680 



Subsurface Geologic Methods 



hole or pay section. Such changes may involve sand, lime, dolomite, or 

 primary or secondary anhydrite. 



2. New wells in which saturated zones of various permeabilities are 

 present. 



3. Wells showing excessive gas-oil ratios, but with the possibility of 

 additional available oil present in tighter saturated sections. 



4. Wells in which it is desirable to make additional gas available. 



5. Wells with intermediate water zones. 



B Ml 



OIL 



OIL 



ACID 



ELECTRODES 

 IN OIL 



ELECTRODES 

 IN OIL AND ACID 



ELECTRODES 

 IN ACID 



Figure 366. Operation of an interface locator. 



6. Wells deepened to new pay zones. 



7. Old wells with depleted zones but with oil remaining to be re- 

 covered from the tighter sections. 



8. Wells in which it is desirable to hold acid away from plug-back 

 or other repair work, including "Securaloy" lines, plastic or cement 

 squeeze, and redrill jobs. 



9. Wells in which conditions are such that it is important to keep 

 acid away from the casing seat. 



10. Injection wells in which it is desired to have uniform distribu- 

 tion of the gas or water being returned to the formation or in which it is 

 desirable to alter existing injection characteristics. 



In general, selective acidizing is applicable to wells producing through 

 perforations in casing set through the pay section, together with those wells 

 producing from open hole with the casing set above or on top of the pro- 

 ducing zone. In some instances, however, there may be conditions present 



