446 Subsurface Geologic Methods 



This wire is small in diameter and assumes the temperature of the fluid 

 around it rapidly. Changes in temperature produce changes in the resis- 

 tance of the wire, which are detected by a bridge circuit in the electrode. 

 Alternating current from a 500-cycle generator is supplied to the bridge 

 terminals. The signal terminals of the bridge are transformer-coupled 

 to the grid-cathode circuit of an a.c. amplifier circuit in the tool. The 

 amplified a.c. signal is rectified and sent to the surface as a d.c. signal, 

 where it is calibrated in degrees. In order to buck out the static value of 

 this signal, a known matched signal of opposite polarity is placed in 

 series with the electrode signal. The resultant d.c. signal is amplified in the 

 instrument tray and recorded in the camera. A switch is provided in the 

 tool for changing signal points at the a.c. bridge, so that the bridge can 

 always be operated close to the balance point. This is necessary for two 

 reasons, to reduce noise and to keep the electrode system from being 

 saturated. The resultant log comes as a plot of temperature versus depth. 



The standard electrode may be obtained in two temperature ranges, 

 from 20° F. to 280° F. and from 60° F. to 340° F. Tools capable of 

 being run in tubing are also made. 



A fundamental knowledge of temperature gradients and temperature 

 anomalies, as expressed in drilled holes, is necessary before the myriad 

 uses of temperature logs can be fully realized. 



Measurements made by a thermometer lowered in a drill hole give 

 the temperature of the drilling fluid. Unless the hole has not been cir- 

 culated for several weeks, the temperature of the mud is very different 

 from that of the formations. The mud is usually colder at the bottom 

 and hotter at the top of the hole than is the surrounding strata. Thus, 

 when circulation is stopped, the mud will warm up in the lower part of 

 the hole and cool at the top. The speed of this heat exchange will depend 

 on the lithology of the bore hole. 



To illustrate this, take a well 8000 feet deep, as illustrated in figure 

 210. The geothermal gradient can be represented by a straight line, as 

 shown in curve 1. The temperature of the mud, when circulation ceases, 

 is almost the 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 conductivity 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 



