Miscellaneous Subsurface Methods 



757 



the exposed section by formation testing prior to acidizing. This procedure 

 gives an index to the initial productive characteristics of the well, permits 

 formational flushing, and leaves an oil column opposite the exposed sec- 

 tion. Formation testing at this stage permits definition of oil-gas and oil- 

 water interfaces which should be established before acidization. Initial 

 flushing also permits removal of drilling fluid present in cavities and solu- 

 tion channels. Following this stage a spinner-type survey may be made. 

 The next step involves acid washing of the hole wall. This stage may be 

 eliminated if bottom-hole pressures are sufficiently high. 



DRILLINS TIME 

 MIH. PER FT. , 



POROSITY A 

 SURFACE PRESSURE FOR 



POROSITY B 



SURFACE PRESSURE FOR 



FORMATION BREAKOOAN. 



1200-2600PSI 



NON- POROUS 

 SET PACKER 7065-91 



URFflCE PRESSURE FO 



FORMATION BREAKOOIV 



0- lOOO PSI 



NON -POROUS 



Figure 405. A — Typical producing interval. B — Circulating acid in place opposite 

 formation. (From Moore and Adams, World Oil.) 



Following the foregoing preparation procedure, the well is then ready 

 for selective acidizing. There are several methods for isolating the indi- 

 vidual zones for treatment. One method follows the two-pump procedure 

 of controlling the displacement and injection of the acid. A second method 

 makes use of a packer or packers to control the acid placement and injec- 

 tion. The packer method has the advantage of more positive control of the 

 acid under high injection rates and pressures. Other methods involve 

 acidization through selective gun perforations through solid cemented 

 casing and by use of temporary plastic and gel seals. 



