778 Subsurface Geologic Methods 



Secondary Methods of Oil Recovery 

 Vacuum 



Although the application of vacuum is not regarded as a secondary 

 method, according to the definition accepted in this paper, brief mention 

 of vacuum is justified because it was one of the first methods to be applied 

 purposefully to wells to increase the production of oil. Vacuum consists of 

 creating a pressure differential in the annular space in an oil or gas well 

 by applying suction, thereby causing a movement of liquid and gaseous 

 hydrocarbons toward the well bore. The amount of pressure differential 

 that can be created by the application of vacuum is, of course, very limited 

 in comparison to the injection of gas under pressure into the reservoir. 

 It was soon recognized that production benefits resulting from the use of 

 vacuum were not permanent, but it was maintained on many properties 

 because of the increased richness of casing-head-gasoline production. 



Air and Gas Injection 



Gas repressuring, in contrast to pressure maintenance, usually is ap- 

 plied in a field when the point of depletion by primary methods of produc- 

 tion has been reached and oil no longer can be produced profitably. For 

 this reason, in the older fields, many repressuring projects have a very 

 humble beginning; and the engineer who may be called upon to supervise 

 the secondary operation frequently will be confronted with a dismaying 

 collection of antiquated and worn-out equipment, junked holes, wells in 

 bad condition, and poor records. 



Unitization of the field for air- and gas-injection operations generally 

 is desirable and will result in lower development costs, maximum economy 

 of operation, and better control of reservoir performance. 



In northwestern Pennsylvania, where air- and gas-injection operations 

 have been conducted on an extensive scale for many years, it is common 

 practice to use old holes for producing wells and to drill and core new 

 holes for injection operations. Wide variations in permeability, which 

 many times are present, are controlled by segregation of the sand body into 

 two to five sections. This segregation is accomplished by setting packers 

 in such position that the air or gas can be injected into each sand section 

 separately and under different pressures. 



Usually an attempt is made to locate the intake wells so as to form as 

 symmetrical a well-spacing pattern as possible in relation to the producing 

 wells. However, more frequently the well patterns are irregular and the 

 well spacing is variable. In some of the more recent, intensively developed 

 projects in Venango County, Pennsylvania, the intake wells are located in 

 the center of a hexagon formed by six producing wells at the corners. The 

 common distance between the intake and producing wells is from 150 

 to 250 feet. 



