This paper describes a method to provide identification of particular depths ina borehole through the use of radioactive markers. The correlation of a marker, placed in the wall of a borehole, with known points of the electrical log andwith the casing collars in the cased hole permits accurate positioning of toolswith respect to a formation, regardless of absolute depth. Such a process isparticularly useful in gun perforation of a casing in a well. Technique andequipment are discussed and illustrated. Examples are given of practicalapplication in the field.
During the past decade the search for petroleum has increased the importance oftesting zones at depths in wells which have become progressively deeper. Thedevelopment of new techniques, such as electrical logging, has permitted theidentification of producing formations which consist of comparatively thinstrata. When thin beds are to be produced at great depths, the problem ofpositioning tools accurately to place the well in production becomesacute.
Continuous consideration is being given in the petroleum industry towards theimprovement of depth measurements in a borehole. The accuracy in absolute depthmeasurements, whether made in an open or cased hole, whether determined by a cable or a drill pipe, depends upon a number of factors, such as tension, temperature, and calibration.
For example, the effects of tension from the weight of 10,000'Ft of drill pipeand the thermal expansion of this length of pipe, where its average temperaturehas increased 50'F, will produce an elongation as much as 8 ft. The effects ofsuch factors can be minimized through the use of care in depth measurements, the application of corrections based on experience, and continuous calibrationor checking.
The fact that different methods and different tools are used to determine thedepths of formations in wells will sometimes give rise to a difference betweenmeasurements. It is evident that an important requirement in a well is theability to locate at will any subsurface point. While absolute depthmeasurements have improved in recent years, it is reassuring to have othermeans to verify, for example, that a casing is perforated at a particular placewith reference to a zone within a formation. Such a check on measurements maybe had by placing a reference marker at a point known with respect to theelectrical log of the borehole. That point is usually chosen to be in proximityto the zone to be perforated. Thus, only short relative depth measurements aremade and any inaccuracy becomes very small and of minor importance.
T.P. 2261
