Miscellaneous Subsurface Methods 549 



pass was held in place and indicated magnetic north. The drift of the 

 hole was read from the level of the gelatin. 



When oil-well drillers felt the need for such survey information, the 

 development of devices for this purpose was accelerated and resulted in a 

 number of types and makes of well-surveying instruments now widely used. 



The three basic types of instruments in general use are drift indi- 

 cators, single-shot instruments, and multiple-shot instruments. 



Drift indicators record the angle of inclination from the vertical 

 (drift) of the well bore but do not indicate the direction of this inclina- 

 tion. Different methods are used to determine this information. Mechani- 

 cal, photographic, electro-chemical and fluid-operated devices are used 

 to record the drift in the well. These instruments give a permanent 

 record for each run. One of the mechanical types makes a double record 

 to assure the operator that the instrument was motionless at the time the 

 determination was made. 



Single-shot instruments record the drift and direction of the well bore 

 at the depth at which they are run. Most successful instruments of this 

 type are photographic. Mechanical survey instruments are being devel- 

 oped because of the increasing depth and consequent rise of temperature 

 in wells. One reading or "shot," as it is generally called, is taken on a 

 recording disc which can be preserved as a permanent record. 



Multiple-shot instruments take a number of readings of the drift and 

 direction of the bore as the machine is lowered into the well. This in- 

 formation usually is recorded upon photographic film. Pictures are taken 

 at predetermined time intervals by the making and breaking of electrical 

 contacts controlled by constant-running motors or clockwork within the 

 instrument itself. A complete survey of any well may be made in one 

 run, records being taken at intervals as desired. One type of multiple-shot 

 machine is run on conductor cable so that it may be controlled from the 

 surface. A contact on the surface-control panel regulates the photograph- 

 ing of the angle unit from which the reading is obtained. 



The direction of drift is often obtained with a magnetic compass when 

 the machine is used in open or uncased hole. Various methods are used 

 to determine the direction within the casing where the compass is in- 

 effective. Some instruments contain a gyroscope for this purpose. An 

 accepted, long-used method is to start the machine into the well faced in 

 a predetermined direction, after which the rotation of the drill pipe is 

 measured as stands are added. These pipe-rotation angles are added alge- 

 braically, and this correction is applied to the survey film as it is read. 

 This drill-pipe-orientation method has the advantage of allowing the 

 surveyor to compare the oriented and magnetic readings when a machine 

 with a compass is used in open hole. 



The drift indicator probably is the most extensively used well-sur- 

 veying instrument available today. As stated before, it records the drift 

 of the well at the particular depth at which it is run. Many drilling con- 



