Figure 164. Chart showing temperature gradients. 



same from top to bottom, as indicated by curve 2. The temperature gradients 

 cross at point A. 



If the well is left idle for several days, the temperatures will tend to 

 equalize and the mud curve will tend to rotate from 2 to 1 about the axis A. 



The cooling or warming of the mud at a certain depth will depend on the 

 thermal conductivity of the formations and the size of the hole. Experience has 

 shown that equilibrium is reached sooner opposite sands than shales. This 

 can be explained by the fact that (1) hole size is smaller opposite sands than 

 shales, and, therefore, the volume of the mud is less, and (2) the thermal con- 

 ductivity of sands is greater than that of shale. 



Thus it will be seen that during thermal evolution, sands will exhibit a 

 lesser temperature than adjacent shales in the top part of the hole and a higher 

 temperature in the bottom part. Curve 3 of Figure 16-4 represents the temper- 

 ature of mud about ten hours after circulation and illustrates this point. 



353 



