BORE-HOLE INVESTIGATIONS 1075 



because the normal temperatures of these formations are higher than 

 106°. This well will be discussed further in connection with the photo- 

 electric method. 



Photoelectric Method. — The photoelectric method locates water 

 flows by measurements based on variations in the translucency of the drill- 

 hole fluid. Usually, the static column standing in the hole will have clear 

 fluid in the upper part. Also, formation water entering a well is more 

 translucent than muddy water or oil. Hence, it is necessary that the well 

 fluid in the interval to be surveyed be made opaque by suitable condi- 

 tioning in order to offer a contrast with the clear formation water that 

 will enter when the fluid level is reduced by bailing or swabbing. 



Conditioning of the well is done by one of two methods. In flowing 

 wells, it usually is necessary to stop the flow and circulate a well-mixed 

 muddy water. This usually gives the required opaqueness. In pumping 

 wells, the interval to be surveyed can be conditioned by the use of a special 

 dump bailer that can be tripped either at the bottom or at any desired depth. 



The instrument for measuring the translucency of the liquid in the 

 hole is about 2" in diameter and 30" long and consists, essentially, of a 

 constant source of light, which is energized by dry cells, a selenium photo- 

 electric cell, and louvers through which the fluid circulates. As the instru- 

 ment is raised or lowered in the hole, the fluid passes between two windows 

 which define the path through which the light rays reach the selenium cell. 

 Variations in the translucency of the mud cause variations in the amount 

 of light reaching the photoelectric cell, with corresponding variations in 

 the current output. The cell is connected by means of an insulated cable 

 to a continuous recording meter at the surface. The recording film, which 

 is moved in synchronism with the cable, uses an arrangement similar to 

 that described for the resistivity logging method. The photoelectric log 

 is a plot of depth versus a parameter which is proportional to the trans- 

 lucency of the liquid passing through the instrument. 



The practical application and interpretation of the photoelectric data* 

 may be best illustrated by actual examples. The photoelectric data shown 

 in Figure 670 were obtained in a well in which mud was dumped through- 

 out the open hole. A conditioning run with the instrument gave curve 

 number 1. The linear character of the curve showed that all the fluid 

 in the zone that was being examined was opaque and would therefore 

 offer a contrast to any translucent formation water which might enter 

 the drill hole in that zone. Fluid was then bailed from the well and 

 subsequent runs (Nos. 2, 3, 4) indicated that translucent water was entering 

 from the bottom ten feet of the exposed formation. This determination 

 allowed the operators to confine the cementation to the water-bearing 

 zone, thus preventing the loss of oil resulting from cementation of a portion 

 of the oil-bearingf zone. 



* The analysis is comparable to that employed in the electrical resistivity drill 

 hole methods. 



