782 Subsurface Geologic Methods 



of secondary-recovery methods. Consequently, it is just as important to 

 provide information which will assist in avoiding failures as it is to en- 

 courage the use of secondary methods in fields where the probability of 

 success is high. Some of the things that will control the success or failure 

 of secondary-recovery operations, and some of the factors which should 

 be considered in the formulation of plans for a secondary recovery project 

 are discussed briefly hereafter. 



It is notable that most of the successful secondary-recovery opera- 

 tions in the United States are restricted largely to fields where primary 

 recovery of oil has been obtained by the action of dissolved-gas drive. 

 Fields in which active natural water encroachment has been effective are 

 generally not so well adapted to secondary-recovery operations after the 

 primary-recovery phase, on account of low residual-oil content. It is also 

 pertinent to emphasize that better reservoir performance efiiciency in cer- 

 tain of the more recently discovered dissolved-gas-drive fields should 

 eliminate, in part, the necessity for future application of secondary 

 methods. If oil had been produced efficiently in many of the older fields, 

 it is doubtful whether there would be such a high present activity in sec- 

 ondary-recovery operations. Therefore, secondary recovery, in a certain 

 sense, is a salvaging operation which may be partly avoided in the future 

 by the more efficient development and operation of oil fields. In this, 

 studies of primary pressure control, undoubtedly, will play a very impor- 

 tant part. 



It will be obvious that geologic factors will have an important bear- 

 ing on the adaptability of an oil field to secondary-recovery operations. 

 In order to evaluate these factors properly, all available subsurface geo- 

 logic information and production data should be plotted on a structure 

 map, showing the configuration of the top of the oil-producing formation 

 being investigated, and on an isopachous map of the pay zone. A num- 

 ber of graphic cross-sections of the pay zone, both parallel and at right 

 angles to the long axis of the field, will assist greatly in the interpretation 

 of the subsurface geology, and will permit taking better advantage of all 

 favorable geologic features which may be utilized for the improvement of 

 oil recovery. Homogeneity and continuity of the reservoir rock will pro- 

 mote uniform movement of injected fluids, and the depth and thickness of 

 the oil-bearing formation will have an important influence on the econom- 

 ics of the recovery operation. The distribution of gas, oil, and Avater in 

 the reservoir rock is most important, and must receive careful considera- 

 tion. Faults, which may seal off segments of the field, may seriously ob- 

 struct 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 de- 

 sign of a secondary-recovery operation. However, where oil and gas accu- 

 mulation 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, 



