Secondary Recovery of Petroleum 779 



As a secondary-recovery operation, the injection of gas into a partly 

 depleted oil reservoir is essentially a continuous circulation of the gas 

 into and through the producing formation. Since the gas is a nonwetting 

 phase, it will pass through pore channels already opened by the previous 

 removal of fluids from the reservoir and will tend to move the remaining 

 oil by viscous drag rather than by direct displacement, such as takes place 

 by the action of an expanding gas cap. 



The rate of movement of a gas drive will be proportional to the pres- 

 sure gradient established in the reservoir. The movement of the gas 

 through the reservoir serves continually to reduce the oil saturation and 

 results in a very rapid increase in permeability to gas, producing what 

 may soon become a prohibitively high gas-oil ratio. This effect is respon- 

 sible for the relatively low efficiency of the gas-drive recovery process, and 

 limits definitely the amount of oil which can be obtained by gas injec- 

 tion. 



The chief justification for the injection of gas is that it will make 

 available additional oil at a commercial rate that probably would not be 

 obtained otherwise because of the low rate of production which prevails 

 toward the end of the primary-production phase. In other words, gas in- 

 jection serves to accelerate recovery during the late life of a field by re- 

 tarding the normal rate of decline. 



The history of air and gas repressuring in many parts of the United 

 States is very similar. When primary production of oil by conventional 

 pumping methods is no longer economically justifiable, the operator must 

 make some change in practice or abandon his property or field. Since a 

 minimum investment in new wells and production equipment is generally 

 required for the injection of air or gas, repressuring by these fluids has 

 been found to be the cheapest method for maintaining or increasing the 

 production of oil. As a result, profitable production can be maintained, 

 and the ownership of the working interest in wells, properties, and possibly 

 entire fields preserved to the producers until conditions may become more 

 opportune for the application of some other secondary method, such as 

 water flooding. 



Water Flooding 



The process of water flooding consists in applying water under pres- 

 sure to an oil-bearing formation by means of specially equipped intake 

 wells. It has been most successful in fine-grained, tightly-cemented sands, 

 which are frequently characterized by high residual oil content after the 

 primary phase of production. In the initial stage of water flooding an 

 oil bank is formed ahead of the advancing water if the mobility ^ of the 



' "Mobility" is defined as the effective permeability of a reservoir rock to the fluid phase divided by 

 the reservoir viscosity of that phase. 



