856 GEOPHYSICAL WELL TESTING [Chap. U 



curve may be calculated. In this manner, IngersoU, et al.,^* estimated 

 that 30,000 years have elapsed since the last glacial period. 



4. Oil. Opinions were expressed in the earlier literature that chemical 

 processes associated with the formation of oil deposits (polymerization, and 

 the like) should have given rise to geothermal anomalies. However, no 

 definite evidence of such anomalies has .yet been found. Of particular 

 significance have been (a) the measurements in the Durban k poof^ where 

 no rock deformation is in evidence and where no temperature anomaly is 

 found, in spite of extensive oil accumulation, and (h) observations on salt 

 domes, whose geothermal reaction appears to be the same regardless of the 

 presence or absence of oil. Even in oil deposits which occur so near the 

 surface that they are mined by shafts, as in Pechelbronn,^^ there is no 

 evidence of a direct heat influence of the oil. On the other hand, it is 

 possible to determine the influx of oil into a well by a slight drop in the 

 temperature curve," which is probably caused by the expansion of the gas 

 dissolved in the oil. In drilling wells, the well is conditioned with a light 

 mud, and the survey is made several hours after circulation. In producing 

 wells, it is preferable to swab the well down and to take the temperature 

 run while it is filling up. 



5. Gases in subsurfac^e formations are generally confined under con- 

 siderable pressure. When they are tapped and liberated by a well, the 

 reduction in pressure produces a decrease in temperature in accordance 

 with the relation 



/ 273°C. Y 

 V273 + 0/ ' 



where c is a constant depending on the gas, AP is the difference in pressure, 

 and e is the temperature at which the gas escaped. As Fig. 11-26 shows, 

 the effect is quite noticeable and may readily amount to several degrees 

 (F.) drop in temperature. The location of the base of a gas formation is 

 of considerable practical importance for determining the depth at which 

 to set casing. If it is set too low, part or all of the oil formation may be 

 cased off; if too high, a well with a high gas-oil ratio, or even a practically 

 dry gas well, will result. In some areas, such as West Texas, temperature 

 well surveys for gas are an indispensable adjunct to electrical logging. 



6. The location of water in wells is likewise of importance in connection 

 with shutoff and cementation depth control. The interaction of formation 



" A.I.M.E. Tech. PubL No. 481, Feb., 1932. 

 "K. C. Heald, A.P.I. Proc. Bull. No. 205, 1930. 

 '^ Haas and Hoffmann, loc. cit. 



" W. J. Gillingham and W. B. Steward, Petroleum Engineer, 9(7), 52-55 (April, 

 1938), and 9(8), 84-92 (May, 1938). 



