a directional hole, and costs of installation and maintenance of pumping equip- 

 ment are less. 



The "kick off" depth, or depth at which deflection of the hole is to begin, 

 is chosen so that it is not necessary to increase the angular deflection of the hole 

 by more than 2y 2 degrees per 100 feet of hole. This depth is also such that the 

 minimum deflection angle necessary to hit the target can be used. 



It can be noted on the idealized well proposal plat, Figure 32-1, that both 

 plan and vertical views of the well are given, together with the coordinates of 

 the proposed bottom-hole target, and the true vertical depth and measured 

 depth at various points along the well course. The vertical section is also marked 

 with the points at which whipstocks are to be set or other deflection methods 

 used. As the well is being drilled, the true course of the well is plotted beside 

 the proposed course. Naturally there will be deviations from the proposed 

 course which are unavoidable in the actual drilling of the well. It may be more 

 practical, for instance, to set a whipstock a few feet higher or lower than planned 

 in order to avoid making a round trip to replace a bit which is too worn to 

 make the last few feet, or to avoid wasting the footage left on a bit which would 

 be too worn to send back into the hole after setting the whipstock. 



Other factors than the above also can cause minor deviations in the well 

 course. The resistance and dip of the formations, and the pressure or weight 

 on the bit must be taken into consideration. In order to set a limit on the allow- 

 able deviation, the directional-drilling proposal will usually set up a hypothetical 

 cylinder around the proposed well and specify that the actual well must not 

 deviate out of that cylinder. The radius of this allowable cylinder will vary with 

 field conditions, but it will usually be within 25 to 100 feet. Difficult surface 

 locations with other wells close by may require a smaller cylinder near the sur- 

 face, but allow a wider target at depth, whereas a small reservoir target may 

 require a cylinder which tapers to a small radius near the bottom of the hole. 



During the course of a well, the actual control of the directional drilling 

 is in the hands of a directional-drilling engineer employed by the service com- 

 pany which has contracted to supervise this phase of the drilling. The engineer, 

 and through him the service company, is responsible for the drilling of the well 

 within the limits called for in the drilling proposal. The engineer supervises 

 the settings of whipstocks, recommends the type and number of drill collars 

 and other weighting and stiffening members in the drill string, and he advises 

 the drilling crew concerning the speed of drilling and the weight placed on the 

 bit — all measures to insure that the well follows the plan intended. 



The steps involved in setting a whipstock and beginning the deviation 

 of a well from its previous course are pictured in Figure 32-2. The whipstock 

 itself is a rather long tapered piece of metal which has a sharpened or chisel- 

 like lower end. A special drill bit is attached to the whipstock by soft pins 

 which are sheared by pressure exerted on them as drilling begins. Before the 



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