BORE-HOLE INVESTIGATIONS 1089 



track corresponding to a cross-sectional hole area of 250 square inches. The recording 

 is usually made on two film tracks, the first trace indicating hole diameters up to 25.2 

 inches. When this trace goes off-scale on the right side of the second film track, 

 another trace appears which indicates larger hole sizes up to about 32 inches of 

 diameter. 



It is interesting to observe that in the figure a correlation can be made between 

 the electrical log and the section gauge log. It is frequently observed, when the for- 

 mations are not too firmly consolidated, that the shale sections will wash out, leaving 

 an enlarged hole, whereas the hole through the sand sections is likely to more nearly 

 maintain its gauge. 



Evaluation of Hole Volume from the Section Gauge Log. — It is commonly 

 desired to know the volume in cubic feet of the hole between specified depths in order 

 to determine the cement required for cementing the casing. This data may be obtained 

 by measuring the area with a planimeter, and then using the proper factors which 

 depend upon the scale value and the depth scale of the recorded log. 



Field Operations with the Section Gauge. — Section gauge measurements are 

 performed with conventional electrical surveying trucks equipped with multiconductor 

 cable and a photographic recorder. The length of the unit is 8' 7", its diameter (without 

 springs) is 3^", and its weight is 133 pounds. The maximum diameter of expansion 

 of the springs is 36". Before the survey, the calibration of the instrument is checked 

 by placing a calibrating ring over the springs. The recording is generally made coming 

 up-hole, with a surveying speed of 60 to 100 feet per minute. 



Devices have been proposed by Baysf and Johnson^ wherein the primary electrical 

 log is automatically corrected for variations in hole diameter as determined by calipers, 

 or for variations in well fluid resistivity by means of auxilliary conductivity electrodes. 



RADIOACTIVE MARKER BULLETS 



With the increasing depths of oil wells, the problem of accurate depth 

 measurements becomes more important. The accuracy in absolute depth 

 measurements, whether made in an open hole or a cased hole, and whether 

 determined by a cable or a drill pipe, depends upon a number of factors, 

 such as tension, temperature, and calibration. Errors due to these factors 

 can be minimized by carefulness in actual measurements of depth and the 

 application of corrections based on experience and frequent calibration or 

 checking. 



Although the accuracy of depth measurements has improved in recent 

 years, other means may be employed to verify, for example, that a casing 

 is perforated at a particular place with reference to a certain producing 

 zone. Such a reference point may be a radioactive bullet shot into the 

 formations from a sample-taker or a perforating gun.§ The bullet contains 

 a radium salt and is therefore a compact source of gamma rays. These 

 rays are able to pass through several inches of cement and steel, and can 

 be detected easily by suitable equipment. The location of the radioactive 

 marker, after casing has been set, identifies the depth of a point in the hole 

 whose position is known with reference to the formations. That point may 



t G. S. Bays, "Well Logging," U. S. Patent 2,392,357, Jan. 8, 1946. 



t E. A. Johnson, "Method and Apparatus for Logging Wells," U. S. Patent 2,415,636, Feb. 11, 

 1947. 



§ H. G. Doll and H. F. Schwede, "Radioactive Markers in Oil-Field Practice," A.I.M.E. Petro- 

 leum Technology, Vol. 174, 1948. 



