Miscellaneous Subsurface Methods 525 



very low drift angle or an unnecessarily high angle is not recommended. 

 Drift angles from 15° to 45° have been successfully maintained for thou- 

 sands of feet. It is within this range that most economical directional 

 drilling is accomplished. Very low angles definitely increase the cost of 

 a well because of the greater amount of directional hole which has to be 

 drilled and the difficulty of controlling the direction of a low-angle hole 

 during drilling. Extremely high-angle wells present special difficulties 

 in logging, surveying, and running casing. The depth at which the deflec- 

 tion work is started materially affects the cost. If a reasonably high angle 

 is used, the directional work can be started deeper, and more straight 

 hole may be drilled first. The formations to be drilled and their rates of 

 penetration should be studied in order to select the best zone in which 

 to do directional drilling. Whenever possible deflecting tools should be 

 set in formations which are not badly fractured. The starting point, the 

 bottom depth, the desired deflection, and the drift angle chosen all bear 

 a definite relationship to one another. 



A directionally drilled well is not a "crooked hole" but is a directed 

 well wherein all bends are controlled to stay within safe limits. Frequent 

 causes of mechanical trouble in both vertical and directed wells are ex- 

 cessive "dog-legs" and unnecessary wandering of the well course. A 

 "dog-leg" may be eitlier a change in drift or a change in direction or a 

 combination of both. Calculated graphically, "dog-legs" are expressed 

 in degrees for a certain specific length of well. Different formations vary 

 in their tendency to keyseat. It would not be practical to attempt to de- 

 termine the maximum permissible "dog-leg" for each formation; there- 

 fore, a safe angle is chosen which will suit the requirements for efficient 

 drilling and production. Recommended practice is to limit the average 

 increase in drift to 2° 30' to 3° 00' per 100 feet drilled, and to limit the 

 maximum "dog-leg" caused by deflection tools to 3° 00' in any 50-foot 

 section and 5° 00' in any 100-foot section of well bore. 



Directional wells are of two general types. In one, the drift angle 

 is increased at a uniform rate to the desired maximum deflection angle, 

 which is maintained until the oil zone is reached. In the second type, the 

 angle is increased at a uniform rate and maintained until the desired 

 deflection is obtained, at which point the well is brought back to vertical 

 at a uniform rate of decrease in angle. These two basic types of wells 

 are shown in figure 254. The first type of well generally is easier and 

 less expensive to drill. It also has the advantage that the oil zone is pene- 

 trated at an angle, increasing the exposure of sand in the producing zone. 

 The second type of well is used in situations where deeper horizons are 

 to be explored or in drilling when a well is to be bottomed very near a 

 lease boundary or along salt domes. Owing to the fact that the straighten- 

 ing process in the deeper portion of the well is often a slow process, this 

 type of well generally is more costly. 



The casing program for a directional well will vary only slightly 



