1090 EXPLORATION GEOPHYSICS 



be correlated with the casing collars, which thereby provide another perma- 

 nent set of references. As a result, regardless of absolute depth, a tool or 

 device may later be placed in the hole at any specified short distance from 

 these known reference points. 



Apparatus. — The active part of the radioactive marker is a small brass capsule 

 containing 1/10 milligram of radium salt. The brass pellet is placed inside a steel 

 bullet which can be shot out of a sample-taker gun or a perforating gun. 



The detector for radioactive markers consists of a casing collar locator and a 

 radioactive marker detector sonde assembled together, to locate simultaneously the 

 position of the radioactive markers and the position of the casing collars. 



Field Operations. — Radioactive markers are usually placed immediately following 

 the electrical survey. If side-wall samples are required, the taking of cores and the 

 placing of radioactive markers can be accomplished during the same round trip in 

 the hole by means of a sample-taker gun loaded partly with markers and partly with 

 sample-taker bullets. 



For each zone to be perforated, it is advisable to place three or more markers 

 at intervals of about SO feet over a section that straddles the zone. 



During the placement of the markers an S.P. log is recorded, and the bullets are 

 fired as the gun travels past the desired placement points. The firing of the bullet 

 makes an indication on the log in the form of a sharp pip on the curve. (Figure 681.) 

 The position of these pips with respect to the recorded curve determines exactly where 

 the bullet lies in the formation. The placement of radioactive markers is a quick 

 operation. The necessary rig time, according to the depth, may vary from one to two 

 hours. 



The later location of the radioactive markers is a separate operation, made usually 

 after casing has been set. It ordinarily involves the detection of the markers and their 

 correlation with the depths of the casing collars in their vicinity. The radioactive 

 marker detector has a low sensitivity, so that the natural gamma radiation from the 

 formation will have no measurable effect on the recording galvanometer. When the 

 detector passes by a marker in the hole, a sharp deflection of the galvanometer occurs 

 and a peak is recorded on a photographic record. (See location and collar log, on 

 right side of Figure 681.) 



It is important to verify that the intervals between the three or more markers 

 placed in a zone are the same as at placement, so as to make sure that the markers 

 have not moved. Since the positions of the radioactive markers and the positions 

 of the casing collars are recorded simultaneously, it is evident that the positions of 

 the collars are determined accurately with respect to the formations. The positions 

 of these collars may then be used as reference points for the accurate placement of 

 a perforating gun or other instrument. Like the placement of radioactive markers, 

 the location of radioactive markers and casing collars requires little rig time. 



Applications. — The use of radioactive markers is valuable where accuracy in 

 depth measurements is important, in particular: (1) production from specific zones 

 of an oil-bearing formation, and in known relation to any oil-gas or oil-water contact; 

 (2) increased efficiency in placement of perforations for cement squeeze jobs or tests; 

 and (3) testing and production from thin beds at great depths below the surface. 



Radioactive markers also provide permanent and consistent reference points for 

 all production problems throughout the life of a well, regardless of any change in 

 the equipment at the surface.f 



Paleontological Studies. — Paleontology is of course invaluable in 

 the recognition of markers. Paleontological markers, in addition, are in 



t H. G. Doll and H. F. Schwede, "Radioactive Markers in Oil Field Practice," A.I.M.E. Tech. 

 Pub. 2261 (1947). 



