588 Subsurface Geologic Methods 



hole below the "fish." Needless to say, this is a delicate operation depend- 

 ing upon a good survey. 



Single-shot surveys are made at frequent intervals on all directional- 

 drilling operations. The exajct course of the hole must be known at all 

 times as drilling proceeds, since plans for the use of deflecting tools, a 

 change in drilling setups, and the like are dependent on knowledge of the 

 present position of the well and its relation to the objective. In high- 

 angle-deflection work, the difference between the measured and vertical 

 depths of a well is often hundreds or thousands of feet. 



Legal aspects of drilling are such that wells near the boundary of 

 a lease or on very small leases or strips must be surveyed to assure the 

 adjoining property owner that none of the course of the hole trespasses 

 upon his property. Costly lawsuits, court-order surveys, the loss of pro- 

 duction, and the possible redrilling of a well to keep it on the correct 

 property are eliminated by surveying as the hole is made. Under existing 

 laws some wells on narrow strips and very small leases never could have 

 been drilled without benefit of surveying. 



Some operators prefer to use a drift indicator while drilling, running 

 a magnetic multiple-shot survey of the drilled hole just previous to setting 

 casing. Often an oriented survey is made after the casing is cemented. 

 Such surveying consumes less rig time than if a single shot is used as 

 the drilling proceeds and is, therefore, less costly to the operator. How- 

 ever, the information is available after the hole has been made; hence, 

 any remedial operations taken will be costly. Such surveys serve as a 

 permanent record for future geologic and engineering reference. 



Legally one of the advantages of a multiple-shot survey is that the 

 information on the whole well is obtained in one run. Since machines used 

 for this purpose are run by service-company employees only, they are 

 able to attest to the authenticity of the survey record. Single-shot surveys 

 necessarily are made by the driller and crew on tour at the time the hole 

 was drilled. No one man could give an affidavit as to the accuracy of the 

 survey. Both oriented and magnetic multiple-shot surveys have been 

 accepted by courts dealing with subsurface trespass cases. 



Positive assurance that the subsurface portion of the well is on the 

 lease is given if a well is checked with a drift indicator while being 

 drilled and surveyed before being put on production. 



Many wells drilled and producing for years are being surveyed when- 

 ever the necessity of redrilling is indicated by loss of production or 

 water encroachment. Operators often are surprised by the crookedness 

 of older wells; some have been surveyed which attained drift angles over 

 45 degrees. The reason some wells are water cut and others always have 

 been poor producers often is self-evident from the surveys. At times two 

 well have been found that have been bottomed within twenty or thirty feet 

 of each other. The survey, plotted on a map of the subsurface structure 

 along with the surveys of other wells on the lease, often reveals the desir- 



