Subsurface Logging Methods 449 



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 determina- 

 tion of a cement top behind casing by means of thermal measurements. 

 The magnitude 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 forma- 

 tion 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 oper- 

 ators prefer not to release the pipe pressure until after the initial set of 

 the cement, which is a function of the type of cement, tlie 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. However, the survey does not indicate where the cement is. It is, 

 for all practical purposes, impossible to detect channeling on a cement 

 job by means of a temperature survey. 



Numerous other uses have been made of temperature records in both 

 cased and uncased holes. As gas enters a well, either through a hole in 

 the casing or into a bore hole on an uncased well, it expands and cools. 

 This characteristic has been used to find oil-gas contacts or a hole in the 

 casing. However, any use of a temperature log presupposes that the tem- 

 perature anomalies being measured are of a greater magnitude than four 

 or five degrees, these being the order of formation thermal differences. 



WELL LOGGING BY DRILLING-MUD AND CUTTINGS ANALYSIS 



ARTHUR LANGTON 

 Today the determination of the fluid content of the porous forma- 

 tions penetrated by the bit so that no oil- or gas-bearing formations will 

 be overlooked constitutes one of the most important problems in drilling 

 for oil and gas. Modern mud-analysis, well-logging equipment, and im- 



