1066 



EXPLORATION GEOPHYSICS 



Measurements Made Before Thermal Equilibrium 

 is Established 



This method, which is more commonly used than the one described 

 in the last section, utilizes the evolution or change in the thermal gradient, 

 measurements being made before the well is in equihbrium. 



In practice, the mud of the well is circulated for several hours before 

 the survey. During this circulation, the mud from the surface is forced 

 down through the drill pipe and returns upward between the pipe and the 

 formations, t At the beginning of this cycle, the mud is cooler than the 



TEMPERATUflE ~ °F 



Fig. 600. — Evolution of temperature gradient. (After 

 Leonardon, Geophysics.) 



MORMAL 

 TEMPERA TURE 

 QRADJEMT 



TEMPERATURE 



Fig. 661. — Temperature curve illustra- 

 ting change in thermal gradient at gas 

 and water sands. 



normal formation temperature at the bottom of the hole and warmer than 

 the formation temperature near the surface. During circulation, there- 

 fore, the formations near the bottom of the hole give up heat to the mud, 

 and the formations near the surface absorb heat. Finally, when circulation 

 is stopped, the temperature curve for the mud in the hole is approximately 

 a straight line, having temperature values which are lower than the 

 temperatures of the formations in the lower portion of the hole and higher 



t E. G. Leonardon, "Thermometric Measurements in Drill Holes," Geophysics, Vol. I, 1936, 

 pp. 115-126. 



