444 Subsurface Geologic Methods 



can be set and successfully cemented without channeling and bridging. The 

 value of a caliper log in locating these zones has been demonstrated many 

 times. 



Modern drilling practices presuppose the use of many scratchers, 

 centralizers, and guides welded to the casing to assist in obtaining a 

 better cement job. All these tools have proved extremely useful in obtain- 

 ing better cementing jobs and eliminating costly squeeze jobs, but they 

 are useless unless properly positioned in the hole with the aid of a caliper 

 log. 



In plugging back with cement, plastics, or gravel packing, a knowl- 

 edge of hole size is invaluable. In remedial work of this kind, well records 

 are usually inaccurate or nonexistent, and it is only through an application 

 of the knowledge gained from a caliper log that a successful job may be 

 performed. 



In the field of drill-stem testing, a knowledge of caving conditions 

 has saved oil operators an untold amount of money. Figure 209 is a 

 caliper log of a well in Smith County, Texas, where the operator wished 

 to find a packer seat to drill-stem test the Paluxey sand at 7140 feet. A 

 glance at the log will show that, using trial-and-error methods, the chances 

 of successfully setting a packer are relatively small. By applying the 

 information to be gained from the use of a caliper log, the proper point 

 to set the packer and the proper size of packer are easily determined. 



Many other uses have been found for caliper logs. The successful 

 completion of a fishing job may depend on a knowledge of the size of the 

 hole above the junk. A log made after a fishing job is completed will pro- 

 vide the necessary data successfully to resume drilling operations. 



A knowledge of hole size is essential in evaluating an acidizing job, 

 picking a zone to side-wall-core, evaluating the results of shooting with 

 nitroglycerine, and finding a proper zone to gun-perforate. The present 

 caliper log has fulfilled all these functions. 



Temperature Logging 



As early as 1869 Lord Kelvin conducted experiments 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 quanti- 

 tative 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 five-sixteenth-inch conductor 

 cable. 



The temperature electrode is a three-inch rubber-covered tool about 

 six feet long. In a groove in the rubber coating of the electrode is a 

 twenty-inch length of platinum wire, which is exposed to the mud column. 



