Miscellaneous Subsurface Methods 681 



that will cause complications in wells completed in either of these ways. 

 Channeling behind the pipe and vertical fissures or fractures in the pro- 

 ducing formation are two such conditions that might be mentioned. Often- 

 times control can still be effected in some instances under these conditions, 

 however, through the proper application of materials that tend to bridge 

 and build up an effective filter cake, thus blocking the movement of fluid 

 into the channels or fractures. In other instances, the conditions mentioned 

 may be too dilfcult to permit effective control through the use of these 

 materials. 



It might be well to mention a few items that, if given thought and 

 study in specific cases, may go a long way toward assuring satisfactory 

 results from the selective type of treatment. All available bottom-hole 

 data should be plotted and correlated. Permeability surveys have been 

 found to be of primary importance for comparison and correlation with 

 geologic information, and, under certain conditions, temperature surveys 

 are quite valuable. Other factors for determination and consideration are 

 the possibility that emulsion troubles may be encountered, the gravity of 

 the oil to be used, and the solubility of the formation to be acidized After 

 considering all the available information, including past experience with 

 conventional and selective acid work in the area, where available, the 

 proper procedure for the individual well can be determined. 



Figure 367 illustrates several typical selective-acidizing applications. 



Permeability Surveys 



In determining the permeability profile of a section of well bore with 

 the electric pilot, the results obtained are in terms of relative permeability, 

 i.e., the permeability of any given zone is expressed in a percentage rela- 

 tive to the permeability of the entire open section. The relative perme- 

 ability of an exposed section is determined by measuring the fluid-injection 

 rate into all parts of the entire exposed formation. Various techniques 

 have been used for these relative-permeability surveys, but the one that 

 seems to give the best results in most of the wells consists in introducing 

 sufficient salt water into the bottom of the well completely to cover all the 

 section to be surveyed and then forcing this salt water into the formation 

 by the introduction of oil into the well. The rate of fall of the salt-water- 

 oil interface is determined by means of the electric pilot during the injec- 

 tion of salt water into the formation. 



In permeability-profile work the hookup for the electric pilot is 

 different from that used in selective acidizing in that it is necessary to 

 follow the downward-moving two-fluid interface with the electrode assem- 

 bly. In this procedure the electrode assembly is not seated at the bottom 

 of the tubing, no tubing being required in the well for this type of job. 

 In permeability-survey work, furthermore, the pumping rate at the surface 

 must remain constant throughout the survey. Figures 368 and 369 illus- 



