586 Subsurface Geologic Methods 



them to productive formations or in exploring for oil near salt domes, 

 complete knowledge of the course of the well is essential. The more par- 

 ticular the type of formation encountered, the more necessary accurate 

 data of this kind become. Surveys of old wells have shown that wells 

 intersecting faults at acute angles have a tendency to continue in the 

 plane of the fault. Some were productive; others were not. In any 

 event, they usually wandered far from the bottom-hole location originally 

 intended by the geologist. 



If a single shot is used, the wandering course of the hole is known 

 as the well is drilled. Inspection of the survey plot gives ample time to 

 correct the tendency of a well to wander from the course intended. If it 

 is found impossible or impractical because of cost to straighten the course 

 of the well by natural means, a deflecting tool may be set to correct it. 

 Some operators will permit the well to wander (provided no "dog legs" 

 are in the hole) within limits until it nears the producing zone, when a 

 directional-drilling crew is called and the bottom section of the well is 

 deflected to the point desired. This has often been found to effect a con- 

 siderable saving of rig time as compared to drilling slowly to keep the 

 hole very nearly vertical. A well rarely is bottomed directly under the 

 derrick unless it is deliberately completed there by controlled directional 

 drilling. Unless wells are surveyed to determine their positive under- 

 ground location, it would appear that geologic and engineering conclu- 

 sions based on surface locations and spacings will be in error in some 

 ratio similar to the deviation of the wells. 



An acceptable bottom-hole-spacing plan sometimes can be made, 

 even after some of the wells on a lease have been drilled and are produc- 

 ing. By surveying the producers and plotting their bottom locations, new 

 wells may be drilled to provide correct drainage. This can be done by 

 directional drilling or by drilling nearly vertical wells at the appropriate 

 surface locations, in which case the surface appearance of the lease will 

 not be uniform. The pattern of wells at the oil sand will be as desired, 

 however, which is the important consideration. 



The collision of a drilling well with a neighboring well on the same 

 lease or on a nearby lease has sometimes occurred. The bit has at times 

 cut the casing of a producing well and caused expensive repairs and 

 loss of production while the work was in progress. Delay in drilling, 

 the running of a cement plug, and the sidetracking of the new hole 

 materially increased the cost of the newer well. At times a drilling well 

 has mudded-off" another well, damaging the older well and necessitating 

 the redrilling of a considerable portion of the new well to make a satis- 

 factory producer. Production of wells in low-gas-pressure sands has been 

 impaired permanently in this way, and a few of the wells have been 

 abandoned. Surveys of both wells concerned would have prevented such 

 costly errors. 



From the standpoint of the geologist, whose work it is to take the 



