780 Subsurface Geologic Methods 



oil concurrently is greater than that of the water. The initial stage is 

 followed by a viscous drag stage, at which time the permeability of the 

 reservoir surfaces to oil is greatly reduced and the permeability to water 

 is greatly increased, resulting in high produced water-oil ratios. 



Most water-flooding operations are developed on what is known as the 

 "five-spot" pattern, with the producing oil well located in the center of a 

 square formed by water-input wells at the four corners. Other patterns 

 have been employed to a lesser extent, but the theoretical flooding efficien- 

 cies of the various patterns are so close that the convenience of the 

 "five-spot" usually encourages its use. 



The spacing between water-input and producing wells has been de- 

 termined in the past largely by long experience. In some of the earlier 

 "five-spot" developments in northern Pennsylvania the distance between 

 input and producing wells was as low as 150 feet. More recently, distances 

 of from 225 to 250 feet have become common practice, which results in a 

 considerable reduction in development expense. Experience in the Brad- 

 ford field (Pennsylvania) has shown that there has been no appreciable 

 decrease in the oil recovery obtained from the wider-spaced floods, al- 

 though a longer period of time has been required to obtain the total re- 

 covery. However, in many fields the lack of continuity of individual beds 

 of the reservoir rock might cause the trapping of considerable oil if much 

 wider spacing should be employed, and for that reason there seems to be 

 rather definite practical limitations to further expansion of well-spacing 

 patterns. 



The time required to deplete the wider-spaced flooding projects, of 

 course, can be reduced by the use of higher injection pressures. However, 

 there is a limit to the pressure that can be applied because of the tendency 

 of the rocks to break or rupture under excessive pressures. It has been 

 found that pressure-parting of the rock can be avoided if the bottom-hole 

 injection pressure does not exceed about 1.25 pounds per square inch per 

 foot of depth. Variations from this breakdown pressure can be attributed 

 to differences in the strength and rigidity of the producing formation and 

 the overburden. 



Delayed drilling of producing wells for a predetermined period after 

 the injection of water into the reservoir has been commenced has resulted 

 in a much improved recovery of oil where a wide range of permeabihty 

 exists. However, the water must be introduced into the sand at a balanced 

 rate through each intake well so as to prevent an off-center concentration 

 of oil within the pattern. 



When the producing wells of some of the first delayed floods were 

 drilled, it was found that they would flow on account of a buildup of pres- 

 sure in the reservoir. This discovery immediately suggested tjiat complete 

 secondary recovery might be obtained by flowing if some back-pressure 

 could be maintained on the producing wells. Flowing secondary production 



