Subsurface Logging Methods 



443 



balanced by means of a large rubber diaphragm, which acts as a volume 

 equalizer between the oil and the mud, compensating for the difference 

 caused by the motion of the push rods and by changes in temperature. 



A constant direct current is supplied to the rheostat in the tool, and 

 the resultant potential drop across it is measured and recorded as a caliper 

 log. These logs take the form of a continuous galvanometer trace recorded 

 on film showing the average diameter of the bore hole, recorded as a func- 

 tion of depth. 



A study of numerous caliper logs soon leads one to the conclusion 

 that caving patterns exist. Certain generalizations may be made as to the 

 relative ability of rocks to stand up to bit size. These generalizations are 

 shown in graphic form in table 23. 



TABLE 23 

 Ability of Rocks to Stand Up to Bit Size 



Although similar drilling conditions and similar muds tend to stand- 

 ardize caliper logs, some astonishing long-range correlations may be 

 made by using them, even for wells drilled under entirely different cir- 

 cumstances. In the west Texas area, a number of horizons may be recog- 

 nized on caliper logs, from Upton County, Texas, to the Hobbs field. 

 Lea County, New Mexico, a distance of 120 miles. Here caliper-log cor- 

 relations are more trustworthy than those made with electric logs. 



The most obvious use for the caliper log in the oil industry is as a 

 tool to calculate the proper amount of cement necessary to fill up the 

 annular space between the casing and the open hole to a desired point. 

 The actual amount of cement necessary for a desired fill is often two or 

 three times the amount one would use from theoretical calculations. 

 Figure 208 is an actual caliper log of a well in Smith County, Texas. The 

 hole was drilled with an 8f-inch bit, with S^-inch casing set on bottom. 

 Theoretical fill-up is 22 sacks of cement per 100 feet of hole. For more 

 than 1000 feet of section 220 sacks of cement is the theoretical amount 

 necessary to fill back to 9500 feet. The actual amount of cement needed 

 is 544.4 sacks, or more than twice the theoretical quantity. 



Another problem encountered in the successful completion of an oil 

 well is the location and reaming of tight spots in the hole, so that casing 



