TEMPERATURE LOGGING As early as 1869 Lord Kelvin conducted ex- 

 periments in measuring earth temperatures at 

 a depth of 350 feet in the ground. Since then, geologists have speculated on 

 the geothermal gradient in the earth's crust. Even with such an early start, little 

 has been done in the way of quantitative work with earth-thermal measurements. 



At the present time thermal measurements in either a cased or open hole 

 are usually obtained by means of a continuous-recording, extremely accurate, 

 electronic thermometer. Such a tool is standard equipment on an electric- 

 logging truck and is run on a 5/16-inch conductor cable. 



The temperature electrode is a 3-inch rubber-covered tool about 6 feet long. 

 In a groove in the rubber coating of the electrode is a 20-inch length of platinum 

 wire, which is exposed to the mud column. This wire is small in diameter and 

 assumes the temperature of the fluid around it rapidly. Changes in temperature 

 produce changes in the resistance of the wire, which are detected by a bridge 

 circuit in the electrode. Alternating current from a 500-cycle generator is sup- 

 plied 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 cancel 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 

 20F to 280F and from 60F to 340F. 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 circulated 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 800 feet deep, as illustrated in Figure 16-4. 

 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 



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