548 Subsurface Geologic Methods 



A section of a well drilled for subsurface exploration from a shore- 

 line location to bottom under the Pacific Ocean is shown in figure 270. As 

 far as is known, this well deviates the greatest distance of any well ever 

 drilled. It maintained a drift of over 70 degrees from a 3,000-foot depth 

 to a 9,500-foot depth. With a measured depth of 12,127 feet, it deviates 

 horizontally 9,882 feet to bottom at a 5,772-foot vertical depth. Directional 

 drilling commenced at a 400-foot depth. The drift was increased at the 

 average rate of 3° 00' per hundred feet drilled. It was not possible to 

 survey the course of this well by using a single-shot run on sand line. Be- 

 cause of the extreme angle of the well, 15 magnetic surveys were run 

 on drill pipe to determine that the course of the well was as originally 

 proposed. 



Work-over operations on old wells usually are accompanied by direc- 

 tional work of some kind. Collapsed casing or liners can be sidetracked 

 with casing whipstocks and subsequent slant-hole drilling of the bottom 

 to a new, more productive location. 



Wells that have become unprofitable from edge-water encroachment 

 are deflected in their lower portions upstructure to result in productive 

 wells. 



Surveys of all wells producing on a lease often show incorrect well 

 spacing, allowing the drilling of one or more slant holes to attain the 

 greatest yield from a lease. 



It is expected that directional-drilling practices, now well established, 

 will assist drillers of wells even more in the future than in the past. The 

 oil industry has found that deflected wells have made it possible to de- 

 velop oil resources that might otherwise never have been developed. 



OIL-WELL SURVEYING 

 J. B. MURDOCH, JR. 



Initial interest in the surveying of bore holes began long before the 

 turn of the century in connection with deep exploratory diamond-drilling 

 operations in South Africa. It became increasingly evident that holes 

 started straight at the surface did not continue to be vertical as the bit 

 penetrated steeply dipping beds of diff"erent hardness. Geologic correla- 

 tions often showed improbable changes in the formations in holes drilled 

 in close proximity at the surface. Early attempts at measuring the angle 

 from the vertical assumed by holes were made with acid bottles. A small 

 amount of a dilute solution of hydrofluoric acid was poured into a glass 

 culture tube, which was lowered into the hole and allowed to remain 

 motionless for ten to fifteen minutes. A meniscus was etched in the tube, 

 from which an indication of the angle of the bore could be ascertained. 

 Another method was to float a magnetic needle in fluid gelatin and allow 

 it to remain at rest for a time. The drift (inclination from the vertical) 

 and the direction were obtained. When the gelatin hardened, the com- 



