1064 EXPLORATION GEOPHYSICS 



The technique of intermittent readings has not been commercially 

 applied, however, because of the length of time necessary for the well to 

 reach thermal equilibrium and because of the relatively few points which 

 can be read in a reasonable period of time. 



Direct Temperature Measurements. — Direct temperature measure- 

 ments are usually conducted with the aid of an electrical resistance ther- 

 mometer which is suspended in the drill hole by a two-wire insulated cable. 

 The thermal element of the resistance thermometer is composed of materials 

 whose resistances change rapidly with the temperature. In commercial 

 work, suitable materials are iron, nickel, silver, and certain alloys which 

 have high temperature coefficients. 



When employing D.C. measuring or recording methods, the apparatus 

 consists essentially of a small size wire wound on a cylindrical tube of 

 small diameter in a single spaced layer. Because the electrical resistance 

 of this unit is very high, changes in the cable resistance due to temperature 

 changes constitute a relatively small part of the total change in circuit 

 resistance. The thermal unit may be housed in an oil-filled copper container 

 with a small separation between the winding and the walls of the container 

 to minimize thermal lag. Any suitable recorder may be employed. The 

 speed at which the instrument is lowered into the drill hole will be depend- 

 ent primarily upon the response speed of the thermal unit. A surveying 

 speed of 2000 to 3000 feet per hour is usually possible. Measurements 

 must be made going down in the hole in order to avoid disturbing the mud 

 with its perturbing effect on temperatures. Should a second run be required, 

 it usually is necessary to wait from 6 to 12 hours in order that the original 

 temperature conditions may have time to reestablish themselves. 



Measurements Made After Thermal Equilibrium 

 Has Been Established 



In this method, a graph is made of depth versus temperature along 

 a drill hole which has been undisturbed for a sufficiently long time to 

 allow the hole and surrounding formations to be in thermal equilibrium. 

 Because of differences in thermal conductivity and temperature, the 

 gradient will tend to be different for different formations, each gradient 

 change corresponding to the boundary between the major thermal zones. 

 The general form of the temperature curve after thermal equilibrium has 

 been reached is illustrated in Figure 658. 



Probably the most successful application of temperature measurements 

 has been the location of cement behind casing. The method t depends 

 on the heat produced by the chemical reactions involved in the "setting" 

 of the cement. It has been found that the temperature of the cement. 



t C. Schlumberger, "Thermometric Method of Locating the Top of Cement Behind a Well 

 Casing," U. S. Patent 2,050,128, issued Aug. 4, 1936. 



