will absorb 

 heat faster than it is generated by the setting cement, thereby cooling the mud. 

 As a rule the best time to run a temperature survey is from four to eighteen 

 hours after the cementing plug has hit bottom. The exact interval depends on a 

 number of factors. Most operators prefer not to release the pipe pressure until 

 after the initial set of the cement, which is a function of the type of cement, the 

 type of water, the temperature, the pressure, and other variables. 



The quantity of cement also affects the magnitude of a temperature anomaly. 

 The amount of deflection tends to vary as does the amount of cement, which is 

 a function of hole size. The joint use of a caliper log when trying to interpret 

 a temperature log is often useful in this respect. 



Several precautions should be taken when obtaining temperature logs in 

 a cased hole. Circulation after cementing has been completed, results in the 

 heat evolved by setting cement being dissipated, and a trustworthy record is not 

 obtained. 



Temperature surveys in their present form will tell how high cement is in 

 the annular space and, comparatively, how much cement is behind pipe. How- 

 ever, the survey does not indicate where the cement is distributed. It is, for all 

 practical purposes, impossible to detect channeling on a cement job by means 

 of a temperature survey. 



354 



