rotations of the drill pipe gives the orientation of the reference mark on the 

 multiple-shot instrument at all times. 



The second solution to the orientation problem in cased wells is through 

 the use of a gyroscopic compass in place of a magnetic compass. This method 

 eliminates the necessity of measuring drill-pipe rotation. 



Multiple-shot surveys are customarily run by service company crews who 

 are trained to run the instrument. Unlike drift indicators and single-shot instru- 

 ments, multiple-shot instruments are not leased to drilling companies for opera- 

 tion by drilling crews. 



Frequently a multiple-shot survey is run at the completion of a well as a 

 double-check on the well course as determined by single-shot surveys taken 

 during the drilling. If the calculated course and position of the well from each 

 of the two surveys agree, the results of one of the surveys are taken as the true 

 course of the well. The accepted survey is frequently the single-shot survey, but 

 the engineer or geologist in charge may choose the multiple-shot survey if he 

 believes it to be more accurate. If the results of the two surveys are in wide dis- 

 agreement, another multiple-shot survey may be made of the well — frequently 

 by a different service company using a different instrument — before one survey 

 is accepted as correct. 



TECHNIQUES OF There are two basic types of directionally 



DIRECTIONAL DRILLING drilled wells: (1) after the maximum angle 



of deflection has been reached, that angle is 

 maintained to the bottom of the hole; or (2) the maximum angle of deflection 

 is maintained for a distance after it is attained, then the angle is decreased and 

 the hole becomes closer to vertical or essentially vertical near the bottom. The 

 techniques of drilling both types of wells are essentially the same except that 

 more deflection tools and more care must be used in drilling the latter type. 

 The normal sequence of steps in the drilling of a directional well will be out- 

 lined in the following paragraphs. 



A written well proposal is prepared first, outlining the anticipated course 

 of the well. The typical proposal includes the depth at which deflection of the 

 hole is to start, the rate at which angular deflection is to be built up, the bottom- 

 hole target for the well, and the maximum allowable deviation from the proposed 

 well course. The written proposal is accompanied by a plat, an example of which 

 is shown in Figure 32-1, which depicts in plan view and cross section the course 

 which the well is expected to take. 



It is best to drill vertically as much of the upper portion of the hole as 

 possible, consistent with the necessity of starting to deflect the hole at such a 

 depth that a uniform and gentle rate of angular increase may be maintained 

 to the bottom of the hole. Vertical hole is drilled more cheaply and faster than 



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