Miscellaneous Subsurface Methods 551 



tracts specify that the angle of the well will at no point exceed 3 or 5 

 degrees of drift. This limitation helps to assure the well owner that no 

 sharp kinks or "dog legs" will render difficult the running of casing and 

 that mechanical difficulties will not be encountered in producing the well. 

 On a large lease where wells are spaced far apart, some assurance of 

 correct bottom-hole spacing is obtained. Where productive formations 

 are not dipping steeply and hence the bottoming of the well is not es- 

 pecially critical, the drift indicator gives as much information as is 

 necessary. Consistent use of the instrument at intervals of not over 100 

 feet warns the driller in order that the well may be straightened before 

 it has exceeded the contract limit. 



The Eastman Oil Well Survey Company manufactures two types of 



Figure 273. Type "W" 

 drift-indicator disc. 



drift-indicator instruments. The most popular kind is the type "W" self- 

 checking instrument (fig. 271). It is a mechanical machine which uses 

 no bulbs, batteries, or sensitized discs, 



A contact-timing watch is mounted in the bottom of the drift indi- 

 cator. It controls the movement of the disc cup containing the record 

 disc (fig. 273) by means of an ingenious arrangement of cams and 

 levers. A plumb bob suspended in a universal-joint mounting houses a 

 pointed marking stylus buffered by a spring. The plumb bob points 

 directly downward when it is motionless. 



To operate it, one unscrews the plumb-bob section from the watch 

 section. A soft paper disc, on which concentric rings representing de- 

 grees of drift have been printed, is pressed into the disc cup. The knurled 

 disc cup is rotated manually as far as possible in the direction of the 

 arrow and held in this position. A contact-timing watch of special con- 

 struction is both set and wound in one operation by turning the watch- 

 setting stem. The amount of time set is observed through a window in the 

 case. Functioning of the clockwork timing mechanism is checked by in- 

 spection through a second window. Enough time is set so that the instru- 

 ment will reach the bottom of the well before the time indicates zero. A 

 surface watch (fig. 274) is set at the same time for reference. The drift 

 indicator is screwed together, slid into the barrel, and run into the well. 

 When the time set has elapsed, the disc cup is lifted and turned 180 de- 

 grees in azimuth by a cam device. A tiny punch mark is made on the 



