Secondary Recovery of Petroleum 783 



Homogeneity and continuity of the reservoir rock will promote uniform 

 movement of injected fluids, and the depth and thickness of the oil-bearing 

 formation will have an important influence on the economics of the re- 

 covery operation. The distribution of gas, oil, and water in the reservoir 

 rock is most important, and must receive careful consideration. Faults, 

 which may seal off segments of the field, may seriously obstruct uniform 

 fluid movement. Where the rocks are horizontal or where the rate of dip 

 is low, the effects of structure may be disregarded in the design of a 

 secondary-recovery operation. However, where oil and gas accumulation 

 has been controlled by a steeply dipping anticline or is associated in any 

 way with steeply dipping beds, the injection of fluids with reference to 

 structural position is most important. Because of differences in gravity, 

 it is preferable to inject gases at high structural position and liquids at low 

 structural position in the reservoir. The shape and geologic pattern of the 

 reservoir, developed by the conditions controlling the deposition of the 

 sediments involved, and the presence of shale partings or beds of low or 

 negligible permeability, may have very definite influence on the locations 

 best suited for producing and injection wells. 



The permeability of the reservoir rock will influence the distance 

 between wells, the well-spacing pattern, and the pressures which must be 

 exerted to promote the effective movement of fluids through the reservoir. 

 The principal use of permeability determinations is for the prediction of 

 rates of flow through the reservoir. In secondary-recovery operations 

 fluids are injected into the reservoir and other fluids are withdrawn from 

 the reservoir. The control of the amount and rate of fluid injected and 

 produced is very important in order to insure the maximum economic 

 recovery of oil, for in order to produce oil from a reservoir in which the 

 original energy has been dissipated, it must be displaced and moved by 

 extraneous force. Control of injection rate, therefore, is one of the factors 

 which help to control recovery. 



Although uniformity of the permeability profile is, admittedly, a most 

 desirable condition, secondary-recovery operations have been conducted 

 successfully in formations having a wide permeability range. Methods have 

 been developed which enable the plugging off of zones of extraordinary 

 permeability, and a concentration of oil around the periphery of producing 

 wells can be induced by a delay in their drilling until the voids of the 

 producing formation approach complete fluid saturation. 



It should be evident that the amount of oil that may be recovered from 

 a partly depleted sand by secondary methods, using either gas or water 

 injection, is dependent upon the amount of oil remaining in the sand. Of 

 almost equal importance is the amount of gas or the amount of water which 

 will have to be injected in order to obtain a given amount of oil. Observa- 

 tions in both the laboratory and the field have demonstrated that when a 

 sand contains less than 20 percent oil saturation, practically no oil will 

 flow through it. It is, therefore, virtually impossible to obtain oil by 



