524 



Subsurface Geologic Methods 



carried out are very important. The engineer must have an idea of the 

 productivity of the formation, as well as laws and regulations controlling 

 bottom-hole spacing. Directional wells should be planned to take advan- 

 tage of multiple-zone completions, possible further deepening to lower 

 zones, or the perforating of casing at a shallower depth. For instance, 

 when geologic conditions are such that a directional well must penetrate 

 a number of oil sands all lying close to a salt dome, a proposal can be 



Figure 253. Bottom-hole-orientation reader 

 and disc in place. 



drawn so that the well will penetrate the zones parallel to the side of the 

 salt dome. This can be accomplished by allowing the well to become 

 vertical before entering the first zone, or by directing the course of the 

 well in a plane parallel to the side of the salt dome. The dip and strike 

 of the formation and its physical properties affect the drilling of a direc- 

 tional well. Usually wells drilled in a direction normal to the strike will 

 be the easiest to control, while those which parallel the strike will cause 

 the greatest difficuty. 



The proper starting point for the deflection depends upon the forma- 

 tion characteristics, the maximum drift angle selected, and the rate of 

 increasing the drift angle. The foregoing factors also are governed by the 

 total vertical depth to the oil sand and the horizontal deflection required. 

 The well proposal must be laid out so that the rate of increase of angle will 

 be feasible in the formation to be drilled. An average rate used in many 

 instances is 2° 30' to 3° 00' per 100 feet of hole drilled. Some directional- 

 drilling work has required an increase in the drift angle of from 6° to 8° 

 per 100 feet until a maximum drift angle from 60° to 80° has been 

 attained. The best practice is to increase the angle at a lower rate and 

 drill a little more directional hole to reach the objective. Maintaining a 



