ments. Single-shot and multiple-shot instruments record both the amount and 

 direction of inclination, whereas drift indicators record only the deviation from 

 the vertical. 



Well proposals and drilling instructions for most of the straight holes drilled 

 at the present time require that the holes be drilled within a very few degrees 

 (generally 3 to 5) deviation from the vertical throughout the course of the well. 

 This requirement is necessary if the well is to reach the reservoir at the desired 

 location and if there are to be no dog-legs which would give trouble when pump- 

 ing equipment is installed. For this type of situation, the use of the drift indi- 

 cator gives all the information necessary during the drilling of the well to satisfy 

 the deviation requirements. 



The simplest type of modern drift indicator, which utilizes a sharp-pointed 

 plumb bob, is entirely mechanical in operation. As with most drift indicators 

 and single-shot instruments, this indicator is operated usually by the drilling 

 crew and does not require that a service company representative be present on 

 each run. When a reading is to be taken, a timing device on the instrument is 

 set for an interval long enough for the indicator to be run into the hole and to 

 become stationary before the actual reading is taken. The reading is taken by 

 means of a spring mechanism that forces a paper disc against the sharp point of 

 the plumb bob. This disc is marked with a number of concentric rings that are 

 calibrated to give the deviation in degrees from the vertical. Some models of 

 this type of instrument are arranged so that after the first reading is taken, the 

 disc is automatically rotated 180 degrees and a second reading is taken. The 

 second reading should be esssentially the same as the first; otherwise the in- 

 strument has been in motion while the readings were being taken, and the 

 readings are erroneous. Because of its simplicity and because of the safeguard 

 against erroneous readings, this type of drift indicator is the most commonly 

 used. 



A modification of the above instrument uses a disc of light-sensitive paper 

 upon which an image is made by a hollow plumb bob and a light source. There 

 is no provision for a second reading in this instrument, but the slow exposure 

 speed of the sensitized disc is an assurance against erroneous readings. No 

 image will appear on the disc if the plumb bob is in motion during the exposure. 



The single-shot instrument is generally run at intervals as the hole is being 

 drilled. In this way a check of the well's direction can be made about every 

 hundred feet, and correction can be made if the well is not going as planned. 

 As with the drift indicators, single-shot instruments are operated usually by the 

 drilling crew. Normally a record of the angle and direction of drift is made by 

 means of a simple camera which photographs on a disc of sensitized paper the 

 image of a skeleton-type plumb bob, together with the concentric circles repre- 

 senting degrees of drift and a compass card. The angle of drift is indicated by 

 the distance of the plumb-bob image from the center of the disc, while the com- 



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