18 DEEP BOREHOLE SURVEYS AND PROBLEMS 



to new holes near those old holes which have been shattered 

 at their base by time charges to increase the yields as was 

 first done in the Pennsylvania oil field. 



/, Fissured Strata.'^ — These may direct a borehole in any 

 direction. 



g. Pressure on the Rods. — In many boreholes, particularly 

 in diamond drilling, the tool tends to turn against the dip 

 of the strata and this is greatly affected in the case of a 

 hole nearly meeting the strata plane, i.e., nearly flat strata 

 in vertical holes; that is to say, ''face on" in inclined holes. 

 Hydraulically fed drills in these cases are best, like the 

 Sullivan, which control the rod pressure and adapt it to 

 keep the crown pressure constant. Thus in soft strata 

 the water escape in the hydraulic cylinder being more 

 rapid the drill descends more quickly and vice versa in hard 

 strata. On the other hand, screw feed drill speeds are set 

 between fixed limits regardless of petrologic changes in the 

 hole. In harder strata greater pressure on the rods tends 

 to produce a screwlike action. 



h. Reduction of Borehole Diameter. — The necessary peri- 

 odical changes in diameter to lessen the weight on the engine 

 and crown no doubt affect the plumbness of the hole. 

 The upper parts of the hole being wider allow the rods 

 more latitude, and the rods tend to curve by displacing 

 the center of the crown bit from the hole center. Alterna- 

 tions in hardness supplement this eccentricity. Longer 

 core barrels up to 50 ft. have in places been adopted to 

 ameliorate this tendency. 



i. Oversetting the Diamonds in the Crown. — It is considered 

 good practice to set the diamonds so that the hole is about 

 Ke in. more in diameter than the core barrel; i.e., M2-in. 

 projection for the diamonds over the crown. 



Any greater overset makes too much play between the 

 core barrel and hole or between drill rods and hole so tending 

 to set up lateral movement. 



j. Weak Core Barrels and Small Holes. — Weakness of the 

 barrel especially at the crown screw tends to twist the tool 



1 Dickinson, Joseph, F. G. S., Trans. Inst. Min. Eng., Vol. 35, p. 397. 



