Miscellaneous Subsurface Methods 679 



lower electrode and with the lower half of the body of the assembly, and 

 oil is in contact with the upper electrode and the upper part of the body. 

 As acid is a conductor of electricity, the electric circuit is completed be- 

 tween the lower electrode and the body assembly. Current now flows in 

 the circuit from the battery to the top electrode, through the resistance to 

 the bottom electrode, through the acid to the body of the assembly, to the 

 ammeter, and back to the battery. A definite amount of current is now 

 flowing in the circuit, and the ammeter will show about a one-half-scale 

 deflection. 



In the right-hand drawing of figure 366, only acid is in contact with 

 the assembly. In this case the electric circuit is completed between both 

 the lower and the upper electrodes and the body of the assembly. The 

 current now flows in the circuit from the battery to the top electrode, 

 through the acid to the body of the assembly, to the ammeter, and back 

 to the battery. As the resistance between the two electrodes now has been 

 shorted out, considerably more current will flow than was possible with 

 the circuit as shown in the center illustration. The ammeter now will indi- 

 cate practically a full-scale deflection. 



The illustrations in figure 366 are analogous to what actually hap- 

 pens during a well treatment. The engineer observes an electrical instru- 

 ment during the treatment, and when this instrument reads zero, the engi- 

 neer knows that the acid is below the bottom electrode. When the instru- 

 ment gives approximately a half -scale deflection, the engineer knows that 

 the acid level is between the bottom and top electrodes. When the instru- 

 ment gives a full-scale deflection, the engineer knows that the acid is at 

 or above the top electrode. The relative acid- and oil-pumping rates em- 

 ployed during the treatment are then determined by the readings on the 

 electrical instrument. If the acid level is too high, as indicated by a full- 

 scale deflection on the instrument, either the acid pump will be slowed 

 down or the oil pump speeded up or both. If the acid level is too low, as 

 indicated by a zero reading on the electrical instrument, the acid pump 

 will be speeded up or the oil pump slowed down or both. 



Selective Acidizing 



In the past many methods such as the use of packers, plugging-back, 

 and two-pump treatments have been developed for use in acidizing proc- 

 esses where conventional treating was not applicable. While these meth- 

 ods improved upon the conventional technique, they still left much to be 

 desired, inasmuch as there was no definite check means to indicate where 

 the acid was actually going during the treatment. With the electric pilot 

 the engineer has knowledge of the location of the acid and of what portions 

 of the formation are taking the acid during the entire treatment. 



The following outline briefly points out some of the reservoir and 

 well conditions under which selective acidizing should be considered. 



1. New wells in which a change in lithology is present in the open 



