858 



GEOPHYSICAL WELL TESTING 



[Chap. 11 



similarity of the temperature and the self-potential curves taken before 

 the casing was run. 



Close supervision of temperature conditions in a well is valuable in con- 

 nection with water encroachment problems. , It is frequently observed 

 that the temperature of oil flowing into a well remains fairly uniform until 

 the well turns to water; then a rapid increase in temperature of the flow 

 is noted. This may be explained on the basis of the difference in heat 

 conductivity of oil and water. The temperature of the latter is likely to 



be higher than that of the 

 -mm 20ohmm former but not felt Until the 



oil envelope separating it 

 from the well becomes too 

 thin. 



In shallow wells water 

 does not always produce an 

 increase in temperature ; de- 

 scending surface waters may 

 produce negative deflections 

 of the temperature-depth 

 curve. In areas of volcanic 

 activity circulation of ther- 

 mal waters may produce 

 unpredictable anomalies in 

 the records (see paragraph 

 1). Since water-bearing 

 beds are better heat conduc- 

 tors than dry formations, 

 depth-temperature curves 

 flatten at times near the sur- 

 face (increase in reciprocal 

 gradient) as the ground- 

 water level is approached.'* 

 In some instances, temperature curves indicate that water leaves a well 

 at one level and enters it at another. In such cases, virtually uniform 

 temperatures may be maintained for appreciable depth intervals. 



7. Cementation problems are successfully handled by well-temperature 

 surveys. To seal off water-bearing formations effectively, it is important 

 to know at what point behind the casing the cement ring begins (Fig. 

 11-29). There is an appreciable rise in temperature during thp setting 

 period of the cement. For most Portland cements the temperature rise, 



Temperature 



170' m' m°r 



Fig. 11-28. Temperature Anomalies of water 

 sands (after Guyod). (Note similarity of tem- 

 perature and self-potential curves.) 



^* Van Orstrand, Problems of Petroleum Geology, p. 999 (1934). 



