Miscellaneous Subsurface Methods 725 



tains a small percentage of water, will usually revert to the viscosity of 

 the original fluid within 24 to 48 hours; however, it is customary to use 

 the gel-breaker chemicals to assist breakdown of the gel. 



At the conclusion of the treatment the well is allowed to remain shut- 

 in, under the final pressure remaining when displacing the breaker fluid, 

 for a period of approximately 24 hours, after which it may be placed 

 back on production (fig. 387, step III) without any clean-out, as is neces- 

 sary after shooting with nitroglycerine. The breaker fluid, the gel medium 

 which has reverted to its original viscosity, and the new oil may all be 

 conveyed to the tank battery without further treatment. 



The gel used in the Hydrafrac process carries into the well, quan- 

 tities of graded sand of approximately 0.02 inches diameter, which are 

 added when the gel is prepared. The sand acts as a propping agent to 

 hold open the fractures produced by the application of hydraulic pres- 

 sure; thus when it is released and the fractures tend to close because of 

 the overburden, they are held at least partially open by the sand. 



It is important when considering the application of Hydrafrac to any 

 well that several factors be thoroughly studied to arrive at a decision as 

 to whether the application of the process is practical and economical. 

 These factors are principally: 



1. State of depletion of the well. There must be potentially pro- 

 ducible oil in place, with its movement to the well bore restricted 

 by low eff'ective permeability, but with suflScient bottom-hole pres- 

 sure to cause flow of oil into the well bore if permeability can be 

 generated. 



2. Formation permeability. Although Hydrafrac procedures can be 

 applied to almost any formation permeability, they are usually 

 applied to wells of rather low permeability. The permeability of 

 the zone being treated is one of the factors that controls the vis- 

 cosity of the gel to be used and the rate of input to produce the 

 pressures necessary to fracture the formation. The procedure has 

 been applied to wells with permeability of almost zero and on 

 other wells with permeabilities of larger values, as for example, 

 certain parts of the east Texas sands wherein permeabilities may 

 run above 1,000 millidarcys. As a general rule, the commercial 

 application of the process has been on zones of permeability of 

 less than 300 millidarcys. 



3. Formation thickness. With producing zones of great thickness it 

 is often necessary to isolate and treat portions of the zone one at 

 a time in order to gain maximum benefit from the process. With 

 thin zones, if closely associated with water or gas, complications 

 may be introduced, for it is quite possible that a fracture may be 

 produced in the water- or gas-bearing section rather than in that 

 which will produce oil. Further, the thickness of the formation, 

 coupled with permeability, controls the viscosity of the gel to 



