784 Subsurface Geologic Methods 



known secondary methods from sands possessing a residual oil content of 

 20 percent or less of the effective pore space, and for this reason secondary- 

 recovery methods have not been successful in some fields, even though the 

 total oil content of the reservoir may amount to many thousands of barrels 

 per acre. As a practical matter, the recoverable oil may be estimated by 

 subtracting the minimum saturation determined by laboratory tests from 

 the total saturation, with a correction allowance to account for lenticular- 

 sand conditions, heterogeneous permeability, or other factors which may 

 affect the uniform displacement of the remaining oil. 



Experience derived from studies of fluid saturations of oil-bearing 

 rocks has shown that the higher the oil saturation, the larger the percent- 

 age of oil in place that is recoverable, and this is a factor that would en- 

 courage early injection of gas or water into the reservoir to insure the 

 maximum economic recovery. 



One of the chief physical quantities entering into the process of re- 

 covery of oil by secondary methods is the viscosity of the crude oil. Un- 

 fortunately, all of the effects that a change in viscosity may have on the rate 

 of flow of fluids from an intake to a producing well cannot be predicted, 

 for the change may not be uniform throughout that part of the reservoir 

 in which a differential in pressure has been created. However, at any 

 particular point in the reservoir the rate of flow of a fluid phase is in- 

 versely proportional to its viscosity, if the pressure gradient and perme- 

 ability to the same phase remain constant. 



The time required for any given recovery process, therefore, will 

 depend on the viscosity of the crude oil, and an increase in viscosity may 

 result in a corresponding diminution in total recovery. The viscosity of 

 crude oils which have responded successfully to secondary-recovery oper- 

 ations has ranged from two to thirty times the viscosity of water. 



The phenomena of capillarity and surface tension are important fac- 

 tors in determining the efficiency of the recovery processes of gas and 

 water injection into oil-bearing formations. Oil is held within the pores of 

 the reservoir by the action of capillary forces and it is also adsorbed on 

 the reservoir surface as a film. Much remains to be learned about the 

 effects of these phenomena on secondary-recovery operations, but some 

 conclusions are rather generally accepted. 



Displacement of the oil from the reservoir involves the disturbance 

 of a state of equilibrium that exists between the interfacial tensions of 

 four phases: gas, oil, water, and solid minerals; and the fluid with the 

 greater adhesion tension with reference to the solid phase will tend to 

 displace the fluid with the lesser adhesion tension. 



In most petroleum reservoirs it is believed that both oil-wet and 

 water-wet surfaces exist, with the latter predominant except in a few 

 fields. It is likely that hydrophobic sands will not respond successfully 

 to water flooding. 



