Miscellaneous Subsurface Methods 539 



dome. Slant-drilled from D, the well was converted from a dry hole to a 

 producer. Well ^ is a successful but expensive vertical well. The cap rock 

 and salt or mineralized sand, through which it was necessary to drill, pre- 

 sented costly difficulties. Some cap rock is very hard and makes the drilling 

 of such a hole very costly owing to the time and bits consumed in pen- 

 etrating this formation. The mineralized sand is very porous and full of 

 cavities, which often results in loss of returns from circulation. Location 

 of well E permits the well to be drilled in better formation and directed 

 under the overhang. When geologic conditions are such that a directional 

 well must penetrate a number of oil zones all lying close to a salt dome, 

 it can be directed so that it will penetrate the zones parallel to the side of 

 the salt dome. It may be allowed to become vertical before entering the 

 first zone, or its course can be made parallel to the side of the dome. The 

 last method permits penetration of a number of horizons at points equi- 

 distant from the face of the salt dome without the necessity of changing 

 the drift of the directed well. 



Since the subsurface geology may be very complex near salt domes, 

 it is not uncommon to redrill a well three or four times in exploring for 

 producing formations. A well will be drilled, taking into consideration 

 the best information available. If it penetrates salt, it is plugged back and 

 directed to a new location, possibly again striking the salt. If this occurs, 

 the well is redirected to a point thought to be more favorable from the 

 geologic information previously obtained. Much exploratory work of this 

 kind has been done in the Gulf Coast area with the assistance of directional 

 drilling. 



Adverse surface topography has caused the adoption of directed drill- 

 ing. An operator in developing a mountainous lease must balance the 

 cost of access roads for wells drilled vertically against the cost of segregat- 

 ing surface locations at conventient locations and resorting to directional 

 drilling to space their bottoms correctly. 



It was found that, on a number of leases in Ventura and Santa Bar- 

 bara counties in California, the cost of building and maintaining roads for 

 the life of the wells (say 25 years), together with the expense of laying 

 oil lines to each of them over the rough terrain, was much greater than 

 development by slant drilling. Figure 265 is a typical example. Wells 

 have been grouped in valley locations for ease of access and convenience 

 in drilling and servicing. The courses of the wells were directed to correct 

 bottom-hole spacing in the oil-bearing sand. 



The development of oil resources under tidal swamps and bayous 

 presents situations in which it is impossible, or at best very expensive, 

 to drill vertical wells. The cost of piling and mat foundations is great, 

 and they are sometimes unsatisfactory. Directional wells as drilled on the 

 Gulf coast of Louisiana and Texas offer the solution to this problem. 



Directional wells permit the tapping of oil reservoirs beneath wide 

 rivers, lakes, and similar bodies of water. Wells directed under the Mis- 



