568 



Subsurface Geologic Methods 



the drill collar, as is done with a drift indicator. Figure 289 illustrates 

 a "go devil" barrel in a K-Monel drill collar. 



At times a small single shot is used for surveying in open-hole run 

 on a sand line in the same manner as the regular single shot. In small- 

 diameter holes, such as the "rathole" made in drilling off a whipstock, 

 some operators consider it safer to use the small instrument. In exception- 

 ally hot wells an inside single shot is sometimes run in a regular (3J-inch) 

 single-shot barrel. Spacer rings position the smaller instrument in the 

 barrel. The air in the annular space between the single shot and the 

 barrel acts as an insulator. Thus, pictures have been taken when other- 

 wise it would have been impossible. Discs are similar to those used by 

 the regular single shot except that they are 1^ inches in diameter. 



The small single shot is a rental machine serviced at regular intervals 

 and run by the drilling crews themselves. 



Table 30 gives pertinent information on the small instrument, barrel, 

 and accessories. 



Most operators take single-shot readings at depth intervals not greater 

 than 100 feet. At critical points in drilling, in checking deflecting-tool 

 runs, and whenever the situation requires, pictures are taken at shorter 

 distances. After reading the discs for drift and direction, computations 



TABLE 30 

 Small Type Single Shot 



Diameter 



Length 



Weight 

 (lbs.) 



Instrument (12° and 20° units) 



Instrument (90° unit) 



Survey barrel 



Kit and supplies 



iy2in.0.D. 

 iy2in.0.D. 

 21/8 in. O.D. 

 7 in. X 7 in. 



23 in. 



25% in. 



8 ft.-17 ft. 



30% in. 



3Vi 



4 



80-170 



28 



are made for the vertical depth and deviation of each course. Rectangular 

 coordinates for each depth at which a reading was taken are derived by 

 use of the directions read from the discs. After vertical depths for each 

 measured depth have been obtained, a plan and vertical sections of the 

 course of the well may be plotted on cross-section paper. The advantage 

 of a single-shot survey is that the foregoing data can be obtained quickly. 

 If the course of the well is not correct, measures can be taken immediately 

 to deflect or straighten the well. The position of the well as it is being 

 drilled is information essential to the conduct of controlled directional- 

 drilling operations. Plans for future action affecting the course of the 

 well can be made from this information. Since "dog legs" in a well are 

 usually a combination of both change in drift and change in direction, 

 the amount of "dog leg" can be ascertained quickly when the single-shot 

 readings are available. 



A type "A" single-shot instrument is in process of manufacture at 

 the present time. It is a further development in which the most desirable 



