Miscellaneous Subsurface Methods 527 



from that which would be chosen for a straight hole. Bending stresses 

 set up in the casing by curvature of the hole are small; however, it is 

 good practice to increase the thickness of the casing throughout the curved 

 portion of the well bore to resist wear by the drill pipe. Some operators 

 use one grade better casing on a directional hole than they would in 

 a comparable straight hole. On very deep or difficult directional wells 

 it may be necessary to use a protective string of casing to safeguard the 

 upper portion of the hole. The constant use of wall reamers in directional 

 drilling keeps the well bore in good condition, and usually casing is run 

 without difficulty. Flush-joint casing is seldom used in slant holes, as it 

 has a tendency to stick. Some flush-joint liners have been run successfully 

 when drilling in the oil zone was done with oil as the circulating fluid. 



Having considered a number of varying factors in planning a direc- 

 tional well, definite decisions must be made and a proposal for opera- 

 tions drafted. Tables and charts similar to the one illustrated in figure 

 255 are used by engineers for this purpose. The chart shown gives the 

 vertical depth and deviation for a hole, the angle of which increases at 

 the uniform rate of 2° 30' per 100 feet of hole drilled. A short section 

 of vertical hole should be drilled from the surface to prevent future 

 difficulty in pumping the well. Whenever possible it is advisable to set 

 surface casing in vertical hole and start deflecting the well 100 feet or 

 so below the shoe. In some cases, where maximum deviation was desired 

 at a shallow depth, deflection has been started at 100 feet below the 

 surface. After a tentative starting point has been chosen, the average 

 deviation from this point to the objective is computed by considering the 

 vertical depth to the producing zone. This gives a rough approximation 

 of the maximum angle of the well. As the maximum angle will be at- 

 tained by increasing the angle at a uniform rate, the actual maximum 

 angle in the well will be somewhat greater than the average. It is advis- 

 able to assume two or three logical starting points and figure the maximum 

 angle for each. Other factors aff"ecting the drilling of the well may be 

 evaluated in relation to each of these starting points. The theoretical 

 course of the well is computed by means of special charts and plane 

 trigonometry after the starting point has been chosen. The measured 

 depth of the well course and depths of different formation markers are 

 obtained in this way. In fields where a considerable amount of directional 

 drilling has been done, it will be necessary to plot wells already drilled 

 and those proposed on the same plan and section to give assurance that 

 the course of the wells will not interfere or intersect. 



After the calculations have been completed and consideration has 

 been given to other wells in the vicinity, a drilling proposal is drawn to 

 scale (fig. 256) . A vertical section of the well is made, as well as a plan 

 view showing the course of the well from the surface to the objective. 

 The proposal is drawn on cross-section paper, so that a single-shot survey 

 of the well made as it is drilled may be plotted upon both the plan and 



